NEW ENHANCEMENTS AND TMDL TOOLS FOR GENERALIZED WATERSHED LOADING FUNCTIONS (GWLF) MODEL

We have developed a suite of enhancements and total maximum daily load (TMDL) tools for the legacy Generalized Watershed Loading Functions (GWLF) model. GWLF has been widely used in TMDL development to calculate watershed pollutant loads because of its simplicity, efficiency, and low data requirements on input and calibration. One of the major disadvantages of GWLF is that it lacks a detailed stream routing algorithm. To address this shortcoming and improve GWLF's applicability to TMDL development in multiple subwatersheds that are connected by a stream network, we have developed the following enhancements and tools: (1) an ArcView GWLF interface; (2) a Windows version of GWLF, BasinSim 2.0; (3) a stream routing tool; and (4) a TMDL allocation tool.The ArcView GWLF interface works with various watershed delineation tools (e.g., BASINS) and facilitates the generation of the GWLF input files for each delineated subwatershed using national watershed datasets available from the public domain; it also generates a stream network configuration file that describes the spatial relationship between the subwatersheds. The ArcView interface passes the model input files to BasinSim, which runs the GWLF algorithms and calculates flow and pollutant loads for each subwatershed. The stream routing tool interpolates the daily subwatershed results to hourly input and performs flow routing using the Muskingum-Cunge method. Sediment transport processes, including deposition and channel erosion, are calculated on the basis of stream transport capacity and channel physical properties. Nutrient transport through streams is simulated with simple first-order decay functions.By using the ArcView interface, BasinSim, and the stream routing tool, the user can calculate flow and pollutant systematically from the headwaters to the outlet in a large watershed. The TMDL allocation tool can read stream network output, compare the loads with reference or predefined targets, help the user identify the “critical source areas,” and guide the allocation process. Through the TMDL allocation tool, the load reduction percentage specified by the user for each pollutant source in each subwatershed is used to modify the BasinSim output. The routing tool is then re-executed to obtain new loading results after the load reduction. After several iterations, the TMDL allocation process for the entire watershed is completed, and the effects of the allocations for the upstream subwatersheds are properly taken into account for the allocations in the downstream subwatersheds.Although the new GWLF enhancements and TMDL tools are stand-alone components, they were designed to work together tightly and efficiently in a TMDL project. They were also packed into a single installation file, named Enhanced GWLF, or EGWLF. We have successfully used EGWLF in developing sediment TMDLs for West Virginia.