Climate and Land Use Effects On River Discharge and Base Flow in Tile Drained Watersheds.

Land use changes such as tile drainage, cultivation, and cropping practices have been blamed for recent increases in river flows in the Midwestern United States. This study analyzed river flow data from 42 HUC 8 watersheds in Minnesota and Iowa to decipher climate and land use effects on river discharge and base flow. Base flow was quantified from daily river discharge values using the USGS PART program. The procedure involved comparison of regression relationships between annual river discharge or annual base flow vs. annual precipitation for two periods; prior to 1975 and after 1976. Mid 1970s was used as the break point because plastic tile line started to have wide spread adoption for agricultural drainage during this period. Underlying premise of the regression comparisons was that land use changes, especially tile drainage, will significantly shift the relationship of flow vs. precipitation in the upward direction. Results show that the relationship between annual discharge or annual base flow vs. precipitation for several heavily tile drained watersheds were statistically same for the periods prior to 1975 and after 1976 thus suggesting precipitation is the major driver of annual flows. For some watersheds, the relationship between flow and precipitation was statistically different; however, the upward shift was relatively small compared to the upward shift from recent wet climate. Analysis of the Raccoon River in Iowa showed that evapotranspiration has remained the same since 1916 and recent higher flows are due to higher precipitation and not due to the shift in area from small grains to row crops.  In this presentation, we show that past studies have wrongly characterized climate effects as land use impacts by overemphasizing statistics and by ignoring the physics of infiltration and runoff.