Modeling soil water redistribution during second-stage evaporation

Calculating the dynamics of soil water content ( theta ) near the surface and modeling soil water evaporation (E sub(s)) are critical for many agricultural management strategies. This study was performed to develop a model to simulate soil water redistribution during second-stage evaporation (SSE). In this model, the daily change of theta was estimated from the difference between the initial theta ( theta sub(i)) and air-dry theta ( theta sub(ad)), multiplied by a conductance coefficient (C). The C represents the fraction of the remaining soil water ( theta sub(i) - theta sub(ad)) that can be removed in 1 d during SSE and is a power function of soil depth. Testing the dependency of C and alpha (the slope of cumulative evaporation [E sub(c)] vs. square root of time [t super( one half )]) on soil characteristics was done using theoretical and laboratory data. Then the whole model was evaluated in laboratory and field conditions by measuring theta for different soils at different depths during SSE. Linear relationships with zero intercept were found between alpha and drained upper limit theta ( theta sub(dul)) with slope and r super(2) = 1.19 and 0.69 and 1.39 and 0.95 for laboratory and theoretical data, respectively. Conductance coefficient and theta sub(dul) were correlated with r super(2) > 0.9. Root mean square error (RMSE) between measured and estimated theta in the field was highest (0.014 cm super(3) cm super(-3)) at depths of 3 and 6 cm and lowest (0.005 cm super(3) cm super(-3)) at the 9-cm depth. The model gave reasonable estimates of both water redistribution and E sub(s) during SSE and is expected to work well for soils for which the diffusivity theory holds.