Trends in evapotranspiration and its drivers in Great Britain: 1961 to 2015

Abstract. In a warming climate, the water budget of the land is subject to varying forces such as increasing evaporative demand, mainly through the increased temperature, and changes to the precipitation, which might go up or down. Using a verified, physically based model with 55 years of observation-based meteorological forcing, an analysis of the water budget demonstrates that Great Britain is getting warmer and wetter. Increases in precipitation (3.0 ± 2.0 mm yr −1  yr −1 ) and air temperature (0.20 ± 0.13 K decade −1 ) are driving increases in river flow (2.16 mm yr −1  yr −1 ) and evapotranspiration (0.87 mm yr −1  yr −1 ), with no significant trend in the soil moisture. The change in evapotranspiration is roughly constant across the regions whereas runoff varies greatly between regions: the biggest change is seen in Scotland (4.56 mm yr −1  yr −1 ), where precipitation increases were also the greatest (5.4 ± 3.0 mm yr −1  yr −1 ) and smallest trend (0.29 mm yr −1  yr −1 ) is seen in the English Lowlands (East Anglia and Midlands), where the increase in rainfall is not statistically significant (1.1 ± 0.7 mm yr −1  yr −1 ). Relative to their contribution to the evapotranspiration budget, the increase in interception is higher than the other components. This is due to the fact that it correlates strongly with precipitation which is seeing a greater increase than the potential evapotranspiration. This leads to a higher increase in actual evapotranspiration that the potential evapotranspiration, and a negligible increase in soil moisture or groundwater store.