DYNAMIC MODELING AND MONITORING OF WATER, SEDIMENT, 
NUTRIENTS, AND PESTICIDES IN AGRICULTURAL WATERSHEDS 
DURING SEVERE STORM EVENTS

Deva Borah, Maitreyee Bera, Susan Shaw, and Laura Keefer

Illinois State Water Survey
 

Abstract

A dynamic watershed simulation model (DWSM) was developed at the Illinois State Water Survey (ISWS) using physically based governing equations to simulate rainfall-runoff, propagation of flood waves, soil erosion, entrainment and transport of eroded soil particles (sediment), and commonly used agricultural chemicals for agricultural and rural watersheds. During the early period of the study (September 1997-March 1998), the hydrology component of the model was developed and tested. It was tested on the 925-square-mile Upper Sangamon River basin, which drains into Lake Decatur in central Illinois, using data collected earlier by the ISWS. Monitoring was started at the Big Ditch station, draining a 38-square-mile subwatershed of the Upper Sangamon River or Lake Decatur watershed. Study objectives, literature review, hydrologic model formulations, initial monitoring of the Big Ditch station, and model results were reported in the 1998 conference proceedings of the Illinois Groundwater Consortium (IGC) (Borah et al., 1998). 

During the middle period of the study (April 1998-March 1999), detailed data on rainfall, flow, and concentrations of sediment, nitrate-nitrogen, phosphate-phosphorus, atrazine, and metolachlor were collected at the Big Ditch station. The data were analyzed to develop relationships of pollutant concentrations with water discharge. Two different hydrologic algorithms of the DWSM were tested on the Big Ditch subwatershed using the monitored data. The DWSM was expanded to simulate soil erosion and sediment transport in a watershed, and tested on the Big Ditch subwatershed using the monitored data. The monitored data and their analyses, soil erosion and sediment transport model formulations, and the model results were reported in the 1999 conference proceedings of the IGC (Borah et al., 1999b). 

During the final period of the study (April 1999-September 1999), the same types of data, except metolachlor, were collected at the Big Ditch station and at two other stations on the main stem of Sangamon River--Fisher and Mahomet, draining respectively 240 and 360 square miles of the Upper Lake Decatur watershed. Rainfall data were collected from five newly established rain gages throughout the Upper Lake Decatur watershed above Mahomet, in addition to the one near the Big Ditch station established in 1998. The hydrologic, soil erosion, and sediment transport model developed and reported earlier was verified on the Big Ditch subwatershed for the April 15-16, 1999, storm, the only major storm that occurred during spring 1999. In this paper, the monitored data and model results from the final study period are presented and discussed. The study provides an extensive database, new relations of constituent concentrations with water discharge, and the DWSM, an advanced tool for engineers, scientists, and public policy makers working on land use and groundwater protection issues in floodplain areas. Dynamic modeling of nutrients and pesticides is currently in progress in a follow-up study as ongoing research at the ISWS.