This chapter provides an overview and assessment of the appropriateness of current measures in place to support drought resilience. This includes demand-side measures (e.g. awareness raising, technological solutions, water allocation regimes, price signals) as well as supply side measures (water infrastructure, innovative water re-use measures). It briefly mentions the importance of measures meant to improve water quality.
Adapting the Paris Metropolitan Area to a Water‑Scarce Future
Abstract
Drought resilience measures address both supply and demand for water resources. Raising public awareness of the risk of water scarcity and the potential for water savings plays an important role in regulating water demand. The use of technology such as water-saving kits can help reduce water consumption. Water pricing and allocation schemes can also encourage consumers to rethink their usage in certain situations. At the same time, measures designed to optimise resource management have an impact on water supply; for example, encouraging nature-based solutions to recharge groundwater increases available water supply to prevent drought. Infrastructure can also be used to harness unconventional resources (rainwater, greywater or wastewater), helping to reduce demand for primary water by facilitating reuse or the use of currently underutilised resources.
This chapter assesses the level and type of adjustments needed to current resilience measures to guarantee the region's future water resilience. It begins by discussing why it is necessary and useful to set a resilience target (4.2). It then provides an overview and discussion of the measures currently in place to address the risk of water scarcity in the region. Measures are presented according to whether they have an impact on water demand (4.3) or supply (4.4). Although quality issues are critical in addressing lower dilution capacity when water levels are low, this chapter will focus primarily on measures to improve the quantity of resources. This is because water quality measures are necessary regardless of climate change, and the water agencies have already clearly identified measurable targets here. Nevertheless, there are no reliable projections to help estimate pollutant concentrations by 2050 and thus make measures coherent.
4.2.1. Aligning measures with a specific target
The resilience measures must be chosen to achieve a specific resilience target. Defining a resilience target is useful to determine the level of effort required and therefore the type of measures to be put in place. For example, the United Kingdom has set a resilience target to avoid hosepipe bans during a drought with a 500-year return period (GOV UK, 2023[1]). A similar objective in France would mean ensuring that no crisis level is ever reached, as defined by the current French drought management plan (Box 4.1). Were this objective chosen for the Paris metropolitan area, it would therefore mean that the region would not permit any restrictions on river navigation, irrigation, industrial water abstraction or watering of urban green spaces and sports facilities in the case of a drought such as the one covered in chapter 1. Yet, in such a drought scenario, in the absence of additional measures, these restrictions would be applied for up to 166 days.1
The aim of drought orders is to anticipate and reduce the risk of water scarcity. Drought orders define a system of gradual restrictions, ranging from awareness-raising to abstraction bans, i.e. reduced water availability for affected users. These restrictions apply when flow rates or groundwater levels dip below thresholds that have been set for various monitoring stations on rivers and aquifers. The graduated scale helps pre-empt the risk by engaging users at the first signs of low water levels. These progressive measures are designed to reduce water abstraction in order to preserve existing resources. There are four risk thresholds:
Warning: Inform and encourage users to save water
Alert and Heightened Alert: Reduce water abstraction by up to 50%
Crisis: Implement bans to protect priority usage
At the national level, there is a target for reducing abstraction, which is intended to improve the resilience of water basins. The National Water Plan developed by the French government proposes a series of targets, including a 10% reduction in water abstraction by 2030, and a number of projects for unconventional water reuse by 2027. These targets will need to be tailored to each water basin to reflect their specific challenges.
However, this target does not reflect an acceptable level of resilience or risk at either the national or local level, and it may prove insufficient. Although targets to reduce water abstraction and to protect and harness resources could help increase resilience in French regions, they do not reflect a precise risk assessment or a specific level of protection. If the region reduces current abstraction by 10%, it may still require the water usage restrictions for the crisis level defined in the current drought management plan, as covered in chapter 1. Furthermore, these targets do not take into account the long-term impact of climate change on resources.
Setting a resilience target represents a trade-off between the costs of adaptation and the costs of inaction, both of which are complex to assess. Targets for reducing abstraction volumes may vary significantly depending on the accepted level of risk of water scarcity. For example, a target of avoiding reaching crisis level in drought orders means ambition is lower in terms of reducing abstraction than if the aim were never to exceed the warning threshold). Similarly, setting the levels of reduction required depends on the approach adopted. These volumes can be calculated as the water deficit that would occur in the event of exceeding the crisis or warning threshold and which would need to be prevented. Another approach, which is more systemic but also more conservative, involves observing long-term water trends, such as groundwater recharge, and planning a uniform reduction in withdrawals over the course of the year. The approach should be chosen following in-depth discussion with the water agency. The resilience target must be designed to be socially, environmentally and economically optimal, and to form part of a long-term approach to climate change adaptation.
4.2.2. Identifying a combination of measures appropriate to the users and the area
The climate change adaptation strategy for the Seine–Normandy river basin sets out a suitable hierarchy of resilience measures to reduce abstraction in the basin (Agence de l'Eau Seine Normandie, 2023[2]). To reduce human pressure on water resources, and consequently water abstraction, the adaptation strategy prioritises measures that encourage demand reduction. By reducing water consumption, these measures reduce users' dependence on water and thus their vulnerability in the event of drought. This also protects upstream resources at the same time as reducing the risk of water scarcity. The resilience strategy for the basin then moves on to solutions more related to water supply. The second priority is nature-based solutions, because of their multiple benefits and because they are "no regret" solutions. Finally, if necessary, the use of technology can be considered as a last resort.
The Seine–Normandy water agency proposes a pathway for reducing abstraction per user, which gives an indication of the effort expected from each user. The Seine–Normandy water agency has translated the goal of reducing water abstraction by 10% by 2030 into reduction pathways for drinking water, industry and irrigation. Given the progress made by drinking water and industrial users, the agency recommends reducing abstraction by these two sectors by 14% and 4% respectively (Agence de l'Eau Seine Normandie, 2023[2]). Since agricultural abstraction is marginal, and needs are expected to increase to cope with climate change, the water agency plans to maintain irrigation abstraction at its current levels. This target is a challenge, as the current trend of increasing abstraction would result in a 45% increase in abstraction by 2050 (Chapter 2).
A focus on demand reduction measures first requires accurate information about consumption and the efforts users are already making. For example, the regional Water Syndicate (SEDIF in French), assessed the water consumption of some of the local authorities it serves and found that some facilities, such as schools and associated sports halls, were suffering from major leakages and had unnecessarily excessive water consumption. This contrasted with a recent assessment, also by SEDIF, of household water consumption within its area. It estimated consumption to be 100 litres per person per day (INSEE, 2023[3]), with large disparities depending on standard of living or municipality (ranging from 78 to 138 litres per person per day). As a result, there may be more limited scope for reducing water consumption by households than by local authorities. In the remainder of this report, due to lack of data for the region, the allocation of drinking water will be based on assessments for the City of Paris, e.g. 60% for residential use, 9.1% for economic activities such as offices, shops and tourism, and 1.1% for hospitals (APUR, 2022[4]). Meanwhile, although water usage for irrigation may be justified due to the impacts of climate change, there may be varying degrees of effort to reduce use depending on the type of farm or crop.
There is no analysis of current consumption that helps identify the demand reduction efforts needed according to the type of user. With the exception of the INSEE study of the SEDIF area, or that of the Agence d'urbanisme de Paris [Paris Town Planning Agency], which calculates drinking water consumption for the whole of Paris (APUR, 2022[4]), there have been few studies allowing demand reduction policies to be tailored to users. This lack of data is partly explained by the absence of systematic metering system to monitor water consumption in the region. Similarly, an analysis of consumption would show the difference between companies that are already being careful and those that could reduce their water usage. Finally, for a target of stable abstraction for irrigation to be achieved, there will need to be a clear policy supporting irrigation for certain crops, along with an appropriate allocation of water resources.
However, even without this analysis, it appears that demand reduction measures may not be enough to achieve the reduction in water abstraction intended by the water agency. Residential consumption of drinking water in the region is below the French average of 148 litres,2 and relatively low compared with other OECD countries (Chapter 1). The World Health Organisation considers sufficient consumption of drinking water to be between 50 and 100 litres of water per person per day (United Nations, n.d.[5]). Therefore, households could reduce their consumption and still be within these limits. For other users, such as municipalities responsible for road cleaning, there is probably more limited scope to reduce consumption, as cleaning requirements are not likely to decrease. At a time when France is pursuing a policy of reindustrialisation, it could prove difficult to reduce water consumption through demand reduction policies alone. Similarly, as the region is seeking to improve its food sovereignty (for example through developing market garden crops), a pathway of reducing agricultural abstraction could prove difficult without additional measures to manage the water supply.
Increasing the region's resilience requires a strategic combination of measures to manage both water supply and water demand. Given the limited scope for demand reduction by industry, farmers and certain consumers of drinking water, local authorities in the region are considering complementary measures to harness additional resources such as rainwater or swimming pool water, as well as nature-based solutions.
However, these measures need to be tailored to the geographical context. For one thing, some parts of the region are more vulnerable, as demonstrated by the spatial variation in water restriction measures during the 2019 drought (Figure 4.1). The soil type affects water infiltration capacity and effective groundwater recharge. For example, soils with higher levels of organic matter have better water-holding capacity (Agence de l'Eau Seine Normandie, 2023[2]). In rural areas, therefore, it is probably a priority to target measures towards agricultural practices, given the potential reductions in abstraction. Conversely, in urban areas, one of the priorities could be to combat the increase in built-up land. Likewise, there is great variation in water consumption profile between areas with dense multi-dwelling units (e.g. Paris) and those with predominantly detached houses (e.g. rural areas).
Source: Authors, based on Propluvia data.
A cost-effectiveness analysis of the various measures could prove useful in identifying priorities. Examples of such analysis to select measures are shown in Box 4.2.
United Kingdom, South East Region
In the South East of the United Kingdom, an alliance of six water operators (Water Resources South East) has devised a programme of priority measures to achieve the region's resilience targets. This programme identifies measures to be implemented progressively, under a range of demographic and climate scenarios. For each future scenario, the programme identified the quantity of water to be provided in order to achieve the resilience targets set at the national level. It evaluated 1 400 measures to determine which combinations of options were the most cost-effective. Initially, operators are focusing on measures to manage water demand, i.e. demand reduction. For example, Thames Water, the operator for London, plans to reduce water abstraction from certain rivers by 2025, reduce leakage from 700 km of the water network using cutting-edge technologies, and carry out 400 000 water consumption audits, while continuing its efforts to raise public awareness. Alongside this, Thames Water plans to harness new resources by 2030, through groundwater recharge, reuse of treated wastewater and inter-basin transfers. Efforts will be stepped up after 2030 by continuing with these measures and also creating new reservoirs.
The Netherlands
Preparation of the Freshwater Delta Plan included a cost-effectiveness analysis of 150 measures under a range of climate and socio-economic scenarios. The aim was to assess the impact of these measures on the risk of drought, i.e. to compare the costs incurred in the event of drought in 2050 (taking climate change into account), both with and without the adaptation measures. This cost was then compared with the cost of the adaptation measures. This study helped identify the most economically appropriate measures for each scenario. For example, under a "moderate" scenario, the study showed that certain measures had positive impacts on reducing agricultural losses but did not have sufficient impact on water resources compared with the cost of the measures. They would therefore not be financed by the state but could be funded by farmers. By contrast, certain measures are considered a priority because of their positive impact, such as protecting Lake Ijsselmeer, one of the country's main freshwater reservoirs, which is suffering from saltwater intrusion.
