Time Scales for Surface-Subsurface Flow Modeling

Traditionally, interactive free surface flow and subsurface flow are analyzed separately, using infiltration rate described by an infiltration formula as the common boundary condition. The temporal and spatial scales used for the surface and subsurface models are selected separately and they are different. In reality, the infiltration rate is a part of the results being sought in the conjunctive surface-subsurface flow. Recent advances in numerical techniques and computer capability together with improved understanding of the surface and subsurface flow equations make it feasible for conjunctive modeling of these equations. In the computation instead of using the same time and space scales for the coupled model, the obviously different physical characteristics of the surface and subsurface components permit different time and spatial scales to improve computational efficiency. Subsurface flow, being much slower, allows the use of a much bigger computation time interval than for the surface flow. With the aid of order-of-magnitude analysis of the flow equations, allowable scales can be estimated.