Effects of steady-state assumption on hydraulic conductivity and recharge estimates in a surficial aquifer system

The ability of a calibrated now model to predict the behavior of a surficial aquifer system is governed by the quality of the hydraulic conductivity and recharge estimates used. Reasonable lateral and vertical hydraulic conductivities can be estimated by steady-state simulations driven by effective recharge rates that approximate the net effects of evapotranspiration, and water released from storage during periods of recession. Results from a hypothetical, transient, cross-sectional model indicated that most of the water was contributed uniformly from storage from five to 25 days after a recharge event. Results also showed that a steady-state, snapshot calibration approach can be used on aquifers in a humid climate with diffusivities between 20 and 500 m(2)/d. Most estimates of the lateral and vertical hydraulic conductivities of the hypothetical aquifer system were within 30 % of the actual values. Estimates of hydraulic conductivity from the transient cases were similar to those from the snapshot calibration cases. The long-term recharge rate could be identified by calibrating to multiple synoptic surveys that were sampled over the range of drier to wetter conditions. The effective recharge rates estimated for the driest and wettest conditions bracketed the long-term recharge rate, Results suggested that the effective recharge rate estimated for the synoptic survey with the lowest water level root-mean-square (RMS) error was the best estimate of the long-term recharge rate. A field application of the snapshot calibration approach simulated the surficial aquifer system beneath Cecil Field Naval Air Station well and provided reasonable estimates of the long-term recharge rate (0.4 mm/d) relative to the range of recharge rates that were independently estimated by the chloride concentration ratio method (0.2 to 0.6 mm/d).