A Fully Coupled Surface Water Storage and Subsurface Soil Water Dynamics Model for Characterizing Hydroperiod of Wetlands

Hydrological modeling of wetlands is important for reliable estimation of biogeochemical processes in soils subject to periodically inundating conditions. The present study has developed a wetland module in the Richards-equation-based SWAT model to fully couple the surface water storage and soil water dynamics. The wetland module was tested using observed daily water level data from four wetlands (including restored and natural wetlands with and without impermeable soil layers) in the Choptank River Watershed, Maryland, USA. After the wetland module was calibrated, simulated daily water level and observed data were compared and evaluated using three statistics, i.e., percent bias (P_bias), coefficient of determination (R²), and Nash-Sutcliffe coefficient (NS) from 2016 to 2017. The results showed that, in general, the wetland module regenerated hydroperiods for both restored and natural wetlands with and without impermeable soil layers; specifically, the module was able to accurately model saturation conditions for different soil layers corresponding to wet and dry periods in plant growing seasons; the wetland module had the tendency to generate better results for natural wetlands because restored wetlands tended to have mixed plant types which caused difficulty for accurate estimation of evapotranspiration; the ability to accurately describe inundation conditions for wetlands is important for biogeochemical modeling so that the newly developed wetland module has a great potential in enhancing simulation of biogeochemical cycles not only at the site scale but also at the watershed scale.