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Quantifying the interconnected impacts of climate change and irrigation on surface water flows is critical for the proactive management of our water resources and the ecosystem services they provide. Changes in streamflow across the Western U.S. have generally been attributed to an aridifying climate, but in many basins flows can also be highly impacted by irrigation. We developed a 35-year dataset consisting of streamflow, climate, irrigated area, and crop water use to quantify the effects of both climate change and irrigation water use on streamflow across 221 basins in the Colorado, Columbia, and Missouri River systems. We demonstrate that flows have been altered beyond observed climate-related changes and that many of these changes are attributable to irrigation. Further, our results indicate that increases in irrigation water use have occurred over much of the study area, a finding that contradicts government-reported irrigation statistics. Increases in crop consumption have enhanced fall and winter flows in some portions of the Upper Missouri and northern Columbia River basins, and have exacerbated climate change-induced flow declines in parts of the Colorado basin. We classify each basin’s water resources sustainability in terms of flow and irrigation trends and link irrigation-induced flow changes to irrigation infrastructure modernization and differences in basin physiographic setting. These results provide a basis for determining where modern irrigation systems benefit basin water supply, and where less efficient systems contribute to return flows and relieve ecological stress.

The Western United States (U.S.) relies heavily on scarce water resources for both ecological services and irrigation. However, the response of irrigation water use during drought is not well documented. Irrigation decision‐making is complex and influenced by human and environmental factors such as water deliveries, crop yields, equipment, labor, crop prices, and climate variability. While few irrigation districts have plans to curtail water deliveries during droughts, water rights, fallowing patterns, crop rotations, and profit expectations also influence irrigation management at the farm scale. This study uses high‐resolution satellite data to examine the response of irrigators to drought by using a novel measure of irrigation management, the Standardized Irrigation Management Index. We assess the state of drought at the field and basin scales in terms of climate and streamflow and analyze the importance of variations in crop price and drought status on decision‐making and water use. We show significant variability in field‐scale response to drought and that crop type, irrigation type, and federal management explain regional and field‐scale differences. The relative influence of climate and prices on crop transitions indicate prices more strongly drive crop planting decisions. The study provides insights into irrigation management during drought, which is crucial for sustainable water supply in the face of the ongoing water supply crisis in the U.S. Southwest.

Catchment hydrometeorology and the organization of shallow subsurface flow are key drivers of active contributing areas and streamflow generation. However, understanding how the climatic water balance and complex topography contribute to these processes from hillslope to catchment scales remains difficult. We compared time series of vapor pressure deficits and soil moisture to the climatic water balance and topographic variables across six zero‐order catchments in the Lubrecht Experimental Forest (Montana, USA). We then evaluated how local hydrometeorology (volumetric water content and atmospheric vapor pressure deficit) affected the spatial occurrence of shallow subsurface flow. Generalized linear mixed model analysis revealed significant, temporally stable (monthly and seasonal average) patterns of hydrometeorology that can be predicted by the topographic wetness index and the dynamic climatic water deficit (CWD = potential evapotranspiration − actual evapotranspiration). Intracatchment patterns were significantly correlated to the topographic wetness index, while intercatchment patterns were correlated to spatiotemporal variance in the CWD during each time period. Spatial patterns of shallow subsurface flow were related to the hydrometeorological conditions of the site. We observed persistent shallow subsurface flow in convergent hillslope positions, except when a catchment was positioned in locations with high CWDs (low elevations and southerly aspects). Alternatively, we observed persistent subsurface flow across all hillslope positions (even 70‐m upslope from the hollow) when catchments were positioned in locations with especially low CWDs (northerly aspects and high elevations). These results highlight the importance of considering the superposition of the catchment‐scale climatic water balance and hillslope‐scale topography when characterizing hydrometeorology and shallow subsurface flow dynamics.