Predicting Soil Water Content Using the “Drained to Equilibrium” Concept

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This study was conducted to evaluate the accuracy of a simple model, based on the “drained to equilibrium” concept, for predicting soil water dynamics in the presence of a fluctuating, shallow, transient water table. This concept assumes that soil water is primarily due to capillary rise from the water table. Water table depth varied between 1.8 and 2.3 m (with a mean of 2.0 m) during this study. The model was evaluated under four field conditions: (i) deep soil created by trenching, with tree roots, (ii) deep soil created by trenching, without tree roots, (iii) shallow soil with tree roots, and (iv) shallow soil without tree roots. Collocated water table and soil water content data at the 10-, 20-, 40-, and 60-cm depths were collected in a lychee (Litchi chinensis Sonn.) orchard in Homestead, FL. Hydrostatic conditions were identified and van Genuchten parameters were determined using the filtered data set and the van Genuchten equation; accuracy of the predictions was assessed with the Nash–Sutcliffe coefficient of efficiency (C_eff). The greatest variability in soil water content was measured at the 10-cm depth, while at other depths the soil water content was more consistent. In general, for the highly permeable soils and predominantly dry conditions tested, the drained-to-equilibrium model evaluated in this study proved to be a simple one-dimensional physical expression that predicted soil water content well (median C_eff = 0.85) based on a shallow water table level. This information can be used to improve irrigation scheduling and flood management in shallow groundwater systems.