Evaporation from natural mountain grassland

Evaporation from upland grassland is investigated using the results from two weighing lysimeters. The lysimeters were operated at a site on the Balquhidder catchments at an altitude of 595 m for 2 years (1988 and 1989). The results show an evaporation rate which is significantly below that of the Penman evaporation ( E T), particularly in the early part of the year, when the grass is dormant. The lysimeter data have been used to calibrate and assess a number of water use models designed to estimate grassland evaporation from meteorological measurements. Transpiration models to take account of the dormancy of the vegetation were tested using either a temperature function or a seasonally adjusted crop factor. The introduction of a simple interception model did not improve the model fit. A range of models gave a very good fit to the cumulative total evaporation; the most generally applicable model reduces the evaporation at low air temperatures.