Water demand and management impacts on streamflow under the prior appropriation doctrine in the bighorn basin

Wyoming and Montana (United States) Waters within the Bighorn Basin are over-appropriated and governed by the prior appropriation doctrine, constrained further by the Yellowstone River Compact. This regulatory setting provides valuable context for understanding how water allocation under prior appropriation influence basin streamflow and water availability. Using a hydrologic-allocation modeling framework, we evaluate how the spatial and temporal distribution of water rights and associated management decisions affect streamflow. This study demonstrates how water right network structure, relative to available supply under the prior appropriation doctrine, contributes to streamflow changes. The spatial structure of appropriation and seniority strongly shapes streamflow, particularly at mid-elevation zones (1200–1800 m), where upstream junior curtailment in support of downstream senior diversion demands bolster streamflow. In contrast, return flow is the primary contributor to instream flow at low-elevations. Trans-watershed diversions further compound these effects by depleting streamflow in high-elevation catchments and redistributing water to mid-elevation catchments. Notably, improving irrigation efficiencies reduced streamflow through increased junior water right holder access resulting in increased evapotranspiration. Results demonstrate how water right network structure, appropriation, and priority date affect both streamflow and management decision outcomes. • Improving irrigation efficiency in the over-allocated basin reduced streamflow. • Downstream water right priority drives upper basin in-stream flows. • Return flows bolster lower basin instream flows. • The value of water has restrained development to a limited area within the basin. • The Shoshone River is key in determining Bighorn Basin outlet streamflow.