Modeling Shallow Water Table Dynamics in Unconfined Aquifers Using Drainable and Fillable Porosity

Water table dynamics in shallow unconfined aquifers is usually studied with the help of hydraulic groundwater theory which requires two important aquifer storage parameters, the drainable and fillable porosity. Yet most of the models typically use only drainable porosity although these two parameters tend to be different in field conditions due to the occurrence of unsaturated zone soil moisture fluxes, even when hysteretic effect is neglected. In this study, separate expressions for drainable and fillable porosity were developed and used to simulate the shallow water table dynamics. These analytical expressions account for the unsaturated zone moisture flux to and from the water table through the assumption of steady-state flow in successive time steps. The results showed that when using separate drainable and fillable porosity parameters to simulate respective decline and rise of the water table, water table fluctuation was predicted significantly better than the single parameter model. Water table rise due during rainfall events was also better predicted by the model implemented with fillable porosity parameter. The study suggests that the flux-dependent expressions of drainable and fillable porosity have implications not only in improved modeling of shallow water table dynamics but also in estimating evapotranspiration from diurnal water table fluctuations as well as estimation of water rise during rainfall.