estimate of unsteady evaporation from bare soils by zero flux plane (zfp) method and comparison with water balance model

evaporation from the soil surface has an important role in the water balance model in nature, so that the major part of rain fall and irrigation water in arid and semi-arid regions is lost via evaporation from the soil surface. in such areas, there are some regions with high water table and because of evaporation from the soil surface, salts accumulation also occur in the soil surface. so evaporation from the soil surface not only losses water, but also it is responsible for soil salinization. the main difficulty in accurate estimation of nonsteady evaporation in field conditions is the lack of simple functions with minimum input data for calculating water losses in water balance model. the main goal of this study is to introduce a simple method for calculating evaporating from the surface of the bear soil in arid and semi-arid regions with a deep water table. soil water characteristic curves, water content versus depth curve, nonsteady and one dimensional water movement were used in this method, and the rate of anticipation accuracy was determined by the real conditions of the region. since it is difficult to make the lower boundary conditions similarity, applying the physical model in regions with a deep water table is impossible. applied parameters of this model were determined by using water balance model in different regions and with respect to the existing position and trend of changes. by using these parameters, the final amount of evaporations were calculated for karaj, torbat heidarieh and mashhad 174, 116, and 297 mm respectively during 20, 44, and 62 days periods in june 2003, august and september 2002, july and august 2002. in this study, zero flux plane (zfp) was applied with the initial and boundary conditions dominant in the evaporation process which unlimited depth forms its lower boundary conditions. water content – depth curve was drawn by using the results obtained from field. the physical parameters were obtained from laboratory tests using undisturbed soil samples. first the water characteristic curve was drawn, then the hydraulic potential-depth curve was obtained from those results. the depth of evaporation and final evaporation were obtained based on the above mentioned curves and zfp. the amount of evaporation in above regions are 182, 117, and 297 mm respectively for the above mentioned periods. the differences between the results obtained from zfp and water balance model were less than 5% which is not so important in field experiments, and there is a good crossespondance between them.