Estimating soil surface evaporation under unsteady state and variable moisture conditions

Large areas in Iran which located in arid and semi-arid region are faced with serious water shortage. Because in these regions, from one side the precipitation and access to water resources are limited and from the other side atmospheric factors are intensively influence evaporation. In addition, evaporation from soil surface not only wastes water resources, but also may cause soil salinity (Homaee et al., 2002; Hillel, 1998). The objective of this study was to estimate evaporation from bare soil at presence of different water table levels. The rate of evaporation loss was also estimated, using the parametric model proposed by Zarei and Homaee based on Brooks and Corey’s retention curve.
For this purpose, the cumulative and instantaneous evaporation in a sandy loam soil was measured by weighting method three times a day. The proposed model of Zarei and Homaee was further tested against the collected experimental data. This retention-based model has been proposed to estimate evaporation from water falling table.
Results indicated that the maximum and minimum efficiency of model, obtained under 150 and 75 centimeters of water table depth, were 98 and 81 percent, respectively. The results showed that at water table depths less than one meter, the evaporation rate is increased. The results also indicated a reasonable agreement between the measured data and model outputs. The small difference between the measured and predicted values may be attributed to the moisture movement as vapor, soil shrinkage and errors in digital scale measurements. The predicted instantaneous evaporation rates provided better agreement with the experimental data as water table depths increased.
The parametric model of Zarei et al. (2010) provided a reasonable agreement with the experimentally measured evaporation data. Consequently, this model can be used to estimate the evaporation under similar climatic conditions.