Estimating potential evaporation over a hill

A simple analytical model of the effect of a hill on potential evaporation (i.e., evaporation from well-watered vegetation) is presented. It is shown to reproduce the results from a two-dimensional non-linear numerical model. The analytical model is used to demonstrate the sensitivity of the changes in evaporation to surface resistance, aerodynamic resistance, temperature, slope and sun angle. Methods used by MORECS (Meteorological Office Regiona l Evaporation Calculation System) to estimate the potential evaporation at high elevations, and by Numerical Weather Prediction models to represent su b-grid scale hills, take account of the change in altitude but neglect the slope effects. A slope of 20 degrees, which is typical of upland terrain in the U.K., increases the potential evaporation by about 5%.