This section explores demand management measures that help make users less vulnerable. Among these measures, awareness-raising campaigns and technologies to increase water efficiency can be particularly effective in terms of volumes of water saved. Other measures, such as water tariffs, could prove more worthwhile for business operators than for households. Finally, although it is more difficult to estimate the impact of management measures such as allocation schemes, this type of measure has an important role in encouraging careful use of water, improving resource-sharing and preventing future conflicts of use (Table 4.1). In this section, measures are examined from the perspective of reducing consumption. However, these measures are also very effective tools for encouraging users to implement novel solutions (rainwater, greywater, wastewater, etc.) and thus reduce water abstraction (without impacting on actual water consumption). The use of measures in this context is detailed in the next section.
Reduction potential of the measure for the user concerned, impact on the 10% reduction target, and contribution to the pathway of reduction per user set at basin level
Note: The column "Contribution to target of 10% reduction in abstraction [in the region]" indicates the part played by the measure to reaching the final target of a 10% reduction in abstraction set by the National Water Plan. The column "Contribution to per-user target" evaluates the effectiveness of the measure in achieving the reduction target per user. For example, in the case of the use of water-saving kits, the savings achieved go 47.1% of the way to achieving the target of reducing residential consumption of drinking water by 14%. This accounts for 16.8% of the total savings required to achieve a 10% reduction in abstraction. A 1% reduction in consumption is taken to translate into a 1% reduction in abstraction, without taking account of water losses in the networks. This approach does not take into account the volumes of water returned by consumers and then withdrawn from the river basin downstream. However, from an accounting perspective, this is a logical approximation of abstraction. The impact shown for awareness campaigns and labels is based on a literature review. For the price increases, the variability in demand reduction reflects the range of impacts observed in the literature.
Source: OECD calculations. (Climate Adapt, 2023[9]) (Ehret et al., 2021[10]) (DRIAAF, 2019[11]).
4.3.1. Awareness-raising and support for users
Raising awareness about the drought risks encourages the various users in the basin to reduce their water consumption and shift their consumption or production towards more water-efficient products. Addressing household consumption, for example, can lead to changes in production activities (industry and agriculture). It is also important to provide awareness-raising and training for the agricultural sector, as water is a direct factor of production here.
Public information campaigns
Awareness-raising campaigns inform the population about drought risks and encourage demand reduction. An awareness-raising campaign can be based on motivational or transformational factors to achieve specific consumption targets. These campaigns can be run on a national or local scale. They can promote social norms or identity-based messages about belonging to a particular town or community, emphasise the financial benefits of reducing water consumption, or simply appeal to an assumed sense of environmental responsibility. This type of campaign has proved particularly effective in bringing about behavioural change in Europe, Australia and the United States, for example (Box 4.3).
Both throughout France and in the region, awareness-raising is gradually moving towards risk prevention, although it remains rooted in crisis management. There is a range of tools available to communicate the measures to be taken in the event of a serious drought, such as the Propluvia website (Propluvia, n.d.[12]) which shows the departments currently subject to a drought order, or communication campaigns to be used at the first signs of risk. To complement these tools, in 2023 the French government launched a new tool, VigiEau, which provides users with personalised risk information and suggestions for reducing their consumption. Alongside this, local authorities disseminate information, including lists of simple water-saving tips. The Seine–Normandy water agency also funds training programmes to inform citizens and elected representatives about the risks of degradation of water resources and the challenges involved in preserving them (Agence de l'Eau Seine Normandie, 2021[13]).
Although awareness-raising is an essential way of engaging users and reducing their consumption, the effectiveness of these measures should be looked at in context. Their potential depends on initial consumption levels, the type of environment (whether urban area with dense multi-dwelling units or rural area), the performance of the drinking water networks and the availability of resources (whether the area is used to water stress or abundant resources). The efforts being made, both throughout France and in the region, are important in terms of getting users more involved but may not be sufficient to achieve the water abstraction reduction target set by the Seine–Normandy water agency. It is important to note that these campaigns are also designed to encourage the use of alternative resources, thereby limiting abstraction from the resources available (see Section 4.3).
Melbourne
The Target 155 campaign encouraged Melbourne's citizens to lower their average water consumption to 155 litres per day to meet the needs of a growing population. This was achieved by targeting the 10% of households that consume the most water: questionnaires were sent out to understand their consumption habits and protective measures tailored to their usage were promoted. Implementation was evaluated.
Atlanta
In Atlanta in the United States, a trial compared the effectiveness of two awareness campaigns. The first involved sending water users a comparison of their consumption with that of their neighbours. The second campaign called on users to help preserve the city's water resources. Users in the first group reduced their consumption by 4.2% more than the second group, highlighting the effectiveness of communication involving social norms.
Berlin, Copenhagen, Tallinn and Zaragoza
A study looked at four European cities with low residential water consumption and multidwelling units to assess the effectiveness of a range of water demand management measures on domestic water consumption levels. Based on a survey of managers of drinking water networks, this study identified the most effective measures that explained how consumption had been reduced over a 20-year period. The study compared pricing measures, awareness campaigns about water savings, and campaigns rolling out technologies to make water use more efficient. With the exception of Berlin, the awareness-raising campaigns were identified as some of the most effective measures, along with campaigns encouraging the use of technologies to improve efficiency. In the case of Berlin, upgrading the networks was the most effective measure, suggesting different challenges.
Labels and standards also help make users more aware of their water consumption. According to the French Agency for Ecological Transition (ADEME), water-saving household appliances can use up to five times less water. Given that washing machines and dishwashers account for 22% of domestic water use, this could represent a saving of around 18 litres of water per household per day. Water consumption could also be included in building standards, as suggested in the action plan of the Assises de l'Eau working groups, which proposed incorporating water-saving measures into all new buildings from 2022. Similarly, the water footprint label raises awareness of the quantity of water required to produce something (in agriculture, industry or services) (Box 4.4).
France already includes water-related information on its household appliance labels and is planning to introduce a water footprint to take this further. France has introduced a regulatory energy label for major household appliances that also includes water consumption (Ademe, 2021[15]). The National Water Plan also calls for the water footprint to be displayed by 2024.
Australia: Water Efficiency Labelling and Standards
The Water Efficiency Labelling and Standards scheme assigns a water efficiency score to products such as taps, toilets and showers. Between 2017 and 2018, this scheme saved 112 billion litres of water, representing 6% of domestic consumption and 0.8% of the country's total consumption.
Water footprint
In 2002, the United Nations Educational, Scientific and Cultural Organization (UNESCO) proposed a water footprint indicator for goods produced and consumed. This indicator is designed to make producers and consumers aware of the impact of human activities on water resources. The figure below shows some water footprint scores for agricultural products. These are global averages, and there are major disparities between regions due to different irrigation practices and soil types. The water footprint can help steer local production towards using more-water-efficient resources, in response to increased demand for this type of product. This is particularly true for the region, which exports or processes some of its production elsewhere in France and abroad.
Source: (Mekonnen and Hoekstra, 2011[16]).
Potential of water footprints
In Greece, a study has shown that although young consumers are not fully aware of the environmental challenges associated with water, they would welcome a quantified water consumption label, regardless of their level of education. Even those who were in insecure employment said they would be willing to pay a premium for water-saving products.
Training for the agricultural sector
Training in resilient practices raises users' awareness of practices that are less water-dependent and therefore consume less water. For example, introducing new crops or new farming practices that consume less water is one way of reducing water consumption. Similarly, replacing drought-sensitive vegetation with more-resilient species helps boost the region's resilience. According to the OECD questionnaire, training is one of three areas that need to be prioritised.
The Ministry of Agriculture is exploring initiatives to provide farmers with training aimed at improving water management. The Ministry has drawn up a training plan, Enseigner à produire autrement (Learning how to produce differently). The first version (2014-18) was rolled out to all agricultural education institutions, with support from the Regional and Interdepartmental Directorate of Food, Agriculture and Forestry (DRIAAF) of the Paris metropolitan area. This type of training raises awareness among future farmers of the challenges of adapting to climate change-induced droughts. This training plan was updated in 2019 to incorporate ecological transition and agroecology.
Yet, trends observed in the region indicate a marginal change in the proportion of water-efficient crops. There was a slight increase in areas planted with sorghum and sunflower - two crops requiring limited water - between 2010 and 2022 (Figure 4.3). However, large areas are still planted with water-intensive crops such as maize, potatoes, and sugar beet. A significant proportion of the area used for these crops is reliant on irrigation (7.1% for maize, 18.8% for sugar beet and 69.1% for potatoes in 2020 (Agreste, 2024[21])). Meanwhile, market garden crops, which currently account for a marginal share of the region's agricultural land, may see further development (Région Ile-de-France, 2023[22]). These crops are highly dependent on irrigation (75.4% of acreage is irrigated).
Note: The proportions represent the utilised agricultural area of a given crop, in relation to the utilised agricultural area of the region, excluding meadows, grassland and individual orchards.
Source: Authors, based on Agreste data 2012-22.
Efforts are being made at the local level to help the agricultural sector cope with the impact of climate change on water resources. The region is committed to improving access to local produce, via supply platforms for school canteens and the provision of meal boxes in the Paris metropolitan area. This type of measure adds value to local crops by guaranteeing a market to suppliers who would change their practices. However, water consumption has not yet been identified as one criterion to select food products. The regional Chamber of Agriculture supports farmers by providing them with information and helping them to access grants such as those offered for agroecology through the post-COVID-19 recovery plan France Relance. The DRIAAF is also observing a transformation in farmer profiles, which indicates opportunities for agroecology and climate change adaptation. However, it considers that a disruptive scenario is unlikely, especially as the proportion under irrigation is still marginal and there is significant national and international demand. For example, 10.2% of sugar beet crops are exported (Institut Paris Région, 2021[23]) and the region contributes to make France a world's leading exporter of potatoes (Haut Commissariat au Plan, 2019[24]).
While it is essential to incorporate climate issues into agricultural training to encourage changes in practices, this training must also be adapted to address the region's future challenges. For example, only a limited area of maize is currently under irrigation (7.1% in 2020 (Agreste, 2024[21])). However, future climate conditions in the region could bring this figure closer to regions that irrigate more, such as the Val-de-Loire region, which currently irrigates 19.1% of its area planted with maize (Agreste, 2023[25]). Furthermore, the intention to develop market gardening throughout the region would indicate an increase in abstraction for irrigation. A first step in calibrating and supporting the transformation of the agricultural sector will be to understand how climate change could affect the needs of irrigated crops, and to assess the irrigation requirements associated with developing market garden production in the region. With a view to keeping water abstraction for irrigation constant, informing people of future risks and challenges can help convince them of the importance of reducing water consumption in anticipation of future conditions. This also helps quantify how much of the reduction in abstraction can be achieved by demand reduction measures such as those mentioned above alone, and to see whether these measures need to be supplemented. However, these changes need to be reflected in both national and regional agricultural policy (Chapter 2).
