Chapter 2 relies on a mix of indicators (observations) and indices to assess current and future hydrological and agricultural drought risk, along with their key drivers. The indicators used in this report meet the following three criteria: (i) they are normalised indicators (i.e. standardised indicators or anomalies) to track changes in drought conditions over time and across regions; (ii) they are widely used and recognised in scientific literature for their accuracy in monitoring agricultural and hydrological droughts; and (iii) they are available at a gridded level for both historical and projected periods.
To evaluate changes in drought frequency, intensity, and extremes, this report primarily relies on the Standardised Precipitation Evapotranspiration Index (SPEI), which measures the balance between precipitation and evaporation, a key driver behind the occurrence and severity of droughts. SPEI strongly correlates with both agricultural and hydrological droughts, making it a valuable proxy for assessing long-term trends. Agricultural droughts are further analysed using the Surface Soil Moisture Anomaly (SMA). Historical hydrological droughts are monitored using station-based measurements of river flow and groundwater levels. Finally, precipitation and temperature anomalies, along with potential evaporation indicators, are used to evaluate historical and projected climate change impacts on drought risk. Table A B.1 below summarises the key characteristics and sources of the climate indicators used in this analysis.