Future increases in soil moisture drought frequency at UK monitoring sites: merging the JULES land model with observations and convection-permitting UK climate projections

Concerns exist about the viability of food security across Europe due to multiple, potentially adverse drivers. These include economic, political and climate forcing factors, all of which require quantification. Here, we focus on the climate forcing, and in particular, the soil moisture change component which crucially determines water availability for crop uptake. We estimate future soil moisture levels at 34 sites of the UK COsmic-ray Soil Moisture Observing System (COSMOS-UK) network. We do this by combining three platforms: the Joint UK Land Environment Simulator (JULES) land surface model, field-scale soil moisture observations from the COSMOS-UK stations and 2.2 km convection-permitting UK Climate Projections (UKCP18). We use COSMOS-UK data to optimise key soil moisture-related parameters in the JULES model, based on its performance in the contemporary period. We then force the calibrated model with UKCP18 data to produce future soil moisture estimates. We evaluate the modelled soil moisture for an average soil depth between 0 and 35 cm to match the depth of soil moisture observations. Our main conclusions concern future soil moisture droughts which we compare with equivalent events in the historical period, 1982-2000. We find that on average across all sites, there is an increase in the frequency of future extreme soil moisture drought events of duration above 90 days. In 2062-2080, such frequency increase of between 0.1 and 0.6 events per year (equivalent to at least 2 and up to 12 additional events in a 20-year period) is expected. We also show that, in 2062-2080, there is an increased risk of high or more intense soil moisture drought conditions in months between May and November, with months between June and October being at especially high risk. The UKCP18 data corresponds to a high-emissions future described by the RCP8.5 scenario.