CAN A SCALING RELATIONSHIP BETWEEN PEAK DISCHARGE AND DRAINAGE AREA BE USED TO IDENTIFY TILE DRAINAGE INPUTS INTO AN AGRICULTURAL STREAM?

Though tile drains have been shown to contribute to high levels of nitrate in agricultural streams, the locations of tile drains on a watershed scale are unknown due to tile drains being located on many separate parcels of private property. This study evaluates the capability of a methodology for locating areas of large tile drainage contribution to agricultural streams and examines if there is a difference in the scaling relationship between tile-flow and non-tile flow conditions. A scaling relationship between peak discharge and drainage area will be created and locations that plot above the scaling relationship have a higher peak discharge relative to drainage area and represent a transition break. A tributary entering a main stream channel causes a transition break, so GIS will be used to identify whether this unknown flux is a tributary entering the main channel or if an area of large tile drainage contribution to the stream is replacing the function of a tributary. Stage data were taken every fifteen minutes and discharge measurements were taken twice a week at eight sites, ranging 6.4 km²-77.2 km² in drainage area, along Money Creek near Bloomington, Illinois. Money Creek is the tributary of Lake Bloomington, which serves as the drinking water source for Bloomington, Illinois. From this data, rating curves will be used to compute peak discharge for four storm events at each site. Preliminary results from data taken on July 1^st, 2016 during base-flow conditions show that there is a tight relationship between drainage area and discharge (R²= 0.9875). I expect outliers that plot above the tile-flow scaling relationship to disappear during non-tile-flow conditions. Outliers can be identified in several different ways; these include using confidence intervals for the regression, looking at residuals, or using percent differences. If this method is effective at Money Creek, it could be applied to tile drained areas throughout the Midwest to locate target zones for best management practices in nutrient reduction.