Modelling water content redistribution during evaporation from sandy soil in the presence of water table

An analytical model based on Richards equation is presented to describe water content redistribution during evaporation. Exponential functions are utilized to describe the relation of hydraulic conductivity and water content on pressure head. The water content at any given depth z and time t, together with the evaporation rate at the soil surface are outputs of this model. The proposed is capable of modelling the transformation of the constant rate stage into a falling rate stage of evaporation, moreover it extends the use of analytical model to arbitrary initial conditions. Detailed parametric analysis was carried out to investigate the effect of six governing factors. The main findings are as follows: hydraulic conductivity (Ks) and desaturation coefficient (α) affect the water content profile more than the storage capacity (θs-θr) does; the relation between the duration of the constant rate stage and the dimensionless evaporation rate γ can be expressed by a negative power function; the initial conditions are the most influential factor for the soil water profile among all factors examined. Finally, two evaporation tests conducted on K-7 sand and Fontainebleau sand were introduced to validate the proposed analytical model. Results showed that this model could reasonably predict the temporal water content profile during evaporation process, but some discrepancies still existed in some cases. Nevertheless, the analytical model provides a useful tool for predicting the vertical distribution of water content during evaporation.