EVAPORATION FROM NATURAL MOUNTAIN GRASSLAND

Evaporation from upland grassland is investigated using the results fr om two weighing lysimeters. The lysimeters were operated at a site on the Balquhidder catchments at an altitude of 595 m for 2 years (1988 a nd 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 designe d 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 adjus ted crop factor. The introduction of a simple interception model did n ot improve the model fit. A range of models gave a very good fit to th e cumulative total evaporation; the most generally applicable model re duces the evaporation at low air temperatures.