Co-Authors:
Assouline, S., Institute of Soil, Water and Environmental Sciences, A.R.O., Volcani Center, Bet-Dagan, Israel
Tyler, S.W., Department of Geological Sciences and Engineering, University of Nevada, Reno, NV, United States
Selker, J.S., Department of Biological and Ecological Engineering, Oregon State University, Corvallis, OR, United States
Lunati, I., Research Center for Terrestrial Environment, University of Lausanne, Lausanne, Switzerland
Higgins, C.W., Department of Biological and Ecological Engineering, Oregon State University, Corvallis, OR, United States
Parlange, M.B., School of Architecture, Civil and Environmental Engineering, Ecole Polytechnique Federale de Lausanne (EPFL), Lausanne, Switzerland
Abstract:
Accurate estimates of water losses by evaporation from shallow water tables are important for hydrological, agricultural, and climatic purposes. An experiment was conducted in a weighing lysimeter to characterize the diurnal dynamics of evaporation under natural conditions. Sampling revealed a completely dry surface sand layer after 5 days of evaporation. Its thickness was <1 cm early in the morning, increasing to reach 4-5 cm in the evening. This evidence points out fundamental limitations of the approaches that assume hydraulic connectivity from the water table up to the surface, as well as those that suppose monotonic drying when unsteady conditions prevail. The computed vapor phase diffusion rates from the apparent drying front based on Fick's law failed to reproduce the measured cumulative evaporation during the sampling day. We propose that two processes rule natural evaporation resulting from daily fluctuations of climatic variables: (i) evaporation of water, stored during nighttime due to redistribution and vapor condensation, directly into the atmosphere from the soil surface during the early morning hours, that could be simulated using a mass transfer approach and (ii) subsurface evaporation limited by Fickian diffusion, afterward. For the conditions prevailing during the sampling day, the amount of water stored at the vicinity of the soil surface was 0.3 mm and was depleted before 11:00. Combining evaporation from the surface before 11:00 and subsurface evaporation limited by Fickian diffusion after that time, the agreement between the estimated and measured cumulative evaporation was significantly improved. © 2013. American Geophysical Union. All Rights Reserved.
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