Initiatives to train farming professionals in water-saving practices can also encourage the adoption of less-polluting methods, resulting in valuable co-benefits. Faced with diminishing water resources and hence lower dilution capacity of water bodies it is essential to preserve water quality to avoid incurring excessive treatment costs or being unable to use these resources due to pollutants concentration. In the Paris metropolitan area, the City of Paris authority is committed to supporting the farming profession. Eau de Paris, with the support of the Seine–Normandy water agency, is carrying out a series of initiatives designed to improve the resilience of the drinking water supply, reduce abstraction, preserve the quality of water resources, and improve understanding and monitoring of resources. To achieve this, Eau de Paris is working with farmers to reduce or even eliminate the use of fertilisers and pesticides, adopt sustainable farming methods or cultivate more meadows. Through these initiatives, Eau de Paris is providing technical advice and financial assistance, and setting up short supply chains to guarantee markets for farmers who commit to this approach. While these projects go beyond reducing water abstraction, they also contribute to the region's water resilience by protecting catchments and limiting the risks associated with reduced dilution of pollutants in the water during times of drought.
4.3.2. Technological solutions
Drinking water
Easy-to-implement technologies can reduce drinking water consumption. Examples include water-saving equipment such as aerators or flow restrictors. These different technologies can be brought together in water-saving kits distributed by local authorities or water operators. Depending on the context, this type of kit can reduce overall household consumption by between 4% and 20%, without compromising residents' comfort (SMEREG, 2023[26]) (Ville de Paris, 2013[27]) (Agence Locale de l'Energie Montpellier, 2017[28]). However, the potential varies between densely populated urban areas and rural communities, as highlighted by a trial carried out in Gironde between 2013 and 2015. This recorded a 4% drop in consumption in built-up areas, compared with an average 12% in communities with fewer than 1 000 inhabitants (SMEREG, 2023[26]).
The distribution of water-saving kits is still uncommon in France overall, including in the region. The Brive conurbation, several local authorities in Gironde, and the Rennes basin water association3 have all subsidised water-saving kits for their customers. In the Paris metropolitan area, the regional water syndicate, SEDIF, can distribute kits on request to the most economically vulnerable users (Chambre régionale des comptes Ile-de-France, 2022[29]). The City of Paris authorities have signed a charter with social landlords and private housing operators to promote the installation of water-saving kits in Paris, which are expected to reduce household water consumption by 8% on average (Ville de Paris, 2013[27]).
Installing such kits could nevertheless result in water savings in the region. According to ADEME, personal hygiene accounts for 39% of water consumption and toilet flushing for 20%. Simply installing this type of kit could therefore reduce the amount a resident of the Paris metropolitan area uses for these two activities by between 4.2 and 21 litres per day. This would be enough to reduce drinking water consumption by households not already equipped with these technologies by between 3.4% and 17%. This estimate for domestic consumption could be revised following the national research project Dynamique de la consommation et référentiels de l'eau [Consumption dynamics and baselines for water]. The City of Paris authority Eau de Paris4 is taking part in this project, which aims to improve understanding of domestic water usage in order to tailor water-saving policies. Nevertheless, the potential savings due to these kits remain indisputable and worthwhile considering the minor effort required.
Rolling out these technologies requires to raise awareness on the benefits of these solutions. Local authorities can also detail the benefits of such solutions to raise awareness among users of the potential savings they could make by installing kits. SEDIF indicated that some users had kits but did not always install them, suggesting a major awareness-raising challenge. In addition, the City of Paris has indicated that householders do not always change an aerator that has become inefficient, undermining the long-term effectiveness of a kit distribution policy.
It is also possible to employ technologies that avoid the use of water altogether. For example, the City of Paris has led a project for source separation of urine in the Saint-Vincent-de-Paul eco-district.5 Urine diversion is a treatment method that can be applied directly at toilet bowls or non-flushing urinals. This type of practice was developed in Scandinavian countries in the 1990s and reflects a circular economy approach (Box 4.5). It saves water, energy and reagents used for wastewater treatment and helps to protect the environment, particularly aquatic environments.
However, this type of approach seems more suited to low-density housing (Arc'Eau, 2021[30]) and requires a suitable storage or treatment system, which is costly. Moreover, the benefits may prove too limited in relation to the cost of implementing the infrastructure required for separation in existing buildings.
The Swedish ecovillage of Understenshöjden is one of the pioneers of urine diversion. It consists of 44 homes housing 160 residents who form a homeowners' and tenants' association. The residents chose to install a system of urine-diverting toilets linked to a urine-collection network. This urine is stored in two 40 m3 tanks; when one tank is full, the urine is directed to the other. A farmer collects this urine annually and uses it as a fertiliser for cereal crops.
Source: (Arc'Eau, 2021[30]).
Agriculture
In the agricultural sector, financial incentives can encourage people to adopt technologies to increase water efficiency (e.g. drip irrigation, soil moisture sensors). For example, this has been done in the Murray-Darling Basin in Australia, where improvements to irrigation infrastructure have been funded to enable volumes of water previously used for agriculture to be reallocated to other uses (Wheeler et al., 2020[31]).
However, improving water efficiency must go hand in hand with managing how the volumes saved are used. Financial incentives to improve efficiency can lead to rebound effects, as observed in Australia, where abstraction by irrigators who had received subsidies actually increased by 21% to 28%.
4.3.3. Water allocation regimes
Water allocation regimes can help reduce water consumption by allocating quantities of water in line with the resources available. "Water allocation regime" refers to "the combination of policies, mechanisms, and governance arrangements (entitlements, licenses, permits, etc.) used to determine who is allowed to abstract water from a resource pool, how much may be taken and when" (OECD, 2021[32]). Allocation regimes can take climate challenges into account and regulate future withdrawals in anticipation of reduced resources. Only 57% of OECD countries, including France, take climate change into account in their allocation regimes (OECD, 2015[33]).
Currently, climate challenges are considered in the French water allocation regime only through the lens of the country's drought order system, and thus crisis management (Box 4.1). Although water management in France is guided by the goal of ensuring the good ecological status of water bodies, the departmental prefect authorises any abstraction as long as it does not affect the environment. Therefore, apart from in areas experiencing water stress, the only measures taken to control allocation of use to address the risk of water scarcity are those taken in the event of drought.
Drought orders could prove counterproductive from a climate change resilience perspective. This is because they assume acceptance of risk management based on ad hoc restrictions. Furthermore, drought orders constitute a short-term approach and therefore do not allow for an organised reduction in consumption. In addition, users often perceive the orders to be unfair, as they do not take into account differing user capacity or effort-sharing.
The repeated issuance of orders since the start of the 21st century raises questions around how effective this approach is as the only water allocation regime. If these supposedly exceptional orders are being repeatedly issued, it suggests that the quantities of water available are no longer sufficient for usage in the region, and that usage must therefore be addressed (Figure 4.4). Besides, there is currently no way of assessing the effectiveness of these orders, either in terms of compliance with them or their impact on environmental protection (IGEDD; IGA; CGAAER, 2023[34]). Although it is necessary to have crisis management measures, the recurrence of drought episodes since 2003 suggests a need to step up preventive measures and to better anticipate the phenomenon.
However, the drought order system is changing to reflect long-term challenges. For example, French law (France, 2023[35]) now exempts a group of companies with potential impacts on the environment ("installations classified for environmental protection") from restrictions, on the condition that they reduce their water consumption. This approach may encourage users to reduce their consumption, as they would receive a reduced water allocation in times of crisis.
Nevertheless, a lack of understanding in the region of the risks associated with water abstraction is preventing consideration of water allocation beyond aquifers that are under water stress. For a water allocation regime that would make the Paris metropolitan area more resilient, there would need to be an understanding of the risks associated with the cumulative impacts of abstraction. Studies of abstractable volumes help determine the overall volume that can be abstracted in eight out of ten years, while guaranteeing that aquatic environments are in good condition. The French government circular of 30 June 2008 (Gouvernement, 2008[36]) requires these studies be conducted for water distribution zones, to avoid having to regulate water resources through drought orders and to better anticipate risks. This type of study helps to anticipate water scarcity and to allocate resources, taking into account the cumulative impact of abstractions. For example, a gradual reduction in authorised abstraction over time could encourage more-efficient water use to meet the challenges of climate change.
Maximum number of days under a drought order for large rivers in the Paris metropolitan area (Seine, Marne and Oise)
Note: Larger rivers are less susceptible to drought orders than smaller rivers, which are more often subject to crisis orders.
Source: Authors, based on HydroPortail data on water levels and the drought order thresholds defined in the various framework agreements.
Participatory approaches at the local level can also improve water allocation to make it more resilient to climate change. In France, the goal is for water resources to be managed collaboratively between farmers and other stakeholders in the area, through unitary bodies for collective management and regional water management projects. More locally, given the vulnerability of the Champigny aquifer in the region, the association AQUI'Brie has organised consultations to define allocation thresholds for each user, based on setting a maximum volume of abstraction and the principles of dynamic and flexible water allocation (Box 4.6). This management approach has avoided the need to issue abstraction orders for the aquifer since 2013.
Unitary body for collective management
A unitary body for collective management is responsible for managing and distributing volumes abstracted for agricultural use in a given area. The unitary body submits a joint application for a multi-year authorisation for abstraction to the Regional and Interdepartmental Directorate for the Environment, Planning and Transport (DRIEAT in French). If this authorisation is approved, the unitary body for collective management holds the overall authorisation for abstraction for all irrigators within its management area, covering surface water, groundwater, water bodies, reservoirs and dams. The unitary body for collective management then allocates the water available to it among the irrigators in its area and establishes rules for adapting this allocation in the event of a temporary restriction or suspension of use. The allocation plan is approved by the Prefecture. The unitary body must also submit an annual report to the Prefecture, detailing consumption during the irrigation season and estimating future abstraction requirements. In the event of a water shortage, the unitary body for collective management is responsible for setting rules to limit abstraction.
Regional water management project
The regional water management projects approach is designed to achieve a long-term balance between local needs and resources. It also supports the construction of water storage facilities, where technically and economically suitable. The regional water management project is a tool for local consultation between the various users (drinking water, agriculture, industry, navigation, energy, etc.), in order to prevent conflicts of use. It promotes demand reduction, without removing the option to store water or to transfer it from areas with a surplus in the event of shortages during low-water periods. Regional water management projects should be prepared using a forward-looking approach that reflects the impacts of climate change on the availability of water resources.
AQUI'Brie
The Champigny aquifer is a major source of drinking water for communities in Seine-et-Marne and for part of the Paris conurbation. Of the volumes abstracted, 87% is used for drinking water supply (in the west) and 7% for irrigation (in the east and centre), resulting in territorial disparities. The association AQUI'Brie was set up in the 1990s, in response to deterioration in the quality and levels of the Champigny aquifer. It brings together government departments, local authorities, departments, town councils, the French water agency, agricultural and industrial users, water producers, associations and scientists, with the aim of managing the aquifer collaboratively. AQUI'Brie has proposed an approach to aquifer management that is based on analysing groundwater levels, rainfall, river flow rates and other quality data. For example, on the basis of AQUI'Brie models, the public authorities agreed to reduce authorisations for water abstraction from 164 000 m3 per day to 140 000 m3 per day in 2004.
