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ABSTRACT Thick unsaturated units can have a significant impact on the propagation of groundwater recharge from the soil zone to the water table. The corresponding unsaturated water flow is governed by Richards' equation and existing transient solutions are either numerically based or analytical approximations in the frequency domain. Using such an existing frequency‐domain solution, we derive a time‐domain analytical solution as a convolution with an inverse Gaussian transfer (impulse response) function. The time‐domain solution allows for a straightforward coupling to output from a daily soil water balance model, driven by precipitation and potential evapotranspiration signals. The integrated model contains only three effective parameters corresponding to average recharge flux, temporal delay and dispersion. It is applied to the regionally important (area≈10 5 km 2 ) Urucuia aquifer in Brazil with significant (depths up to 10 2 m) unsaturated zones, for which we perform a systematic analysis of seasonal groundwater level fluctuations at 65 monitoring wells. The empirical results confirm a pronounced damping with water table depth, which is accurately predicted by the calibrated model. Simulated saturation profiles are used to estimate electrical resistivity through Archie's law, revealing detectable seasonal variations in the upper 30 m suitable for future geophysical monitoring and further field validation. Results also suggest that multiple years of recharge may be stored in the unsaturated zone of the Urucuia aquifer before reaching the water table with critical implications for drought assessment and sustainable water resource management of the region.
Leão et al. (Thu,) studied this question.