UNCONFINED AQUIFER FLOW: THE MYTH AND THE TRUTH

The highly nonlinear nature of unsaturated flow resulted in different ways to approximate the movement of the water table either delayed or instantaneous. The nonlinear boundary condition equation is linearized by Neuman (1972) assuming an instantaneous drainage from the unsaturated zone. Moench (2001) considered gradual drainage from the unsaturated zone using a series of exponential terms in boundary condition but the model does not explain the variation in the soil moisture with height above the water table. Mathias and Butler (2006) obtained a new drainage function based on a linearized Richard’s equation but limiting the variation of soil moisture and hydraulic conductivity in the unsaturated zone to exponential functions. An attempt has been made to compare the water table (hypothetical surface with atmospheric pressure) velocity with the velocity of the drainage above the water table during pumping in an unconfined aquifer. To calculate the water table and drainage velocities, an additional module has been included in the existing WTAQ4 model of Moench (2001) which calculates heads for the above three analytical models during pumping of a confined or unconfined aquifer. The velocities obtained from the three analytical models are compared to water table and drainage velocities from a seven day constant rate aquifer test conducted at Canadian Forces Base Borden in Ontario, Canada and the accuracy of each of the three analytical models is studied. Furthermore, the effect of horizontal flow in the unsaturated zone on the velocities is discussed and the extension of capillary fringe is compared with difference between the distanced traveled by water table and unsaturated drainage during pumping.