EVAPORATION REGIMES AND EVAPORATION MODELING IN AN ALPINE TUNDRA ENVIRONMENT

Evaporation rates responded very quickly to surface desiccation, and t he control by surface resistance (derived from the Penman-Monteith mod el) was very pronounced. The absence of an efficient means to transfer subsurface moisture to the surface resulted in an evaporation regime which was strongly moisture-limited only a few days after precipitatio n. However, the high frequency of precipitation events in this environ ment meant that both energy-limited and moisture-limited regimes occur red in quick succession. The range of minimum surface resistances is s imilar to those used in current land-atmosphere climate models, but th ey tend to be considerably greater during drying events. Four differen t physically based evaporation models were compared with hourly or dai ly Bowen ratio-energy budget measurements. Best results were obtained by using an aerodynamic approach. If high-quality sensible heat measur ements are available, then the evaporation rate could be readily estim ated by treating it as a residual in the energy budget equation. The P riestley-Taylor method is potentially valuable, but the objective spec ification of surface moisture availability is difficult in alpine tund ra. An alternative approach using equilibrium evaporation plus a surro gate measure of surface water (daily precipitation) clearly stratified daily actual evaporation into wet and dry regimes, and may have some predictive value.