These participatory approaches are still underdeveloped in the Paris metropolitan area and have not yet improved the region's resilience. The Chamber of Agriculture has suggested that the criteria required to set up a regional water management project are too restrictive and complex (Chambre d'agriculture Ile-de-France, 2023[37]) and it is considering multi-user consultations as an alternative. This observation seems to be supported by a review of around 15 regional water management projects across the country, which highlighted challenges with consultation, cost–benefit analysis and the procedures required to set them up (CGEDD&CGAAER, 2022[38]). For example, regional water management projects require a qualitative and quantitative assessment of water resources and an understanding of the environmental and socio-economic impacts of water abstraction on the environment. The challenges of assessing abstractable volumes to address future pressures make it difficult to efficiently allocate and manage resources via these regional water management projects. Similarly, although unitary bodies for collective management have been established, these initiatives do not legally or strategically provide for a change in resource allocation to reflect climate challenges. For example, on the Beauce aquifer, 118 irrigators have been in receipt of an abstraction authorisation since 2017, which is managed by the region’s Chamber of Agriculture. This is a 15-year authorisation that authorises much higher volumes than the volumes actually abstracted, with no incentive to reduce consumption or to anticipate climate events.
Considerable support is needed to facilitate consultation that encourages more-flexible allocation that takes climate challenges into account. The AQUI'Brie example underscores the importance of facilitation in a consultative approach to water. The association draws on specialist facilitators for each type of user, as well as a consultation manager. The inspection reports commissioned by the government to begin preparing for regional water management projects also emphasise the role of the Prefecture or decentralised government departments (CGEDD&CGAAER, 2022[38]).
Consultation mechanisms in place in France to address the droughts’ challenges posed by climate change also attest to the crucial role of local stakeholders. In the Adour-Garonne basin, demand reduction coefficients are applied when abstraction demands exceed the authorised volumes. Similarly, in Ariège, a unitary body for collective management has established a water allocation system tailored to the local crops and soil types. These examples are still the exception rather than the rule in France, and the Ministry of Agriculture's assessment of the situation is mixed, due to the regulatory status sometimes being unclear or to differing opinions on the scarcity of water, which create a feeling of injustice among some of the agricultural operators involved (CGEDD&CGAAER, 2020[39]). However, where such initiatives do exist, the role of local stakeholders has been highlighted as being key to guiding understanding and developing flexible allocation regimes adapted to the impacts of climate change.
Support for local stakeholders also requires resources. Faced with the challenges of climate change, river associations and study confederations that draw up water management action plans for their rivers have indicated their willingness to work more on the issue of quantitative resource management. However, some of these associations report a lack of human resources.
4.3.4. Pricing policies
Pricing policies are intended to encourage users to consume water efficiently and thus contribute to demand reduction. By increasing the price of water, the aim is to guide and modify consumption by users, who become aware of its scarcity or of their impact on water resources. These policies include both charges paid by users for water abstraction and the rates that drinking water distributors charge households.
The implementation of a pricing policy is heavily dependent on the user and the target group (e.g. household, industry, agriculture). For example, for household consumption of drinking water, meta-analyses indicate elasticities ranging from -0.1 to -1. This means that a price increase of 1% generally results in water demand decreasing by 0.1% to 1% (Reynaud and Romano, 2018[40]), (Sebri, 2013[41]), (Nauges and Thomas, 2003[42]). In the agricultural sector, elasticity depends on the type of crop. For example, in the case of market garden crops with high value added, the price of water represents a small proportion (2-4%) of operating costs, and water price elasticity will be very low (Montginoul and Rieu, 1996[43]). On the other hand, water demand is elastic to the price of water in some industrial situations. One study showed that in France, this elasticity could vary between -0.1 and -0.79 when considering abstraction of untreated water or consumption of treated water. This elasticity increases for industries requiring treated water, such as the food and chemical industries. In chemical industries, elasticity could reach -2.42.
Pricing of drinking water
Price signal policies for consumption of drinking water are already in place throughout France and in the Paris metropolitan area. Some cities, such as Dunkirk, Rouen and Montpellier, have trialled progressive pricing to raise residents' awareness of the need to conserve resources. In the Paris metropolitan area, the two water operators SEDIF and Sénéo have already introduced a two-tier tariff to reward water-saving behaviour. The upper tier offers a preferential rate to households, calculated on the basis of each member of a family of four consuming on average 123 litres per day (Chambre régionale des comptes Ile-de-France, 2022[29]).
In the Paris metropolitan area, there is significant variation in price from town to town, regardless of the standard of living. In France, the price charged for drinking water services is lower than in neighbouring European countries (Figure 4.5). In 2023, the average price, all taxes included, was EUR 2.35 per m3. There were major regional disparities due to population density, network size or choices by municipalities and clauses in water service delegation contracts. In the Paris metropolitan area, again in 2023, this price varied from one department to another. Paris, for example, benefited from particularly reasonable prices (EUR 1.8 per m3), while the inhabitants of Seine-et-Marne paid more than the national and European average (EUR 3.03 per m3 on average)6 despite their median standard of living being 17% lower (INSEE, 2021[44]). The region's average price is one of the highest in France.
Water price (EUR/m3), 2022
Note: The price comparison between countries is based on consumption of 15 m3 per month. The average per country is obtained by weighting the prices charged by operators against the population served. The price for France is therefore not the average of prices in France.
Source: Authors, based on IBNET data.
The impact of pricing policies is not uniform across France and seems inconclusive in the Paris metropolitan area. In the case of Dunkirk, large water consumers (unlike small ones) have reduced their consumption under progressive pricing (Mayol and Porcher, 2019[45]). For water users of the regional water syndicate, SEDIF, the gap between the two price tiers was increased between 2011 and 2020, with no obvious reduction in water consumption. According to SEDIF, its users have little scope for reducing domestic consumption (few gardens, living in apartments, etc.), and little awareness of water rates, which are often grouped within building occupancy expenses.
Beyond these examples, it appears that the price elasticity of domestic demand varies depending on a range of factors such as the composition and income of a household, the duration over which the effects are observed, or the type of use targeted (Reynaud and Romano, 2018[40]). For example, a meta-analysis showed that water consumption for essential needs such as cooking and hygiene was inelastic to the price signal. Conversely, recreational uses such as gardening or filling swimming pools were more reactive. At the same time, in developed countries, lower-income households are more responsive to price than wealthier households. Lastly, the price signal seems to be less effective in Europe than in the United States. Even within the United States, this type of policy appears to be more effective in arid areas than in the rest of the country (Nauges and Thomas, 2003[42]). The population of the region is urban and has fewer swimming pools and gardens than neighbouring regions. Moreover, the region is not particularly arid and already consumes relatively little water. Finally, the Paris metropolitan area is the region with the highest median standard of living in France (INSEE, 2023[3]).
Given the relatively low residential consumption of drinking water in the region, measures relating to awareness-raising, efficient usage and allocation could be more effective in reducing consumption. In fact, to achieve a significant 5% reduction in consumption of drinking water, the price of water would have to be increased by at least 5% up to 50%, assuming an elasticity of -0.1 to -1. Such an increase could penalise the lowest income households.
Agriculture
The charges for agricultural abstraction are not designed to encourage water-saving behaviour. The amounts are determined by the Finance Acts, which set an annual ceiling for the charges collected by the water agencies (Ministère des Finances, 2021[46]). This ceiling determines the funding available to implement the water agency's programme of measures, as set out in the Master Plan for Water Development and Management (SDAGE in French). The charges are based on abstraction, irrespective of the end use of the water resources. They are therefore not an economic instrument, but an adjustment variable for implementing the basin programme.
Moreover, the charges for agricultural abstractions applied in the region do not incentivise water-saving. Although charges for abstraction for non-gravity irrigation are higher than in other French water basins, overall charges remain particularly low (INRAE, 2022[47]). The concept of water scarcity is only recognised through higher water charges in water distribution zones. Charges in Europe could be up to 30 times higher if they reflected the value of water in crop yields (Box 4.7). Underestimation of the value of water has led to major water losses in the agricultural sector (Albiac et al., 2020[48]). This is because these charges were established against a backdrop of abundant resources, with no thought given to allocation issues in the event of scarcity (Rey et al., 2019[49]).
Several studies have assessed the shadow price of water, which should be paid for irrigation to reflect the true value of water. This is based on the ratio between net production yield and the total amount of water used for irrigation. In Europe, the price of water is generally much (up to 30 times) lower than this shadow price, whether considering the charges for abstraction alone or the total price including irrigation services.
This also applies in the Seine basin and in the Paris metropolitan area, where abstraction charges for irrigation average EUR 0.015 per m3. The price of water for irrigation is around EUR 0.19 per m3. By way of illustration, a study has calculated the shadow price for certain crops in Spain. Although the situation is different, the orders of magnitude enable us to appreciate the gap between the price of water and this shadow price. For example, the shadow price is estimated to be EUR 0.15 per m3 for cereal crops, EUR 0.4 per m3 for fruit trees and EUR 0.9 per m3 for vegetables.
Unlike with drinking water in the region, higher water prices may encourage farmers to change their practices over the long term. Demand for agricultural water is expected to be price-elastic where farmers can adapt their activity by switching to less-water-intensive crops or reducing their irrigated area. This is observed to be effective in the long term, but there need to be markets for these new crops (OECD, 2022[52]). However, the price would have to be increased significantly, which raises questions of acceptability and would require support from the Chambers of Agriculture and the government. Neighbouring countries have also chosen to adapt their pricing policies by making them conditional on efficiency targets (Box 4.8). Progressive or multi-tier pricing can be a useful solution (Montginoul and Rieu, 1996[43]), depending on the objective.
Italy (Emilia-Romagna) has introduced a system in which the price of water varies according to the efficiency of the irrigation system.
Bulgaria charges more for water use that exceeds the set volume for a given crop.
This is a measure that has been identified by the government, but it appears difficult to implement. Work has been under way since 2018 on a reform of water charges, with the aim of promoting the "polluter pays" principle, but also better incorporating the concept of quantitative abstraction. The French Budget Bill 2024 provided for a 20% increase in the charge for non-point source pollution and an increase in the charge for abstraction for irrigation, to raise an additional EUR 10 million for the water agencies and for implementing the National Water Plan. These measures were withdrawn7 with a view to considering a multi-year approach from 2025, suggesting major difficulties with acceptability in a context marked by inflation and rising energy prices.
Industry
Depending on the objective, pricing policies can be useful in the industrial sector. Given the elasticity to water pricing in industry, this type of policy could encourage the sector to further reduce its water consumption. One study has shown that, in France, this elasticity could vary between -0.1 and -0.79 when considering abstraction of untreated water or consumption of treated water. This elasticity increases for industries requiring treated water, such as the food and chemical industries. In the case of the latter, elasticity could reach -2.42 (Reynaud, 2003[54]). So, for example, to reduce abstraction in the chemical industry by 4% (the target set by the French water agency for this industry (Agence de l'Eau Seine Normandie, 2023[2])), a 2% increase in water charges would be sufficient.
