Analytical expressions for transient specific yield and shallow water table drainage

[1] New closed-form expressions are introduced to capture the dependence of specific yield on time and depth to water table. The expressions allow the user to convert observations of water table fluctuations to volumes of water released from storage in a shallow water table aquifer. Whereas a linear relationship between water table fluctuations and released volumes holds for a deep water table aquifer, this relationship is nonlinear for shallow water table aquifers. The dependence of specific yield on time stems from the slow drainage of soil water from pores above the water table. The new expressions allow determination of transient specific yield and time to drain the soil water profile for a given water table fluctuation. If the time step in a numerical groundwater model is longer than the time for limiting specific yield, then a constant (time independent) specific yield can be justifiably adopted. The new expressions are easy to use and require knowledge of soil hydraulic properties which are readily available from soil water retention data and surveys.