| Summary: | Water management strategies need to balance water security and food production, particularly in semi-arid regions wherein irrigation is required to supplement rainfall. Irrigated stream−aquifer systems present a unique challenge in this effort, due to complex groundwater−surface water interactions and the high level of human intervention in managing irrigation practices. This paper has two objectives: first, to detail a method for constructing and applying a coupled SWAT-MODFLOW to irrigated stream−aquifer systems; and second, to use the model to quantify the effects of decreasing irrigation on hydrological responses and crop yield. The method is applied to a 734 km<sup>2</sup> study region in the Lower Arkansas River Valley, an alluvial valley in Colorado, USA, which has been intensively irrigated for over 100 years and is threatened by shallow water tables. Therefore, a reduction in applied irrigation amounts has the double benefit of conserving water and decreasing waterlogging, given that crop yield can be maintained for food production. The results indicate that an approximate 10% decrease in total applied irrigation water results in decreases of 6% in surface runoff, 8% in evapotranspiration, and 4% in recharge water. It also results in an increase of 4% in groundwater return flow to the Arkansas River, and an actual increase in groundwater levels due to the decrease in groundwater pumping, pointing to the need for targeted irrigation reduction strategies to decrease waterlogging occurrence. The irrigation reduction yields an average 9% decrease in corn and alfalfa yield. This modeling approach is in general transferable to other similar irrigated river valleys.
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