Although pricing can enable a reduction in water consumption or abstraction, this does not just involve demand reduction in the sense of reducing water requirements. While industrial processes could be made more efficient, the Seine–Normandy water agency has identified a number of examples of closed loop water recycling, which ultimately reduces water abstraction without reducing the water consumption required for industrial activity. The price signal therefore encourages a reduction in primary water demand, by optimising both water consumption and water supply.
4.3.5. Conclusion on demand management
Stakeholders in the region and in the basin have already identified the measures to implement to meet the water abstraction reduction targets. To this end, local authorities and the water agency have launched awareness campaigns in line with crisis management. New tools such as VigiEau, perhaps soon to be accompanied by the water footprint, complement the measures. On the other hand, water allocation remains too static, which may not be appropriate given the challenges of climate change adaptation. Vulnerable people are supplied with technologies to improve water efficiency, but these are not sufficiently used, despite their potential for significant improvements in terms of water savings. In addition, trials conducted with drinking water pricing have had inconclusive results due to low price elasticity for drinking water in the region. Finally, although water prices are still too low in the agricultural sector, it would probably require too large an increase to achieve long-term effects, raising issues of acceptability. It is still worth exploring a reasonable increase in water tariffs to reflect scarcity, while taking into account the possible loss of revenue for drinking water operators and the Seine–Normandy water agency.
However, insufficient understanding of consumption and abstraction is preventing effective calibration of demand management measures. The efforts expected of the various users (households, offices, municipalities, irrigators, industries, etc.) are not based on an accurate analysis of current consumption, nor on future needs. It is therefore difficult to quantify how much demand reduction measures can reduce water abstraction. Similarly, the region lacks understanding about the cumulative effects of water abstraction on the environment and on the risk of water scarcity, making it difficult to allocate water in a way that meets the climate challenges.
Water supply management measures are mainly based on infrastructure. The water supply currently depends on reservoirs and an efficient drinking water network. There are also projects under way in the region reusing unconventional water sources such as mine water and swimming pool water. These are proving particularly effective in reducing drinking water consumption by users such as municipalities. The region is also promoting the use of nature-based solutions that retain water in the soil and reduce the vulnerability of users, such as those in the agricultural sector.
Given the lead times needed to build infrastructure, planning is key. Infrastructure projects carried out now will determine the region's resilience in 30 to 50 years' time, especially in urban environments where modifications or upgrades can be costly and complex to implement. Thus, solutions that may seem superfluous today, such as reusing greywater, merit consideration in light of climate change, given their potential effectiveness (Table 4.2). Furthermore, in terms of using multiple resources such as rainwater, greywater, mine water or other non-potable water, understanding the impact of the use of these resources on other systems such as sanitation, and the benefits derived from this infrastructure depending on geographical factors, makes it possible to calibrate infrastructure needs coherently.
This section assesses the robustness of existing infrastructure and proposes new ways of making the region more resilient.
Note: Data from the Greater Paris Sanitation Authority (SIAAP) (SIAAP, n.d.[55]) were used for the reuse of treated wastewater, assuming a 5% increase in volumes treated by SIAAP. In the case of rainwater reuse, a 26% reduction in household use was considered, representing savings on toilet flushing (20% of domestic use) and on car washing and garden watering (6%). For the reuse of greywater, a literature review indicates savings ranging from 30% to 80% of domestic consumption (Van de Walle et al., 2023[56]). Finally, the impact for leakage reduction was estimated by transposing figures from SEDIF and Eau de Paris to the Paris region.
Source: Authors, based on (Climate Adapt, 2023[9]) (Ehret et al., 2021[10]) (DRIAAF, 2019[11]).
4.4.1. Improving the efficiency of existing infrastructure
Reservoirs for all river usage
The Seine basin boasts four reservoirs that help ensure adequate flow rates in some of the region's rivers when water levels are low. Reservoirs are created to regulate the flow rate of the river and its tributaries, thereby meeting water requirements for drinking water, industry, irrigation and energy production when water levels are low, as well as limiting the risk of flooding. The storage capacity of these reservoirs is 800 million m3 (equivalent to 71% of abstraction in the region, Figure 4.6).
Back-up from reservoirs could be drastically reduced in the event of extreme drought. As mentioned in Chapter 1, this would affect industrial activity, pleasure boating and irrigation, as well as the operation of heating and cooling networks, due to insufficient low-water support. One study, moreover, highlights difficulties in supplying drinking water to the region when reservoirs are only 50% full. Levels could be even lower than this in the event of a drought similar to that described in scenario 1, Chapter 2.6 (Aquavesc, SEDIF, Sénéo, Ville de Paris, 2020[57]).
Adjustments to how reservoirs are currently managed may therefore be required. Adjusting management policies is a recurring challenge for the reservoir management authority, which reports that regulatory targets for reservoir filling are very rarely 100% achieved, which would have confirmed they were appropriate. On the other hand, when the reservoirs are very full, it can become difficult for them to fulfil their flood control role, as demonstrated during the high-water levels in July 2021. Conversely, the reserve proportion designated for supporting exceptionally low water flow rates between 31 October and 31 December is regularly used. The authority regularly brings together users and public decision makers to assess future needs for low-water support and to adjust its management policies in the event of anticipated water shortages.
There are plans for more-flexible management. For example, a recent study (EPTB Seine Grands Lacs, 2022[58]) proposes adjusting the reserve proportion based on the fill level observed between the months of July and October, to account for the risk of late low water levels. In addition to changing the size of the reserve proportions, the management authority is also considering adjusting the drawdown curve to account for the needs of the basin's various users. However, it is difficult to assess how the reservoirs specifically contribute to user needs.
Nevertheless, in view of the dual objective of low-water support and flood control, reservoir filling could remain a challenge, undermining the effectiveness of reservoirs. The flood control objective entails annual emptying of the reservoirs and refilling during winter, which is consistent with climate projections that indicate an increase in total winter precipitation (Météo France, 2023[59]). Returning to the low water levels used in the economic impact assessment in Chapter 2, these levels appear to be the result of a particularly dry winter, resulting in low reservoir filling. Management that avoided emptying the reservoirs would thus probably have limited the impacts calculated in Chapter 2. However, in view of the growing risk of flooding, this does not seem a suitable management approach. Other measures are therefore needed to complement the role of the reservoirs.
Source: Authors based on EPTB Seine Grands Lacs.
Looking ahead, considering different types of reservoirs to complement the existing ones requires a cautious approach to avoid maladaptation risks. Reservoirs refer to any facility or structure used to store water, regardless of their supply source (river, aquifer, etc.) or purpose (agricultural, drinking water, etc.). Reservoirs can worsen drought conditions by significantly reducing annual river flow (by 7% to 35%) and decreasing low water flow rates, particularly in dry years (Association Rivière Rhône Alpes Auvergne, 2020[60]). These effects depend on the climate (e.g., evaporation of stored water), location within the river basin, and the type of water withdrawal (e.g., timing, river diversion, etc.). Moreover, reservoirs can increase users' dependence on water resources and heighten vulnerability to water shortages (Figure 4.7). In north-western France, the development of small reservoirs led to reduced river flows and less efficient reservoir filling (Habets et al., 2014[61]).
The cautious approach taken by the Seine–Normandy river basin regarding the development of reservoirs seems appropriate given the future challenges posed by drought. In the Paris metropolitan area, reservoirs are subject to strict regulations and require authorisation. Reservoir projects must include a goal to reduce water abstraction and consider long-term impacts such as climate change and sediment transfer, which may only become apparent over decades. To qualify for funding from the Seine–Normandy water agency, a project must meet several criteria: it must be located in a high-risk area, be part of a regional water management plan, be supported by a collective body, not increase irrigation volumes, and the water stored must come exclusively from surface water or drainage, without any risk of seepage before reaching the river. These precautions are sensible to avoid creating additional vulnerabilities.
Note: The supply-demand cycle reflects the increasing impact on water demand of new supply, in this case from reservoirs.
Source: Adapted from (Di Baldassarre et al., 2018[62]).
Managing water levels for river transport
The rivers in the basin are protected by infrastructure that is resistant to climate change, ensuring the resilience of river transport. Mobile dams installed along rivers help regulate water levels throughout the year. These dams allow water to be held back during low-water periods at levels consistent with the river's uses, while allowing water to pass through during floods. According to an OECD estimate (see Appendix), a minimum flow rate of 15.5 m3 per second would be sufficient to ensure river traffic on the Seine (for comparison, the average annual flow rate on the Seine is 310 m3 per second (Agence de l'Eau Seine Normandie, n.d.[63])). However, in the worst-case scenario of low water levels envisaged between now and 2100, taking into account difficulties in filling reservoirs, and based on data provided by Voies Navigables de France (the French institution in charge of waterways, VNF) and EPTB Seine Grands Lacs, the average flow rate expected on the Seine would be 26 m3 per second (Appendices). At the time of writing, VNF and EPTB Seine Grands Lacs were finalising an agreement to ensure co-ordinated management of their respective infrastructure during periods of low water levels.
Drinking water network
The drinking water network is supported by a robust supply strategy and infrastructure. The drinking water supply strategy uses diversified sources and thus mitigates the risks associated with climate change. A system of emergency networks enables different operators to ensure access to drinking water in geographical areas potentially affected by water scarcity. Finally, measures to combat leaks in water networks help optimise drinking water abstraction by avoiding losses. For example, operators install network monitoring systems (e.g. acoustic sensors) to locate and repair leaks. They can also upgrade the network.
Drinking water supply scheme
The region's drinking water supply draws on a wide variety of resources rendering it resilient to climate shocks or pollution of water resources. The City of Paris is reliant for half its supply on underground resources, which vary in nature and quantity and in some cases are located as far as 150 km from Paris (Eau de Paris, n.d.[64]). The aquifers vary in characteristics, so the operator can diversify risks in the event that one of the aquifers used has not had time to recharge properly. The other half of the water supply for Paris comes from the surface waters of the Seine and Marne rivers, in line with the rest of the region, which draws most of its resources from the three main rivers in its basin (Aquavesc, SEDIF, Sénéo, Ville de Paris, 2020[57]). Finally, the Paris metropolitan area has an aquifer covering over 100 000 km2, the Albian water table. This strategic resource is heavily controlled (as it is non-renewable) to ensure it can provide emergency assistance to the Paris metropolitan area’s residents in the event of a drought (Eau de Paris, 2022[65]) linked to an external crisis putting the drinking water distribution network out of service.
Although this supply scheme provides resilience, it may require adjustments in the face of climate change. In Paris, the use of underground resources in distant locations where there is competition with other uses could become more controversial in a context of scarce resources. With the region expecting greater climate variability, including alternating floods and droughts, the City of Paris operator (Eau de Paris) could adapt its abstractions, for example to facilitate groundwater recharge during periods of high-water levels. This is one of the options the water agency is more widely exploring to manage resources more flexibly, taking into account the trends in water tables and rivers. Similarly, the 32 municipalities in western Paris served by Aquavesc have access to underground resources, partly fed by the Seine using an artificial recharge technique that enables underground storage and improves the quality of groundwater. This type of approach could be adapted to other areas of the region that are essentially dependent on surface resources.”
