Hydrologic Conditions and Streamflow Change in an Evolving Semi-Arid Agricultural Watershed, Smith River, Montana
Agricultural land and water use has modified natural flow regimes in the western US. Understanding the effect of agricultural water use on streamflow is critical to effective water management and is often limited by the available data record. This investigation evaluates agricultural water use and streamflow in the Smith River watershed (SRW), a semi-arid agricultural watershed located on the eastern slope of the northern Rocky Mountains in west-central Montana. Questions motivating this study include: (1) Has agricultural water use modified streamflow on the Smith River? (2) What aspect of agricultural activity in the Smith River watershed is having the greatest effect on streamflow? (3) What hydrologic properties of the watershed determine streamflow sensitivity to agricultural land and water use? Three approaches are employed to address these questions: (1) construction of an annual water budget, (2) comparison of streamflow to current and historic agricultural water use intensity, and (3) simulation of watershed processes using a precipitation-runoff model. Based on the mean-annual water budget estimate of the SRW, 12 percent of mean-annual streamflow in the Smith River is consumed by irrigation water use. The hydrologic effects of increased irrigated area detected with linear regression analyses of streamflow and agricultural water use include decreased seasonal flows in April, May, and June; increased late-summer flows in August and September; and increased low-flow volumes. Numerical hydrologic model simulations demonstrate that dominant precipitation-runoff processes in the watershed cannot be adequately represented without including agricultural water use.
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