WATER FLUXES IN OXISOLS - A COMPARISON OF APPROACHES

Two approaches to simulating soil water flow were evaluated: a capacity-type water flow model combined with a plant water uptake model based on the concept of plant-available water, and a numerical solution to the Richards equation combined with a potential-driven water uptake model. Irrigation water was applied at rates ranging from 0.3 to 1.6 potential evapotranspiration to an Oxisol under fallow and planted to com (Zea mays L.). Results indicate that a two-parameter soil moisture release curve does not sufficiently describe these soils over the entire plant-available water range. Field saturated water contents were generally only 70% of total porosity. The potential-driven model better predicts profile water content distribution under fallow and cropped conditions than the capacity-type approach (R2 = 0.80 and 0.83 versus R2 = 0.59 and 0.70). When integrated over the growing season, drainage, evaporation and transpiration rates were very similar for capacity-type water movement and potential-driven water movement over the range of irrigation treatments.