A hydrological perspective on UK evaporation: historical trends and future projections

1. Evaporation is an important component of the hydrological cycle. This document presents a review of historical trends and future projections for evaporation in the UK, from a hydrological perspective. 2. Potential evaporation (PE) is generally considered to represent the amount of water that would be lost to the atmosphere if there were no limits to supply. Actual evaporation (AE) can be estimated as a fraction of PE dependent on soil wetness. 3. There are many formulae for estimating PE from meteorological data, of varying complexity, from simple empirical formulae to data-intensive physically-based formulae (like Penman-Monteith). 4. PE is usually required, along with rainfall, as an input for hydrological modelling. MORECS PE, based on Penman-Monteith, is often used in the UK. 5. PE accuracy is generally considered less important than rainfall accuracy for hydrological model performance, as PE is less spatially and temporally variable than rainfall and has a well-defined seasonal pattern. 6. There is relatively little information on historical trends in AE or PE in the UK, or indeed globally. The few UK studies generally indicate increases in PE, and one recent study has shown an increase in national average AE. 7. There is little consensus on the best formulae to derive future PE projections from climate model data. The choice is further complicated when considering possible changes in plant behaviour under higher CO2 concentrations. 8. Most studies presenting future PE projections for the UK indicate increases in annual PE totals, although some studies suggest small decreases in PE for one or two months of the year. 9. The approach used to estimate PE could be particularly important in regions where precipitation and PE are in close balance with each other, but PE uncertainty could be less important than climate model uncertainty for hydrological impacts. 10. Further investigation is needed into which PE formulae are likely to be more (or less) reliable when applied with climate model data, and into the feedbacks between climate change and plant transpiration, plant growth and land-cover.