Emergency links
The drinking water network relies on emergency links to ensure part of the region is resilient in the event of a severe drought. Four water authorities (Aquavesc, Sénéo, Eau de Paris and SEDIF), accounting for 71% of the volume of drinking water distributed, have set up emergency links to ensure continuity of service in the event of one of the operators ceasing production. The operators have tested the resilience of the interconnected zone to critical risks such as heavy pollution, interruption of power supply, major flooding or severe drought affecting an operator's unit output (Aquavesc, SEDIF, Sénéo, Ville de Paris, 2020[57]). This test showed the interconnected zone to be robust in the event of drought, but this may need to be reassessed depending on the level of reservoir filling.
The water authorities are considering how to improve management of the interconnected zone by encouraging greater consultation. Agreements between the water authorities and their operators allow for at least one annual discussion to schedule works over the coming year and guarantee mutual back-up. To optimise resource management in the event of a crisis, there are plans for more-regular discussions to make the best use of safety margins on production capacity. The water authorities are also planning to share inventories of resources for emergencies, back-up, and flood and pollution control.
Network efficiency and leakage control
The drinking water network in the region has a particularly good efficiency rate. The law (Gouvernement, 2012[66]) demands that drinking water networks have an 85% efficiency rate, to limit water losses and make users more resilient to the risk of water scarcity. Thanks to a very dense and well-connected network, which makes it easy to intervene and ensure continuity of service in the event of a leak in part of the network, the Paris metropolitan area has an efficiency rate well above this threshold (89.6% (SISPEA, 2021[67]) on average). By way of comparison, the leakage rate in London is 24% (Thames Water, 2020[68]) and the European average is 25% (Figure 4.8). However, there are disparities between municipalities, with efficiency rates of only 50% on some networks in Seine-et-Marne.
Note: BE: Belgium; BG: Bulgaria; CH: Switzerland; CY: Cyprus; CZ: Czech Republic; DE: Germany; DK: Denmark; EE: Estonia; EL: Greece; ES: Spain; FI: Finland; FR: France; HU: Hungary; IE: Ireland; IT: Italy; MT: Malta; NL: Netherlands; NO: Norway; PL: Poland; PT: Portugal; RO: Romania; SI: Slovenia; SE: Sweden; SK: Slovakia; UK: United Kingdom.
Source: (EurEau, 2021[69]).
The region's water operators are closely involved in the continuous improvement of network performance. This performance is improving every year. Through its water and climate programme, the Seine–Normandy water agency has also helped save nearly 4.08 million m3 (representing 0.1% of abstraction in the basin) by reducing network leakage. SEDIF has achieved a network upgrade rate of 1.19% per year (Chambre régionale des comptes Ile-de-France, 2022[29]), well above the French average of 0.67% (SISPEA, 2020[70]) and the 1% network upgrade target set for rural areas at the Assises de l'Eau conference. Between 2021 and 2022, Eau de Paris fitted 2 750 acoustic sensors to detect and locate leaks, i.e. one sensor approximately every 700 m. The volume of network losses should therefore fall by 4.3 million m3 per year for Eau de Paris (2.3% of the water produced annually by the company) (Eau de Paris, 2022[71]). The aim is to achieve a network efficiency rate of 92%, compared with 90.5% in 2021.
Continuing to upgrade the network is a no-regret strategy for water operators, as long as the costs can be absorbed, given the environmental and economic benefits. It would be unrealistic to aim for a 100% efficiency rate. Firstly, the network is deteriorating, partly due to ageing, but also due to factors such as land deformation and climate events that are affecting the pipelines. Unless the entire network is upgraded in the next few years, zero leakage is not an achievable goal. Where the drinking water network is underground (SEDIF, 2021[72]), working on it involves digging up roads. This does not apply to the Eau de Paris distribution network, 90% of which is located in sewers and accessible galleries, so costs will be lower. However, just because a leak has been repaired in one part of the network does not mean that a new one will not appear the next day. Thus, the target of "zero leakage" would entail repeated works, which raises questions around acceptability, co-ordination and costs, which are passed on to consumers. Nevertheless, water operators can continue their efforts to significantly improve network efficiency where it is low or maintain it at current levels where it is excellent, by combining leak detection and affordable upgrades.
Measures targeted at private networks seem more beneficial. Examples include the leak alert devices (Aviz'eau) used by SEDIF and Aqua'vesc. These devices alert users to anomalies in their consumption. In 2020, a trial of these devices in collective housing saved 1 million m3 of water (Chambre régionale des comptes Ile-de-France, 2022[29]). This promising measure reduces excessive water losses and offers significant savings, both economically for households and in terms of drinking water consumption.
Nature-based solutions to facilitate groundwater recharge and preserve water quality
Nature-based solutions contribute to water retention and the preservation of ecosystems, which in turn help prevent drought episodes. For example, in the natural environment, protected wetlands, grasslands and wooded areas (buffer zones) help purify water by trapping pollutants and fine particles. They are often compared to sponges, thanks to their ability to store large quantities of water (some wetlands can store up to 15 000 m3 of water per hectare (Office francais pour la biodiversité, 2012[73])) and thus replenish groundwater. In times of drought, they release water and thus support flow rates in watercourses. In towns, nature-based solutions such as urban greening significantly reduce rainwater run-off into sewer systems in the event of heavy rain, and the water can be reused for certain purposes. A study in the République district of Paris showed that it was possible to install green roofs on 13% of roofs in the district. These roofs could absorb rainfall and potentially harness it for flushing toilets (APUR, 2015[74]). More generally, urban nature-based solutions help combat heat islands, with indirect effects on water consumption. They also contribute to flood control, where combined sewer systems carry a risk of water becoming polluted and therefore unavailable.
Nature-based solutions also offer valuable co-benefits by addressing both flood and drought risks. For example, nature-based solutions such as green roofs and soil unsealing help limit run-off and flood risk, while also improving groundwater recharge. Similarly, dried out soils and lack of water for agriculture and biodiversity encourage flooding in the event of rain, by preventing water infiltrating the soil.
Nature-based solutions are gradually being implemented in the Paris metropolitan area. For example, the region has created Île-de-France Nature, which will help protect 1 000 hectares of brownfield sites by 2025, support the planting of 2 million trees by 2030, and set up a EUR 1 million regional fund to help forests adapt to climate change (Région Ile-de-France, 2022[75]). Similarly, the Greater Paris metropolis has set up a biodiversity fund with ambitious targets, which will finance projects involving tree planting, reinstatement of ecological corridors, soil unsealing and ecological restoration. The intention is to unseal soils and ecologically restore at least 10 hectares, create or restore 20 pools, and plant 100 000 trees to encourage the development of urban forests (Métropole du Grand Paris, 2023[76]). The Greater Paris Metropolis is also encouraging innovative ways to integrate these solutions into urban planning, through a call for projects under the theme Inventons la Métropole du Grand Paris. The City of Paris has also drawn up a biodiversity plan (Ville de Paris, 2018[77]) which includes around 30 actions, including urban greening projects to create green and blue corridors. In addition, the city is involved in the Oasis Schoolyard Programme, which aims to remove soil sealing in schoolyards and transform them into green spaces to bring nature back into the city and create urban cool islands (Ville de Paris, 2023[78]).
Identifying strategies to protect aquatic environments and their ecosystems requires an accurate inventory of ecosystems present in the region, which is currently lacking. The Parisian Urban Planning Agency (APUR in French) has analysed the potential for soil unsealing in public spaces in Paris (APUR, 2022[79]) and has run a pilot scheme to encourage greening and cooling in part of the metropolitan area (APUR, 2023[80]). Institut Paris Région has identified biodiversity hotspots of national importance in the region (262 000 ha in total in 2018) (Institut Paris Région, 2022[81]). The Greater Paris Metropolis is also carrying out a feasibility study to identify areas at risk and opportunities for ecological restoration of its watercourses and wetlands. Finally, the Greater Paris Metropolis is conducting a study to map the watercourses in its area and their potential for restoration. This study will result in a complete mapping of the watercourses, which will be used as a decision-making tool in identifying priority actions to be taken to restore the aquatic environment. This study should also help identify the amount of setback required for ecological restoration of small watercourses, so that this can be included in Local Urban Development Plans (PLU in French) and Intermunicipal Local Urban Development Plans (PLUI in French) and a land strategy can be devised to make space for aquatic environments. However, barely one in ten wetlands are currently mapped, and therefore likely to be protected. DRIEAT reports that there are potentially 250 000 hectares of wetlands in the region, while only 23 000 hectares have been identified and mapped (DRIEAT, 2020[82]). Unmapped wetlands cannot be protected or restored. Understanding the status of the buffer zones would help identify priority areas to be preserved, as they have the greatest capacity to store water and thus replenish low water levels during the summer.
National and supranational governments also have a crucial role in encouraging local authorities to invest in nature-based solutions. Firstly, the legal framework governing implementation of these solutions can facilitate their use and help supervise their implementation. For example, the Water Framework Directive provides a common regulatory framework for the sustainable use of water and the protection of aquatic ecosystems and environments. Similarly, the European Union Biodiversity Strategy approves and encourages integrating ecosystem services into decision-making. Countries such as Peru have also passed laws to encourage the use of nature-based solutions. In 2014, for example, Peru introduced a law on compensation mechanisms for ecosystem services (World Bank, 2023[83]). This law seeks to promote, regulate and monitor payments for ecosystem services, to ensure that ecosystems can continue to generate benefits. Under this law, those managing ecosystem services receive remuneration for implementing measures to preserve, restore and sustainably use sources of ecosystem services. These measures may include the conservation of natural areas, the rehabilitation of damaged areas, or actions to ensure the sustainable use of sources of ecosystem services.
Rainwater management at source
Classifying rainwater drainage zones helps rainwater infiltrate the soil to improve the resilience of the Paris metropolitan area. This infiltration should be established as early as possible in the urban development process, by limiting soil sealing or removing existing sealing. This helps, for example, retain water in the soil for trees and plants, so they can cool the city and indirectly reduce water consumption in summer. It also helps limit discharge into combined sewer systems (which form the majority of networks in Paris (Ville de Paris, n.d.[84]) and are very common in the Petite-Couronne8 (SISPEA, 2021[85]), reducing the risks of overflow and pollution (APUR, 2022[4]).
Rainwater management at source is a national and local priority, to limit the risk of water resources becoming unavailable due to heavy pollution. It has been the subject of a national action plan for sustainable rainwater management since 2021 and is one of the priorities in the Seine–Normandy basin's climate change adaptation strategy. It is also a priority for local authorities, particularly in view of the water quality issues arising from saturated sewer systems. Rainwater management was also central to planning for the 2024 Olympic Games, which included swimming events in the Seine. The region plans to finance soil unsealing of 5,000 hectares by 2030. The SDRIF mentions integrating rainwater management into urban planning, through the use of green roofs, rainwater harvesting and ditches. The priority is to boost infiltration and retention of rainwater where it falls. Within Paris, the local urban development plan includes a classification of rainwater drainage zones, which is helping integrate rainwater management into urban planning. The city has also drawn up regulations on sewerage zones as part of its Paris Pluie plan. These regulations apply to all developers of infrastructure projects and include an obligation to design a rainwater management system that limits the volumes of water discharged.
