What does the future hold? Using Standardised Precipitation and Evapotranspiration Index (SPEI) to project drought in Scotland.

UN Sustainable Development Goal (SDG) 6.1 aims to achieve universal and equitable access to safe and affordable drinking water for all by 2030. However, even in a developed nation such as Scotland, climate change, and the water systems resilience to it, is putting achieving this goal at risk. Despite being abundantly blessed in terms of water resources, Scotland is facing an accelerated increase in the frequency of extreme weather events. The UK Climate Projections 2018 indicate that Scotland’s climate will become warmer, with drier summers, and increased occurrence of drought events. Recent water scarcity events prove the surge and are evidence for the projected weather patterns. Unlike drought indicators which are parameters describing meteorological, hydrological or agricultural drought conditions, like precipitation amounts, streamflow levels, soil moisture information, drought indices derive value based on statistical calculations. Once such meteorological drought index is the Standardised Precipitation and Evapotranspiration Index (SPEI) which is similar to the Standardised Precipitation Index (SPI). Unlike SPI, SPEI incorporates changes in evapotranspiration as it includes both precipitation and temperature as input data for calculation. Hence, SPEI makes a good choice for projecting future changes in a warming world and allows us to see the impact of climate change in inducing drought. Regional-scale analysis of SPEI across 36 sites using a 50 km grid generated drought scenarios for the longer term 2041-2080 using all 12 model members the UKCP18 dataset using 1981-2020 as the baseline period. These UKCP 18 projections were bias-corrected and downscaled to a 1km grid across Scotland before we acquired the data for analysis, thus enabling the calculation of SPEI at a finer scale. SPEI was then calculated at a 6-month timestep across the 36 sites in Scotland. The number of extreme drought months was computed for the baseline and the future periods. The drought month was defined as any month which has SPEI ≤ -2.  After calculating the extreme drought months for baseline and future periods, the metrics from 1981-2020 were subtracted from the future period for each model member to demonstrate the amount of change in the number of drought months from the baseline period. Results were calculated separately for the individual member and not averaged to avoid incorporating uncertainty associated with projections. The majority of the sites across the spatial extent showed projected increases in the number of drought months for the future period for each of the model members. Sites in the southwest and western Scottish islands showed a greater increase compared to other sites where extreme drought months were observed with little or no change. Results highlighted the need for better preparedness for water scarcity situations which are going to be exacerbated by climate change.