A coupled surface water storage and subsurface water dynamics model in SWAT for characterizing hydroperiod of geographically isolated wetlands

Abstract

Wetlands play an important role in watershed hydrology and biogeochemistry. A geographically isolated wetland (GIW) module for Soil and Water Assessment Tool (SWAT) was developed to couple surface water storage and subsurface water dynamics to characterize hydroperiod. The new GIW module includes the following features: (1) a flexible geometric formula to characterize wetland surface water area, volume, and depth; (2) a revised algorithm to account for evaporation from both water and soil surfaces in wetlands; (3) application of the Richards equation to couple surface water storage and subsurface water dynamics; (4) use of Darcy's law with an effective hydraulic conductivity parameter to simulate groundwater discharge. We tested the GIW module using observed daily water level data from four wetlands at two sites (including restored and natural wetlands with and without a low-permeability soil layer) in the Delmarva Peninsula, USA. The results show that the wetland module reasonably reproduced observed water levels for both restored and natural wetlands with and without a low-permeability soil layer. The module was also able to reasonably simulate saturated and unsaturated portions of the soil corresponding to wet and dry periods. The ability of the GIW module to describe inundation conditions for wetlands holds promise to enhance the understanding and quantification of hydrological and biogeochemical roles of GIWs in a watershed context.