The Illinois River flows from Arkansas to Oklahoma and terminates at Lake Tenkiller (also known as Tenkiller Ferry Lake). The river drains an area of approximately one million acres, and the lake is spread over approximately 13,000 acres. Many reaches in the watershed and Lake Tenkiller are impaired due to excess nutrients and require a TMDL; the watershed is home to intensive livestock operations that have been a significant contributor of nutrients in the watershed. Nutrient loading into the watershed and eventually to Lake Tenkiller is also the subject of an ongoing lawsuit between Arkansas and Oklahoma.
To develop a scientifically defensible watershed and lake model to support TMDLdevelopment for the Illinois River Watershed.
Illinois River Watershed.
The project involved the development of a hydrologic and water quality model for the entire watershed using Hydrologic Simulation Program-FORTRAN (HSPF); this water-shed model was then loosely coupled to a receiving water model for Lake Tenkiller, which the DSI team developed using Environmental Fluid Dynamics Code Plus(EFDC+). The watershed model was calibrated and validated to a 20-year simulation period representing changes in land use, Waste Water Treatment Plant (WWTP) operations, and fertilizer, manure, and litter management. DSI team members applied a detailed nutrient cycling algorithm in HSPF (AGCHEM) for pasture and agricultural land uses. The models were calibrated to multiple monitoring locations throughout the watershed and the lake.
DSI team members added many new features in HSPEXP+ (an Enhanced Expert System for HSPF Calibration) to support the development of the watershed model. The flow and all other constituents from the watershed model were used as boundary conditions for the lake model. DSI team members then simulated detailed nutrient cycling and biochemical processes in the lake. Following the model development and review process, we simulated multiple watersheds and lake scenarios to support the TMDL process.
The final modeling and scenario project report was submitted to the USEPA forTMDL development.