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Spatial patterns of actual evapotranspiration (AE), potential evapotranspiration (PE), and their ratio (AE/PE) in the western United States are examined to describe the annual cycle of moisture availability. Long-term (1971-2000) averages of observed temperature and precipitation, and sunshine data were used to calculate AE and PE. A cluster analysis identifies regions with similar annual cycles of AE/PE. We identify three different spatial scales of variability in AE/PE: (1) broadscale patterns reflect the trade-off between winter-dominated precipitation of the Pacific Northwest and summer-dominated precipitation east of the Rocky Mountains, and the south-to-north decrease in PE related to latitudinal variations in net radiation; (2) mesoscale patterns show the influence of regional-scale features (e. g., the North American Monsoon); and (3) local-scale patterns are related to topography. Understanding the scales of effective-moisture variations is valuable from the perspective of terrestrial ecosystems and water resources management in the mostly arid western United States. Citation: Shinker, J.J., and P.J. Bartlein (2010), Spatial variations of effective moisture in the western United States, Geophys. Res. Lett., 37, L02701, doi:10.1029/2009GL041387.