Dynamic modeling and monitoring of water, sediment, nutrients, and pesticides in agricultural watersheds during storm events

Each year large quantities of fertilizer and herbicides are applied to Midwestern farm fields.In a recent investigation, the White House Committee on Environment and Natural Resources (Goolsby et al., 1999) found elevated concentrations of nitratenitrogen (nitrate-N) in Midwestern streams and rivers.Some central Illinois drinking water supplies (Decatur, Danville, Pontiac, and Georgetown) periodically exceed the drinking water standard of 10 milligrams per liter (mg/L) of nitrate-N that was set to prevent incidence of methemoglobinemia (blue baby syndrome).Fertilizer application is not the only source of these elevated nitrate-N concentrations, other manmade and natural sources such as atmospheric deposition and fixation of N, and mineralization of organic N contribute significantly to the problem.Other drinking water sources, such as Lake Springfield, require expensive water treatments when they periodically exceed 3 micrograms per liter (µg/L) maximum concentration level (MCL) for atrazine, a commonly used herbicide.Upland soil and streambank erosion, and sediment deposition are also critical water quantity and quality issues in Illinois.Erosion causes loss of fertile soil, streambank erosion causes loss of valuable lands, and both contribute large quantities of sediment in water flowing through streams and rivers that cause turbidity in sensitive biological resource areas and fill streambeds and banks, lakes, and reservoirs.Lake Decatur, Lake Springfield, and Peoria Lake are a few of the examples.Eroded soil and sediment also carry chemicals that pollute water bodies and stream/reservoir beds.Runoff water from farm fields, collected in creeks or streams through tile drains or small ditches, contains significantly high concentrations of sediment and agricultural chemicals that pollute receiving water bodies during early stages of planting (late spring or early summer).Agricultural chemicals include chemicals applied through fertilizer, herbicides, and pesticides, and chemicals produced naturally most importantly atmospheric deposition, fixation, and mineralization.Understanding and dealing with these complex hydrologic, soil erosion, and sediment and contaminant transport processes and the associated problems have been quite a challenge for scientists and engineers.Mathematical models are becoming invaluable tools to analyze these complex processes and to evaluate land use and best management practices (BMPs) in reducing the damaging effects of flooding, soil erosion, sedimentation, and contamination on drinking water supplies and other valuable water resources.Existing and commonly used models are limited because they are not physically based and cannot simulate the dynamic behaviors of the water and its constituents' movements.In a 1999 report, New Strategies for America's Watersheds, published by the National Research Council, the Committee on Watershed Management analyzed the current status of watershed modeling for decision making.The Committee concluded that the available models and methods are outdated, and "a major modeling effort is needed to develop and implement state-of-the-art models for watershed evaluation."Existing physically based models are computational and data intensive, and too cumbersome for iiiThe study provides a valuable database of continuous rainfall, runoff, sediment, nitrogen, phosphorous, atrazine, and metolachlor in an east-central Illinois watershed collected during storm events.These data help us understand some of the complex physical and chemical processes in the watershed.Complete understanding would require more research and intensive data collection.The study also provides the model as an advanced tool for engineers, scientists, and public policy makers on watershed protection issues involving both the surface and ground waters and to help make environmentally and economically sound watershed management decisions.Due to its extensive water, sediment, and pollutant routing schemes incorporating most of the dynamic behaviors, the DWSM provides a sound base for further development.For Illinois hydrologic conditions, the model must be further developed with tile drain and base flow routines, which will help improve the predictions of recession and base flow portions of the hydrographs, and also sediment discharges.The model also provides a sound base for further development in the simulations of streambank erosion and detailed stream sediment transport, major problems in many Illinois watersheds.Additional model testing is recommended in various watersheds in Illinois with different hydrologic and climatic conditions.