Subsurface water flow simulated for hillslopes with spatially dependent soil hydraulic characteristics

Water flow through hillslopes consisting of five soil layers, with varying spatial dependence in hydraulic characteristics in the lateral plane was simulated by solving Richards' equation in three dimensions under varying rainfall intensities and for two complexities of terrain. By concepts of similar media the variability in soil hydraulic characteristics was expressed by a single dimensionless parameter, the scaling factor α. The moments of log normally distributed α were set as: Mean = 1.0 and standard deviation = 1.0. Four cases of spatial dependence of α in the lateral plane were selected for simulation, using exponential variogram functions ranging in spatial structure from radom (no spatial dependence) to large dependence (large correlation lengths). The simulations showed that the rates of subsurface flow from the 30° hillslope, during and following rainfall, were significantly enhanced with an increase in spatial dependence. Subsurface drainage was also increased with increases in rainfall intensity and slope complexity. For hillslopes the relative effect of spatial dependence in soil hydraulic characteristics was smaller with 30° horizontal pitching than without pitching. Hillslopes with a random distribution of hydraulic characteristics provided greater opportunity for soil units with differing water capacities to interact than in cases with spatially correlated distributions. This greater interaction is associated with a greater lag in subsurface flow generation. These studies illustrate some of the expected effects of spatial dependence of soil hydraulic characteristics on the integrated hydrologic response of land areas.