375 Borlaug Hall
River flows have increased in the Midwest United States since the mid-1970s. In scientific studies and in popular media, this increase has been blamed on the adoption of row crops especially soybeans and on drain tiles in agricultural landscapes. Most of these studies have ignored the effects of increased precipitation and made unrealistic assumptions that have led to the above conclusions. In a series of studies by the author and his students, we have shown that evapotranspiration has not changed drastically from earlier times in the Midwest and tile drainage cannot increase river flow because water does not drip into air (tile) from porous media (the soil) unless there is an excess supply of water. Most agricultural watersheds in the Midwest are rain fed and thus the supply of water is precipitation. Some people have also compared a tile-drained watershed (the Red River, ND) with a non-tile drained watershed (the Chippewa River, WI) and have attributed the differences in river flows as an effect of tile drainage. Since river flow is a net effect of precipitation, evapotranspiration and change in watershed stored water, river flow increase is only possible if there is an increase in precipitation, a decrease in evapotranspiration, less final or more initial stored water in the watershed. In this talk, I will present river flow-precipitation analysis of the Red River and the Chippewa River watersheds and show that the increased river flow in these watersheds is mainly due to increased precipitation. Furthermore, the temporal variation in river flows is mainly due to more or less precipitation in previous years. Previous dry years (less soil-stored water) buffer to reduce river flows whereas previous wet years (more soil-stored water) prime to increase river flows. The behavior of the Chippewa River flow was atypical relative to the Red River or other Midwestern Rivers. This atypical behavior was not due to an absence of tile drainage as suggested in the literature but due to the presence of substantial lake storage along with several dams/reservoirs that regulated its flow. The above findings suggest that a recent push to store water in uplands of the Minnesota River Basin will not only increase seepage caused bank sloughing but will also result in increased nitrogen loss.