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

Climate change intensifies the hydrological cycle leading to concerns in future water availability. The resulting changes in water availability need to be quantified to determine present and future actions needed with regards to water resources management. In this work, we focus on the soil moisture component of the hydrological cycle which is crucial for agriculture and ultimately for ensuring food security. We model future soil moisture levels under the high emissions RCP 8.5 scenario at 34 sites of the UK COsmic-ray Soil Moisture Observing System (COSMOS-UK) network. We do this by bringing together: the Joint UK Land Environment Simulator (JULES) land surface model, long-term field-scale soil moisture measurements from the COSMOS-UK network and 2.2 km convection-permitting UK Climate Projections (UKCP18). As a first step, we use the COSMOS-UK observations to optimise 12 parameters of the Cosby pedotransfer functions used in the JULES model. We then force the optimised JULES model with UKCP18 data to produce soil moisture estimates in three time periods: 1982-2000, 2022-2040 and 2062-2080. We interpret the results in the context of frequency of soil moisture drought events and the impact on individual months. We find that on average across all sites, there is an increase in future extreme soil moisture drought events above 90 days with respect to the historical period. In 2062-2080, the frequency of these events is expected to increase by a factor of between 1.8 and 2.8. We also show that months between May and November have an increased probability of high or more intense plant water stress in this far future period, with months between June and October being at especially high risk. This work has been published in https://doi.org/10.1088/1748-9326/ad7045.