Abstract
This study aims at evaluating the effects of wetlands on downstream flow, nitrate, organic nitrogen, and phosphate loading in the Fish River watershed (FRW), located in Baldwin County, coastal Alabama. The FRW drains to the Weeks Bay and represents approximately 75% of the freshwater inputs of this sub-estuary of the Mobile Bay. The Weeks Bay is one of the few designed Outstanding National Resources Water in Alabama and has experienced a significant loss of natural land cover over the past two decades. Consequently, it is of the utmost importance to assess the importance of natural ecosystems such as wetlands in the water quality purification of this pristine watershed. We set up a SWAT model that captured the spatial distribution of 44 major wetlands across the FRW. The SWAT model was calibrated using flow and water quality data available at the watershed outlet by performing 500 simulations from 2008 to 2015. The results yielded daily NSE values of 0.87, 0.86, and 0.54 for streamflow, phosphate, and nitrate, respectively. The upper (90%) and lower limits (10%) of the prediction interval along with the medians of the SWAT generated nutrient loadings and discharge to each wetland were fed as input data to the process-based WetQual model. The outputs of WetQual were then incorporated back into SWAT as point sources and SWAT was rerun. The SWAT-WetQual coupling was automated through a FORTRAN routine. The removal efficiencies of the 44 wetlands were found to be 13-15%, 26-45%, and 65-69% for nitrate, organic nitrogen, and phosphate, respectively. The median removal efficiencies at the watershed outlet were estimated at 5.4%, 28%, 56% for nitrate, organic nitrogen, and phosphate, respectively.
