| Summary: | The highly nonlinear nature of unsaturated flow results in different ways to approximate the delayed or instantaneous movement of the water table. In nearly all the approaches, the water table is conceptually treated as a “material surface”. This term defines the water table as having two simultaneous properties: 1) the pressure along the surface is atmospheric pressure, and 2) the water table is fixed to the material, i.e., a set of water particles. This article makes an attempt to explain that the water table, defined as the surface at atmospheric pressure, is not a material boundary, and the water table can move independent of the water particles.
Velocity of the water table and velocity of drainage are compared with three analytical models: the Neuman model, which assumes instantaneous drainage from the unsaturated zone; the Moench model, which considered gradual drainage from the unsaturated zone using a series of exponential terms in the water table boundary condition; and the Mathias-Butler model, which obtained a new drainage function based on a linearized Richard’s equation but limited the variation of soil moisture and hydraulic conductivity in the unsaturated zone to exponential functions. Numerical analysis was conducted with VS2DT and both the numerical and the analytical results were compared with a 7-day, constant rate pumping test conducted by University of Waterloo researchers at Canadian Air Force Base Borden in Ontario, Canada.
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