Water, Sediment, Nutrient, and Pesticide Measurements in an Agricultural Watershed in Illinois During Storm Events

Flow and concentrations of suspended sediment, nitrate-N, phosphate-P, atrazine, and metolachlor were monitored during the spring seasons of 1998 and 1999, primarily during storm events, at a tributary station (Big Ditch) and two main-stem stations (Fisher and Mahomet) of the Upper Sangamon River watershed in east central Illinois. These three stations respectively drain 98, 622, and 932 km 2 of mainly agricultural lands. Rainfall data were collected from six newly established raingage stations. The study provides a valuable database to understand some of the complex physical and chemical processes in an east central Illinois watershed and to calibrate, validate, and evaluate mathematical models. Analyses of the limited data from different sampling methods showed consistencies among the methods. The nitrate-N concentrations in Big Ditch during intense storms of 1998 showed inverse relationships with water discharge. Higher goodness-of-fit of the relationships was found within the data for individual months than for the combined set, showing dependence on varying climate, land cover, and management practices during the growing season. The nitrate-N concentrations at each of the three stations during less intense storms in 1999 showed weak positive relationships with water discharge. The total nitrate-N load in Big Ditch generated by the storms of March, May, and June of 1998 was similar to the average annual load, indicating the importance of single-event storms, and therefore their need for close attention. After the intense storm of April 1999, the less intense storms during May and June after long low-flow periods significantly raised the nitrate-N concentrations in all three stations. A similar pattern was noticed in the 1998 observations before each of the major peaks, when the smaller peaks elevated nitrate-N concentrations followed by dilution with the larger peaks. Results of this study show positive relationships between suspended sediment concentration and water discharge for the smaller watershed (Big Ditch); however, no relationships were found in the larger watersheds (Upper Sangamon River at Fisher and Mahomet). Observed data at each of the stations showed positive relationships of phosphate-P, atrazine, and metolachlor concentrations with water discharge.