RELATIONSHIP BETWEEN INCREASED STREAMFLOW AND ROW CROPS IN A SMALL WATERSHED IN SOUTH CENTRAL WISCONSIN

Analysis of average monthly discharge in Turtle Creek, a 520 km² watershed in south central Wisconsin, showed a decrease in low flows (<100 cfs) and a concomitant increase in mid-size flows (100-300 cfs). This marked change in flow occurs about 1970. Monthly precipitation data from a local weather station showed no correlation with stream flow. Likewise, though population increase and associated land-use change likely had some effect on flow, they would be minimal in this largely rural watershed especially given that the largest population center occurs downstream of the USGS gauging station.

The watershed is dominated (77%) by agriculture, most of which is corn and soybeans. National agricultural statistics for Rock and Walworth Counties were used to estimate the acres of corn and soybean planted within the watershed over time; acreage increased from about 20,000 in 1928 to about 30,000 in 1970; in the early 1970s acreage jumped to 40,000 and climbed to as much as 60,000. Increase in baseflow has been tied to increased planting of corn and soybeans in other parts of the Mississippi River basin; some have attributed this relationship to conservation-farming practices and change in water loss due to evapo-transpiration. I used the Indicators of Hydrologic Alteration program, originally developed by the Nature Conservancy, to compare two parts of the daily discharge record: 1940-1971 and 1972-2011. This analysis shows the increase in flow in the latter period and emphasizes the effects on low flows relative to high flows, all of which are predicted if crops are affecting flow.