Modeling the hydrologic response of groundwater dominated wetlands to transient boundary conditions: Implications for wetland restoration

A variably-saturated groundwater model, based on that of Freeze [Freeze, R.A., 1971. Three-dimensional, transient, saturated– unsaturated flow in a groundwater basin. Water Resources Research 7, 347–366.], was used to analyze the details of surface– groundwater interaction and resulting hydroperiods of a site undergoing wetland restoration (the Lake Station Wetland Restoration Site in Northwest Indiana, USA). The three-dimensional groundwater flow model couples the saturated and unsaturated zones through the use of van Genuchten’s [van Genuchten, M.T., 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Science Society of America Journal 44, 892–898.] characteristic equations. Initial estimates of hydraulic parameters were refined through a calibration exercise aimed at minimizing the discrepancy between simulated and measured water levels in seven wells within the study. Numerical simulations using the calibrated model, and driven by annual time series of rainfall and potential evaporation, were used to generate hydroperiod maps of surface saturation and root-zone saturation over a three-year period of study. This allowed identification of regularly saturated areas that would support hydric plants, as well as more rarely saturated areas that would require more dry tolerant species or additional hydrological remediation. The simulations also revealed the critical roles that topography, rainfall history, and antecedent conditions play in the hydrology of degraded wetlands that have been targeted for restoration.