Evapotranspiration from the Alpine Tundra of Colorado, U.S.A.

Abstract

An electronic load-cell weighing lysimeter was used for measuring daily evapotranspiration from the alpine tundra on Niwot Ridge in the Front Range, Colorado, during the 1987 growing season. Concomitant measurements of net radiation, insolation, ground heat flux, and the gradients of wind, temperature, and humidity in the lower atmosphere are used to evaluate factors that govern temporal variations in evapotranspiration from four sites in a dry alpine meadow. Evapotranspiration was governed by energy availability for 3 to 5 d after precipitation events; thereafter, the rate at which the gradually drying soil could deliver water to the evaporating surface appeared to become an increasingly important control over evapotranspiration. Advection of cold, dry air from the Continental Divide increased both the vertical temperature and specific humidity gradients 100% over what would have occurred if advection were not present. Since the effect of advection on these gradients was approximately equivalent, both the sensible and latent heat fluxes were augmented at the expense of the ground heat flux. A primary effect of advection is to reduce the temperature of the alpine tundra surface.