MODELING THE SPATIAL VARIABILITY OF SUBSURFACE RUNOFF USING A TOPOGRAPHIC INDEX

We propose a new topographic index for use in regions with rapid subsu rface runoff which is spatially variable. By considering field measure ments we suggest that the pattern of recharge to the saturated zone is controlled by both the prestorm catchment-average wetness and the pat tern 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 functio n of both local and catchment-average saturated zone thickness. The in dex predicts patterns of both subsurface runoff and saturated zone thi ckness: spatial patterns change as catchment wetness varies. Subsurfac e 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 si gnificant features of observed spatial patterns of subsurface stormflo w for a variety of prestorm conditions and can also be used as the bas is for a rainfall-runoff model.