An analytical solution of nonsteady evaporation from bare soils with shallow ground water table

Abstract

Evaporation from bare soils plays a significant role in hydrological cycle, particularly in arid and semi-arid regions. In such areas, a large amount of the water that enters into the soil returns to the atmosphere through evaporation. In these regions, there are some areas with shallow groundwater table, evaporating a considerable amount of water into the atmosphere and accumulating salts at the upper parts of the soil profile. One major difficulty for accurate estimate of evaporation in the field conditions arises from the lack of simple function with less needed input parameters to account for water losses in soil-water balance models. The main purpose of this study was to develop and verify an analytical solution for one-dimensional non-steady upward flow from shallow ground water table with minimum input data. Consequently, an analytical solution was developed based on the Richards' equation with the initial and boundary conditions governing evaporation process. In this solution, the amount and instant of evaporation from the soil surface can be estimated as function of water table drawdown, impermeable layer depth, and soil hydraulic functions. The solution is based on the parametric closed form equation of van Genuchten for retention curve. Some lysimetric experiments with packed Silty Clay, Silty Clay Loam and Sandy Loam soils were carried out to evaluate the analytical solutions. The results indicate a reasonable agreement between the collected data and the theoretical scheme. The solution seems to be promising for different soil types, requiring only few accessible input parameters. However, the small discrepancies can be attributed to side gap of shrinkaged soil, evaporation as vapor phase, experimental errors and most importantly the collapse of macropores resulting from soil packing.