Role of subsurface flow in generating surface runoff: 1. Base flow contributions to channel flow

The importance of the subsurface response of watersheds has been vastly underrated in most studies of watershed behavior, both in a quantitative sense and in a generic sense. The mechanism of base flow generation and the nature of watershed response in base flow dominant streams are examined with a deterministic mathematical model that couples three‐dimensional, transient, saturated‐unsaturated subsurface flow and one‐dimensional, gradually varied, unsteady channel flow. The channel flow model uses the single step Lax‐Wendroff explicit technique to solve numerically the full shallow water equations. The subsurface flow model uses the line successive overrelaxation technique to solve numerically the Jacob‐Richards diffusion equation. The results of the simulations on a hypothetical basin suggest a wide variability in watershed response under the influence of variations in rainfall properties, antecedent moisture conditions, and saturated and unsaturated subsurface hydrogeologic properties. This evidence for a wide range of watershed response functions leads to the development of a healthy skepticism toward black box rainfall‐runoff correlations, the concept of basin linearity, and the rationality of hydrograph separation.