Aggregation scenarios to model water fluxes in watersheds with spatial changes in soil texture

Accurate knowledge of water fluxes in the vadose zone of watersheds is important for applications such as water resources management and climate forecasting. Most, if not all, large-scale studies follow a pragmatic approach where simplifying assumptions have to be made regarding problem formulation and estimation of hydraulic properties. This study investigates simplifications in both regards to predict infiltration and evaporation fluxes near or at the surface for a generic, rectangular watershed consisting of sand and silt loam columns. The two-dimensional flow problem (reference scenario) as well as simplifying 1-D problems are solved with the finiteelement method (FEM) for 1, 10, 100, and 1000 m widths of the flow domain and different proportions of the sand and silt loam soils. The hydraulic functions are estimated from soil texture. In the simplifying scenarios, the flow domain is either represented as an equivalent soil using a weighted particle-size distribution as previously applied in physico-empirical predictions of hydraulic properties (a priori aggregation) or as two parallel stream tubes with area-weighted contributions to the total flux (a posteriori aggregation). Substantial differences were found between the fluxes based on the “equivalent” and reference scenarios even though our approach was based on a most favorable situation where only a limited number of texture-dependent hydraulic parameters were different. The “stream tube” scenario typically provided a good description of the flux according to the reference scenario except for infiltration in case of domains less than 10 m wide. No pronounced textural differences are likely to occur over such small distances and the stream tube model appears to be a viable method to describe near-surface fluxes in catchments with a spatially variable soil texture. 1 Currently at USDA-ARS, GEBJ Salinity Lab., Riverside, CA 92507 2 Department of Biological and Environmental Engineering, Cornell University, Ithaca, NY 14853, Tel: (607)255-2476, e-mail: [email protected]