Roles of climate and agricultural practices in discharge changes in an agricultural watershed in Iowa

River discharge represents a vital resource for many human activities. The improved understanding of the physical processes controlling its regime can lead to large economic and societal benefits, such as improved flood warning and mitigation, and improved water management during droughts. This is particularly true for the agricultural U.S. Midwest and Iowa more specifically. Iowa is relentlessly plagued by catastrophic flooding, with the spring and summer river floods of 1993, 2008, and 2013 and the drought of 2012 being the most recent widespread events affecting the state. These natural disasters also come with a very large price tag, both in terms of economic damage and fatalities. During the 20th and 21st centuries, discharge over this area has been changing on a number of temporal scales, from annual to decadal. An outstanding question is related to the contribution of changes in the climate system and in land use/land cover and agricultural practices in explaining changes in discharge. We address this question by developing statistical models to describe the changes in different parts of the discharge distribution. We use rainfall and harvested corn and soybean acreage to explain the observed stream flow variability. We focus on the Raccoon River at Van Meter, which is a 9000-km2 watershed with daily discharge measurements covering most of the 20th century up to the present. Our results indicate that rainfall variability is responsible for the majority of the changes observed in the discharge record, with changes in cultivated area affecting the discharge responses in different ways, depending on which part of the discharge distribution is considered. In particular, land use change exacerbates high discharge during heavy precipitation and low discharge during low precipitation.