Modeling Soil Water Redistribution during Second‐Stage Evaporation

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