Evaporation in the Atacama Desert: An empirical study of spatio-temporal variations and their causes

Summary The Atacama Desert is hyper-arid, and areas where adequate moisture exists for evaporation are spatially highly restricted. Nevertheless, water resources exist and their evaluation requires knowledge of this elusive but important component of the hydrological cycle. Evaporation may occur in four typical areas: rivers and associated riparian zones, localized springs, large playas and extensive areas of bare soil after infrequent precipitation events. Transpiration is locally possible where moisture is sufficiently close to the surface to allow phreatophytes or scarce grass cover to grow, but virtually no information is available for quantification. Pan evaporation data from 11 stations for the period 1977–1991 is analyzed and complemented by analysis of an evaporation study conducted in the Salar de Atacama during 1987/1988. The results show that pan evaporation, and hence maximum potential evaporation may be considered largely a function of maximum temperature and elevation as well as density of the evaporating fluid. Actual evaporation is limited by available moisture and diminishes rapidly as the level of soil moisture saturation drops below the soil surface, extinguishing at ca. 2 m depth. Evaporation is greatest during the summer, but at higher elevations convective cloudiness develops during January and February reducing evaporating rates at a time when significant precipitation may occur. Inter-annual variations in pan evaporation are considerable and weakly correlated with ENSO, but variations in actual evaporation are damped by comparison. Regression equations are developed which have widespread applicability and may be used to estimate evaporation in areas where no site-specific data exists.