Modeling the spatial variability of subsurface runoff using a topographic index

We propose a new topographic index for use in regions with rapid subsurface runoff which is spatially variable. By considering field measurements we suggest that the pattern of recharge to the saturated zone is controlled by both the prestorm catchment-average wetness and the pattern of saturated zone thickness. The major distinguishing assumptions of this index are that (1) the soil lies above an impermeable layer; (2) saturated hydraulic conductivity does not vary with depth; (3) the pattern of recharge can be estimated using a simple nonlinear function of both local and catchment-average saturated zone thickness. The index predicts patterns of both subsurface runoff and saturated zone thickness: spatial patterns change as catchment wetness varies. Subsurface runoff is predicted to be most uniform when the catchment is wettest, and as the catchment dries out, runoff decreases most quickly at the driest locations in the catchment. This index is able to reproduce significant features of observed spatial patterns of subsurface stormflow for a variety of prestorm conditions and can also be used as the basis for a rainfall-runoff model.