Exploring the Relationship Between Groundwater Drought and Evapotranspiration With the JULES Land Surface Model in a UK Chalk Catchment

Hydrological droughts can last months to years and impact large areas, leading to a multitude of ecological and socio‐economic harm. The role of evapotranspiration (ET) in drought is very variable and there is contradicting evidence on the impact of anthropogenic warming on groundwater drought in the UK. We integrated a distributed groundwater model into the JULES land surface model (JULES‐DGW) and simulated a chalk catchment in southern England over the period 1901–2015. The model showed a good match to river flows (Kling–Gupta efficiencies 0.73–0.83) and groundwater levels ( r 2 = 0.92). We found a general trend of drying over time with small decreases in average moisture in the unsaturated zone and average groundwater levels, caused by increases in annual ET and decreases in recharge as a fraction of precipitation. The model suggests drier conditions in the unsaturated zone in late summer/early autumn in the late 20th–early 21st century have led to a delay in the recharge season. No increase in capillary rise was simulated throughout the modelling period and ET was found to decrease in most cases of severe drought, thus acting to limit the fall in groundwater levels.