Calculating potential evaporation from climate model data: a source of uncertainty for hydrological climate change impacts

Evaporation is an important part of the water balance of a catchment, and estimates of potential evaporation (PE) are an important input to hydrological models. When modelling the hydrological impacts of climate change, using data from climate models, present models generally do not provide direct estimates of PE from the land surface. Estimates thus have to be calculated from other climate variables, and many existing formulae can be applied. This paper compares the well-established, more physically-based but data-intensive, Penman-Monteith PE against a simple, temperature-based (T-based) PE, when calculated from readily-available monthly climate model data over Britain (for five global and eight regional climate models). The performance of the two PE formulations is compared to MORECS PE (a gridded dataset derived by the UK Met Office from weather observations, using a modified Penman-Monteith formulation) for the baseline period 1961-1990, and the changes in the two PE estimates between the 1970s and the 2080s are compared. The results show that the T-based PE matches MORECS PE better than does Penman-Monteith PE, for all the climate models studied. However, the changes in the two types of PE between the 1970s and 2080s are different, for each of the climate models, and these affect the modelled hydrological impacts. This is illustrated using three example catchments spread across Britain. The uncertainty introduced by the PE formulation is less than that due to the climate model, but could still be important for some applications.