Application of the Universal Soil Loss Equation to the Big Eau Pleine Watershed

Abstract

The Big Eau Pleine Reservoir in central Wisconsin is subject to rather extensive fish kills during the winter months. In 1974 this occurence sparked sportsmen and lakeshore property owners to request studies of the sources of pollution. Agricultural land use and management was one possible problem area. Soil, nutrients, and organic matter carried downstream and deposited in the reservoir decompose over time and contribute to oxygen depletion. The Universal Soil Loss Equation was applied to 2.8 percent of the watershed area to estimate the extent of excessive soil erosion. Fifteen to twenty-five percent of the area in the basin exceeded the allowable soil loss tolerance limits. Most of this erosion occurred on cropland that was subjected to excessive row cropping, up and down slope farming, and removal of surface residues. Reduction of cropland erosion could be accomplished by application of contour strips, grassed waterways, diversions, and terraces. Modifying tillage methods and crop rotations can also reduce erosion. A cost estimate for implementing such practices on the Big Eau Pleine watershed ranged from $2.3 to $8.6 million dollars. The lower figure would bring erosion below soil loss tolerance levels using Soil Conservation Service (SCS) parameters. The higher value would reduce erosion to levels that may comply with water quality standards. The Universal Soil Loss Equation has been used mainly to compute soil erosion on individual fields for SCS farm conservation plans. The present study is one of the first to utilize the equation in evaluating an entire watershed for its effect on downstream water quality. In the future such application may serve the dual purpose of planning farming techniques that reduce soil erosion to acceptable water quality limits and justifying enforcement of those limits.