Estimating Sediment and Nutrient loads of Texas Coastal Watersheds with SWAT A case study of Galveston Bay and Matagorda Bay

The SWAT (Soil and Water Assessment Tool) model was used to estimate terrestrial sediment and nutrients loads to the Galveston and Matagorda bays from their contributing watersheds. In this report, the term "terrestrial loads" represents the sum of gauged loads from gauged subbasins and model-generated loads from ungauged subbasins. Because we did not have access to the point sources data, the municipal wastewater treatment plants (WWTPs) and industrial point source discharges are not included in this calculation of water quality variables. This information, however, would be required to calculate the total nutrient load actually reaching a bay. The predicted total N and P were compared to the estimated mean annual nutrient loads from fresh water by the previous studies. SWAT is a spatially distributed, continuous model that can be used to estimate flow, sediment, and nutrients at a variety of scales ranging from a small hill slope to a large watershed. SWAT benefits can be summarized into three categories. First, SWAT offers finer spatial and temporal scales, allowing users to observe an output from a particular subbasin within a particular time frame. Secondly, it considers comprehensive hydrological processes at the subbasin level and within the entire watershed, estimating not only surface runoff with associated sediment and nutrients but also subsurface flow, groundwater flow and channel processes. And third, the calibrated model can be developed to analyze scenarios such as BMP (Best Management Practice) usage, land use changes, climate change and more. In this study, two watersheds, the Galveston Bay and Matagorda Bay, were selected for a pilot study because one represents an urbanized watershed (Galveston Bay) and the other a rural watershed (Matagorda Bay). Geographic Information System (GIS) data and other parameters were obtained from several sources: the Digital Elevation Model (DEM) from the Natural Resource Conservation Service (NRCS) provided topography, land cover and soil data; the National Climate Data Center (NCDC) provided weather data; and U.S. Geological Survey stream gauge stations provided flow data series as well as sediment and nutrient samples. The project consists of two parts. Hydrologic simulation was performed in the first phase, and the second phase focuses on the estimation of sediment and nutrient loads. Two separate SWAT models were developed; one for each watershed. SWAT's automatic processes delineated the watersheds, river channels, subbasins, and Hydrologic Response Units (HRUs). Weather station data were enhanced and adjusted using NEXRAD (Next Generation Radar) precipitation data. Two lakes, …