The Seine–Normandy water agency is continuing its efforts to further align urban planning with the management objectives set for the Seine–Normandy basin. For example, although the classification of rainwater drainage zones highlights areas that are more or less vulnerable to the risk of run-off, this assessment does not appear to have been translated into a region-wide action plan. The French water agency has also indicated that targets relating to an increase in built-up land have not been met. Rainwater management is being hampered by a lack of understanding of the issues related to soil sealing and a perception that there are negative impacts from having water in the city. The Seine–Normandy water agency has developed an information portal to help local authorities take better account of rainwater in urban planning schemes.
4.4.2. Increasing the use of alternative water supply measures
Mobilising non-conventional or alternative resources can effectively complement existing infrastructure and technologies. Non-conventional resources are water sources present in the territory but not currently used, such as rainwater or wastewater (e.g., greywater, pool water, etc.). These resources represent an untapped potential to reduce water withdrawals by replacing surface or groundwater use, thereby optimising water management.
Rainwater reuse
Collecting and reusing rainwater allows underutilised resources to be mobilised. By replacing traditional resources with others that are usually lost, reusing rainwater can help make the region resilient. Rainwater can be harvested at the household level, or collectively using storage tanks that serve a building or district. Studies suggest that rainwater harvesting has the potential to supply 50% of a household's water needs, and in some cases up to 80-90% of household consumption (GhaffarianHoseini et al., 2015[86]). In addition to domestic needs, rainwater can, for example, supplement the resources distributed by the City of Paris' non-potable water network to connect more green spaces to this network (APUR, 2021[87]). Similarly, rainwater harvesting systems could meet 16% of irrigation needs (Raimondi et al., 2023[88]). In some regions, such as southern Australia or the semi-arid areas of the United States, rainwater is in fact one of the main water resources.
In France, rainwater reuse has been regulated since 2008 (France, 2008[89]). The law sets out authorised uses, declaration and maintenance procedures, and user responsibilities. Rainwater can be used for any outdoor purpose, but indoor use is limited to toilet flushing, floor washing and laundry, and subject to conditions. Authorised uses reflect sanitary requirements designed to mitigate the risk of water contaminated by having fallen on roofs or other materials containing chemicals.
The region is well suited to this type of approach because of its high population density and hydrology. Water storage tanks need to be able to empty regularly, both to reduce run-off and to optimise resources. Dense development means that tanks remain in constant use and small tanks shared by several users can be installed. Lastly, the region is expected to experience fairly stable rainfall volumes, with intense but brief rainfall episodes, making rainwater harvesting a reliable option (APUR, 2018[90]). Alongside drinking water, rainwater harvesting could offer an interesting option for supporting the agricultural sector to maintain irrigation volumes at current levels.
Rainwater harvesting and reuse require financial incentives or strong regulation. It can be complex and costly to install rainwater collectors on existing buildings, and this can hinder their deployment (Parsons et al., 2010[91]). Firstly, connecting rainwater tanks to the different floors in buildings, especially older buildings, is technically challenging. Furthermore, this type of installation can be costly due to the long networks required. Rainwater meters must also be installed to enable adjustments to be made to the sewerage tax. Finally, rainwater reuse can prove expensive for drinking water suppliers, who will likely experience a drop in consumption, and therefore revenue, and may need to be offered financial compensation based on savings in areas such as water treatment (Table 4.3).
The Paris metropolitan area supports rainwater reuse. The region is offering a 50% subsidy to individuals wishing to install water harvesting systems for sanitary use or for watering gardens (Région Ile-de-France, 2022[96]). In Paris, the Paris Pluie plan informs citizens about the possibility of rainwater harvesting, notably for watering shared gardens or green spaces. Paris is also involved in careful development projects, such as one in the area around Rungis railway station, which reduces water consumption in offices, supplies water for gardens and helps reduce flooding. The city requires that neighbourhood developers install rainwater collectors, which are used for watering building-related spaces and for bathrooms. In the event of excessive rain, water is redirected to a garden and pond to regulate the water level. This harvesting programme will result in water consumption savings of up to 30%.
The use of such infrastructure can be supported by long-term cost-effectiveness studies. For example, where existing infrastructure may be costly to deploy in relation to existing buildings, other solutions are probably preferable. From a longer-term perspective, it makes sense to integrate rainwater reuse infrastructure as early as possible to avoid future modernisation costs. To date, there have been no studies assessing the impact of the strategy implemented on water consumption or on the water cycle generally.
Measures like this do, however, entail risks that have not been assessed in the context of climate change. For the time being, rainwater reuse remains a marginal measure in the region in terms of the volumes mobilised. However, in the event of large-scale collection, this water would not be returned to the environment naturally and could disrupt the water cycle, causing as yet unknown impacts. In the event of summer periods marked by lower precipitation volumes, this resource could prove insufficient, despite the fact that these are the periods when unconventional resources will be most important. Relying solely on these resources could therefore create further vulnerability, particularly in the agricultural sector. It may therefore be necessary to supplement them with other unconventional resources, and to integrate them first and foremost into an approach aimed at reducing groundwater abstraction during periods of high water or heavy precipitation, for example, to preserve these groundwater stocks in anticipation of summer.
Reusing treated wastewater
Reusing treated wastewater is one way of reducing anthropogenic pressure on water. It involves reclaiming wastewater after it has been treated at a wastewater treatment plant, so that it can be used again. Recycling wastewater offers a double saving: water is saved upstream through water reuse, and the volume of polluted wastewater is reduced. In Melbourne, for example, homes equipped with wastewater reuse systems use 30% less water than "conventional" homes (van Leeuwen, 2015[97]). These systems, also known as "third pipes", constitute a parallel distribution network, carrying unconventional water resources for purposes such as sanitary use or watering gardens. More generally, reusing treated wastewater to its full potential would cut primary water consumption by 26% to 48% for all uses combined (Bauer, Linke and Wagner, 2020[98]).
Treated wastewater reuse is not yet well developed in France, especially in the Seine–Normandy river basin. Treated wastewater can be reused for irrigation or industrial cooling, and sometimes for groundwater recharge. The reuse of wastewater in urban areas also has great potential for watering parks, supplying sanitary facilities, cleaning roads, supplying air-conditioning systems or filling tanks for firefighting. In Tokyo, for example, wastewater is recycled for sanitary facilities and watering gardens, as well as to protect biodiversity (Takeuchi and Tanaka, 2020[99]). In France, however, only around 0.2% of wastewater is recycled, compared with 14% in Spain and 90% in Israel and Singapore (CIeau, n.d.[100]). In Europe, Barcelona began reusing treated wastewater in 2002, with the aim of producing 50 hm3/year of recycled water for various purposes. The reason why these countries are further ahead is because they experience more pressure on their water resources.
France is focusing heavily on the reuse of treated wastewater. In 2019, the Assises de l'Eau set́ a target of tripling treated wastewater reuse by 2025. This wastewater reuse target was confirmed in the National Water Plan, which calls for 1 000 reuse projects by 2027. Following a study highlighting the obstacles posed by the complexity of the current regulatory procedure (CEREMA, 2020[101]) (France, 2023[102]), the law has been revised to simplify the process by removing regulatory obligations concerning balance sheets, authorisation procedures and, in certain cases, wastewater quality. Although treated wastewater cannot be used for domestic purposes, it is an interesting solution for cleaning roads and watering gardens, irrigation and some industrial processes (France, 2022[103]).
However, implementing treated wastewater reuse is relatively expensive and requires awareness-raising and incentives due to negative perceptions of treated wastewater. The idea of reusing wastewater can have a negative psychological impact on people (Institut national de l'économie circulaire, 2018[104]). It is therefore important to ensure that the public is well informed about the benefits and risks of reusing treated wastewater. In addition, treated wastewater cannot be used inside homes. This limits its uses and the incentives to acquire the infrastructure needed to make use of it. Reusing treated wastewater would require a dual water network, similar to the non-potable water network used by the City of Paris.
Reusing treated wastewater may also carry the risk of maladaptation in the Paris metropolitan area. Discharge from wastewater treatment plants helps maintain water levels in rivers. On the Seine, the water agency believes that returning treated wastewater can increase the flow of a river during low-water periods by up to 70% (Agence de l'Eau Seine Normandie, 2022[105]). Wastewater reuse can therefore have a significant impact in terms of drying up a watercourse, and adversely affect downstream uses. Similarly, the Greater Paris Sanitation Authority (SIAAP in French) has noted that an inlet flow of 100 m3/s is required to ensure effective treatment of wastewater discharge. This type of approach seems better suited to coastal areas, where wastewater discharge from treatment plants is lost anyway. Given the existing obstacles and the risk of maladaptation, wastewater reuse appears to be better suited to closed-circuit water reuse for industry, and on a case-by-case basis for other purposes such as irrigation.
Greywater
Greywater reuse covers a proportion of the wastewater that can be used to reduce primary water consumption. Greywater use refers to the collection and use of water from household activities, such as showering or washing up. It generally excludes water used for toilets and cooking, which is wastewater. Greywater accounts for 30% to 80% of a household's drinking water consumption (Van de Walle et al., 2023[56]), making it an attractive resource for reducing primary water consumption and thus averting the risk of water scarcity. The use of greywater is commonplace in some US states, Australia, Japan and Spain. California was the first state to standardise the use of greywater (Box 4.9). In Japan, greywater must be used in buildings of over 30 000 m2 or if the potential volume of greywater exceeds 100 m3 per day (Domènech and Saurí, 2010[106]).
In France, greywater is classed as treated water and it cannot be reused for domestic purposes (ANSES, 2015[107]). In practice, there is no formal prohibition but there is an obligation to discharge wastewater into a sewerage system. Greywater reuse is therefore regulated in the same way as treated wastewater reuse. This limits its use to irrigation and watering of green spaces, subject to adequate treatment (Gouvernement, 2022[108]). This approach to greywater primarily reflects a health policy designed to protect the public. The French Agency for Food, Environmental and Occupational Health and Safety (ANSES in French) has, due to the significant health risks involved, recommended restricting this use to regions where water shortages present a persistent and repeated problem.
Greywater requires appropriate treatment to prevent any health or environmental risks. Since it excludes water used for toilets and cooking, greywater naturally contains fewer pathogens than wastewater, but the concentration of detergents and other washing agents can be harmful. Household products, for example, increase water salinity and contain concentrated metals, excessive quantities of which can end up in the soil. Without appropriate treatment, microbial contamination can also occur (Van de Walle et al., 2023[56]). Although quality standards for non-potable water are emerging, greywater filtration and treatment technologies are still underdeveloped. This is mainly due to the fact that, with the exception of a few countries (Van de Walle et al., 2023[56]), there is little enthusiasm for this practice. The lack of quality standards for greywater and the absence of a framework for this practice are holding back the development of technologies that would make it easier to use greywater.
