Low-flow frequency exacerbation by irrigation withdrawals in the agricultural midwest under various climate change scenarios

As a possible outcome of global climate change, reductions in precipitation, while directly decreasing surface water streamflows and reserves, can also initiate or exacerbate the surface water demand by regional agriculture. Driven by the desire to maintain consistent yields and,hedge the risk of drought, farmers may implement or increase supplemental irrigation applications, obtaining water from either surface sources or aquifers. This paper presents results of an investigation of the impact of these effects on streamflows in the midwestern United States. The investigation examines the direct effect of precipitation reduction and the indirect effects of riparian and nonriparian irrigation. The crop-growth and farm-scale hydrologic model erosion-productivity impact calculator is used in conjunction with the routing features of the basin-scale hydrologic model soil water analysis tool to simulate direct and indirect effects of global change on streamflows. The analysis shows that the coupled effects of decreases in runoff and increases in irrigation may be devastating to the streams of Illinois and other midwestern states. For the example basin a 25% decrease in mean precipitation under optimal irrigation practices by both riparian and nonriparian farmers yields an increase in the annual (mean) occurrence of the historical (7)Q(10) from 3 days/year to 13 days/year, despite the streamflow accretion from groundwater-supplied irrigators.