Soil Hydraulic Parameter Upscaling for Steady-State Flow with Root Water Uptake

Abstract

In this study we investigate effective soil hydraulic parameter averaging schemes for steady-state flow with plant root water uptake in heterogeneous soils. “Effective” soil hydraulic parameters of a heterogeneous soil formation are obtained by conceptualizing the soil as an equivalent homogeneous medium. The “effective” homogeneous medium is only required to discharge the same ensemble-mean flux across the soil surface. One-dimensional flow at the local scale has been used as an approximation for various simplified problems under investigation (e.g., a shallow subsurface dominated by vertical flows). The domain is assumed to be composed of homogeneous one-dimensional soil columns without mutual interactions. Using Gardner9s unsaturated hydraulic conductivity model, we derive the effective value for the parameter α. While root water uptake influences the overall water budget, its impact on the effective hydraulic parameter averaging scheme was found to be secondary. Results show that the arithmetic mean of Gardner9s α is usually too large to serve as an effective parameter. Deviations of the effective parameter from the arithmetic mean become larger as the surface suction increases; that is, the flow scenario switches from infiltration to evaporation. The results consistently show a smaller effective parameter for evaporation scenarios than for infiltration scenarios. The effective parameter α eff decreases with an increase in the mean value of α. Spatial variability in α also decreases the effective value of α eff . Alternative root water uptake distributions do not produce significant differences in both the water budget and the averaging scheme as long as total water loss to the plant roots remains the same.