St. Johns River -- Watershed Assessment Project

The purpose of the GIS watershed assessment project was to develop specific criteria to determine the relative impacts of land use, soils, hydrography, and other parameters on the discharge water quality, wetland value, and flood impact. Watershed assessment models have two roles in the overall assessment process: source identification and abatement plan evaluation. This project was the first fully daily routing option of WAM completed 1998. The specific objective was to verify WAM on the Deep Creek watershed and then verify on other gauged watersheds along the Lower St. Johns River. The results of the project showed that process-based WAM simulated the other gauged watersheds extremely well without any calibration and therefore would laid the SJRWMD to have confidence that WAM could be used in ungauged watersheds, which was done. See Lower St. Johns River Subbasins Hydrologic/Water Quality Modeling for additional WAM modeling work in the area.

St. Johns River Watershed Assessment Model

WAM (Watershed Assessment Model) is a GIS-based tool for determining the spatial influence of land uses and soils on the water quality and quantity throughout a watershed. The modeling approach is to overlay land use and soil ARC/INFO Grid coverages (one hectare cell size) to locate every unique soil/land use combination within a watershed. The surface and ground water discharges and their total suspended solids (TSS), nitrogen and phosphorus contents from every cell are then simulated using a land use specific cell model. The USDA GLEAMS model is used for non-wetland and non-urban areas. Wetland and urban areas are handled with separate models. The individual cell discharges are then routed through the watershed based on the GIS hydrography coverage.

Time series of the water and nutrient outflows were of critical concern in this project because of our model's output is being interfaced to a US COE model for the St. Johns River mainstem. Therefore, two separate versions of WAM were developed for predicting either very detailed hourly discharges (WAM-D) or long-term annual average responses (WAM-A). The WAM-D model, which was used in the St. Johns River project, simulates water depths and flows throughout the stream network, but it requires much more data and longer run-times than WAM-A. WAM-A is normally preferred unless the actual time series of constituent loads and flow to a receiving water body is needed for assessments, which was the case here.

A menu driven interface allows the user to easily view the input and output data. Graphical land use editing and management assignment tools allow the user to modify land uses for comparisons to existing conditions. Different best management practices (BMPs) can be easily assigned to land uses to directly assess pollution abatement strategies. Dual screen graphical displays and tabular ranking tables provide both visual and quantitative comparisons of a test scenario.

WAM also provides a simple indexing model for the spatial assessment of BOD, toxins, and coliform bacteria sources, as well as the pollutant assimilative capacity and wildlife diversity of wetlands within a watershed.