Negative user perceptions also point to the importance of raising awareness. A programme to introduce greywater reuse systems in the United Arab Emirates, for example, highlighted residents' scepticism about this process, despite the fact that it is now mandatory (Shanableh et al., 2021[109]). Education and awareness-raising have been key to getting users on board and using greywater. It would appear that consumers confuse greywater with wastewater and have doubts about the quality of these resources for their own use. The idea of contact with greywater is also a barrier for some people. In addition, maintaining these systems requires technical skills and is perceived as a burden. The introduction of greywater reuse in certain countries has shown that the economic impact of reduced drinking water consumption is, on the other hand, an argument that can carry weight with users (Van de Walle et al., 2023[56]). Raising awareness of the environmental and economic benefits is therefore essential to mobilising this type of resource.
Finally, the cost of installing greywater reuse infrastructure is also a barrier to introducing such systems in existing buildings. Greywater use requires the installation of infrastructure that can be complex to fit in older buildings; notably a dual network to distinguish between pipes carrying greywater and those carrying drinking water. Regulatory incentives could be considered, following the example of the city of Barcelona (Box 4.9).
Nevertheless, greywater reuse appears to be a promising option for supplementing rainwater reuse within homes. Greywater reuse could thus lead to significant water savings in sectors such as the hotel and catering industries (March and Gorostiza, 2023[110]), which are major consumers of drinking water. Similarly, while new buildings could incorporate rainwater management systems, it may be worth thinking about greywater systems in advance to avoid the future costs of installing systems that would integrate both types of resources into homes. While rainwater harvesting can help prevent the risk of water scarcity, greywater could compensate for a lack of rain during dry periods. Some homes with roof surfaces or gutter systems that are not suitable for installing rainwater harvesting tanks could also benefit from this system.
The roll-out of greywater reuse infrastructure requires a better understanding of the risks and benefits associated with these measures. Although there are some international examples of greywater reuse, no studies have been carried out to quantify the environmental, social and economic benefits of these measures, or the effectiveness of the various solutions used to address health risks. Such a study would make it possible to take a second look at introducing this practice and facilitate investment in innovative greywater treatment technologies with a view to reuse resources.
California
Southern California is one of the most drought-prone regions in the United States. The state has introduced a number of measures to adapt to water scarcity, focusing in part on greywater use. In 2010, the California Building Standards Commission adopted technical principles for the development of greywater systems for integration into the California Plumbing Code. The Code now permits the installation of greywater infrastructure in homes. In 2019, a law enabling tax credits for any taxpayer wishing to install a greywater reuse system was introduced.
Barcelona
In 2002, municipal ordinances were passed that made rainwater harvesting and greywater reuse systems mandatory. In 2023, however, the picture was mixed. Only five municipalities in the Barcelona metropolitan area include greywater reuse as part of their water conservation plans. Despite proven water savings (almost 22 litres per resident per day for Barcelona residents, i.e. a 20% reduction), there are doubts about the introduction of these solutions, with some preferring other measures, such as reducing water consumption or reusing treated wastewater.
Source: (State of California, 2018[111]); Central Coast Greywater Alliance; (March and Gorostiza, 2023[110]); (Saurí, March and Gorostiza, 2015[112]).
Other unconventional resources: Swimming pool and mine water
Unconventional resources also include mine water and swimming pool water, which account for significant volumes. Across Paris as a whole, it is estimated that swimming pool water accounts for a quarter of the non-potable water currently used by the City of Paris, and which comes from the canals via withdrawals from the Marne River (APUR, 2013[113]). The RATP public transport network has launched pilot projects in some of its stations to assess the potential benefits of using mine water (i.e. clear water considered unfit for consumption, such as river seepage) in metro stations. Mine water could represent up to 16% of the total non-potable water used by the City of Paris (Box 4.10).
Paris is the only city in France with a non-potable water network. It is fed by the Ourcq canal, and by water drawn from the Marne and Seine rivers. Nearly 78 million m3 of water were produced in 2019 (42% of drinking water production in the same year). The network is similar in length to the drinking water distribution network in the City of Paris (1 700 km compared with 2 000 km for the drinking water network).
This untreated water is used to ensure optimal operation of the city's wastewater network, street cleaning, watering of green spaces, and filling of ponds, lakes, rivers and ornamental fountains. The water network will also be used as a heat transfer fluid for heating the future Saint-Vincent-de-Paul eco-district. Finally, the drinking water network is also used by the fire brigade.
However, resources within the non-potable water network could also be reduced in the event of severe low-water conditions affecting the water bodies supplying the main reservoirs. Work was carried out by the Parisian Urban Planning Agency to assess the potential for combining water resources that could, among other things, supply the non-potable water network. This study fed into the discussions that led to the development of the 2022-34 master plan for the City of Paris' non-potable water network. Incorporating new resources could prove very expensive, given the volumes involved. A case-by-case approach is therefore recommended.
Source: Eau de Paris; (APUR, 2022[114]).
Making use of these resources can help achieve the water withdrawal reduction targets that are necessary in order to make the region more resilient. Rather than simply being discharged into the environment or the sewer systems, swimming pool water is a valuable resource that could serve various purposes, such as cleaning public spaces, watering gardens, supplying heating and air-conditioning systems, washing vehicles and washing textiles in laundries. Within the area covered by the regional Water Syndicate (SEDIF in French), this water would meet around one-third of total drinking water requirements for non-domestic use (INSEE, 2023[115]). Some cities, such as Orly, use only swimming pool water for road cleaning (Agence de l'Eau Seine Normandie, 2023[2]). Similarly, mine water is currently pumped, stored and then returned to the sewer systems, whereas it could supply the non-potable water network.
Mobilising these resources may not be cost-effective, however. This suggests the need for a case-by-case approach. In the case of swimming pool water, Eau de Paris, which is also responsible for the city's non-potable water network, cited the high cost of a resource that accounts for just 1% of the existing non-potable water network. With regard to mine water, Eau de Paris warns of the risk of corroding the non-potable water network if this resource is connected and recommends a case-by-case approach (APUR, 2022[114]). As well as making up for any quantitative shortfalls in non-potable water resources through connection to the non-potable water network, this water could also be used elsewhere. It is therefore an interesting resource for municipalities without such networks (see the example of Orly above), or for institutions able to consider a dual network, such as the Quai Branly Museum (APUR, 2013[116]). In the museum's case, however, the network is not used. This is a precaution due to the lack of regulations covering this issue.
4.4.3. Other large infrastructure: Inter-basin water transfers
Inter-basin water transfers allow unaffected regions to support affected regions in the event of a severe drought, which is costly and a source of conflict. Some countries, such as Spain and the United Kingdom, transfer water within a single basin. However, it is expensive to build the infrastructure required for transfers, which can also be sensitive to climate change (Sénat, 2023[117]). Inter-basin transfers are based on the principle of solidarity within a country, between more-water-abundant regions and those that are more water scarce. When resources are in short supply, competition for them creates the risk of conflict (Box 4.11).
This type of approach is not currently envisaged for the Seine River basin, and does not appear to be a priority due to the drought risks anticipated for other French river basins and the potential environmental damage induced by such infrastructure.
The South Korean city of Busan, with a population of 3.5 million, relies mainly on the Nakdong River for its drinking water supply. However, water quality in the Nakdong has declined as a result of pollution by poorly treated industrial and domestic wastewater, including phenol contamination from an industrial complex in Gumi. To alleviate this situation, Busan has been exploring inter-regional water transfers from other regions since the 1990s. In 1996, an inter-regional plan to transfer water from the Hwang River to Hapcheon was drawn up but cancelled due to local opposition. Population growth and urbanisation created water quality and scarcity problems, prompting Busan to seek clean water from the Nam River dam in Jinju in 2018. This plan was also strongly opposed due to the increased risk of water scarcity during the dry season and flooding during the rainy season. Busan finally cancelled this water transfer project in 2019, seeking alternative solutions.
Source: (OECD, 2022[118]).
4.4.4. Conclusion on supply management
The infrastructure that has bolstered the region's resilience by optimising water supply may prove inadequate to cope with future droughts. As a result, reservoirs could have difficulty filling up, and back-up interconnections could suffer. Similarly, the introduction of nature-based solutions is a positive step but is not yet sufficiently strategic to ensure the region's resilience.
This suggests not only that existing infrastructure should be strengthened and the roll-out of nature-based solutions stepped up as a priority, but also that new water supply management measures ought to be considered. Indeed, given the time required to build infrastructure and the density of the region’s-built environment, it is essential to plan the infrastructure that will enable the region to cope with drought risks in the medium-to-long term. Solutions involving the recovery of rainwater, swimming pool water, mine water and greywater seem to offer better prospects than reusing treated wastewater, which presents a risk of maladaptation. However, this type of approach must be considered on a case-by-case basis:
Drinking water and municipal non-potable water uses: While the City of Paris has a non-potable water network, municipal and non-domestic users in other municipalities could benefit directly from the use of unconventional water for purposes where it is difficult to achieve any reductions through conservation measures alone. These uses include businesses that offer services such as car washing, laundry services, supply of water for sports facilities and road cleaning. In addition, given the reduction in the resources available for the non-potable water network in Paris, new resources may be considered to supply this network and meet the challenges of cooling and greening the city. Furthermore, while conservation efforts by households could be enough to reduce their consumption by 14%, the prospect of a diminishing resource in the longer term (2100) and the potentially limited margin of households whose consumption is already very reasonable (INSEE, 2023[115]) make unconventional water an interesting option that should be considered in new buildings or via low-cost additions (e.g. rainwater harvesting tanks).
Agriculture: Given the challenge of maintaining abstraction volumes for irrigation, mobilising unconventional resources seems appropriate. Rainwater reuse, for example, has good potential: almost 16% of irrigation needs could be met in this way. To calibrate these measures and prevent maladaptation, feasibility studies that take account of climate issues and make provision for the conservation efforts that must be made in parallel are needed. Taking the example of rainwater, relying on this resource alone may be risky if droughts become more frequent. One of the risks facing the sector is opting for solutions that are not robust in the face of climate change.
Industry: Industries in the Seine–Normandy river basin have already begun to mobilise unconventional resources by setting up closed loops to reuse wastewater or rainwater. These approaches are highly effective in reducing water withdrawals.
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← 1. OECD 2050 drought scenario as used in Chapter 1
← 2. Data provided by the government via the VigiEau platform: https://vigieau.gouv.fr/
← 3. Ecodo device: https://www.eaudubassinrennais-collectivite.fr/votre-eau/comment-economiser-eau/documentation-ecodo/
← 4. https://www.smegreg.org/index.php?option=com_content&view=article&id=219:elaborer-un-referentiel-des-consommations-domestiques-d-eau-potable-par-usage&catid=11&acm=_73
← 5. Information on urine collection for the Saint-Vincent-de-Paul eco-district: https://www.blog-saintvincentdepaul-leprojet.fr/blog/68/la-collecte-des-urines-un-sujet-serieux
← 6. Data from the National Observatory of Water and Sanitation Services 2024
← 8. Petite-Couronne refers to the area formed by Paris and the three departments nearest to it (Seine-Saint-Denis, Hauts-de-Seine and Val-de-Marne, see Chapter 2).