Achilles' heel of integrated hydrologic models: The stream-aquifer flow exchange, and proposed alternative

Due to the dynamic flow exchange between a stream and a connected water- table aquifer, a physically based hydrologic model is an important tool for the design of conjunctive use of surface and ground waters. It is needed to develop sound plans for sustainable use of the groundwater with minimal undesirable effects. This article reports an investigation of the component (module) that describes the stream-aquifer flow exchange in several groundwater simulators. The study indicates that the module may not have a solid physical basis when used in large-scale regional studies to estimate the stream-aquifer flow exchange. The component relies upon an empirical parameter known as the leakance coefficient. It is demonstrated that neglecting river penetration of the aquifer can lead to non-negligible errors in estimation of seepage. In addition the leakance coefficient (dimension inverse of a time) is not constant in time but varies with the conditions in the river and in the surrounding aquifer. A potential, practical and simple alternative is presented.