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Subsurface moisture was simulated across two important river basins in the arid southwestern United States with the goal of assessing the importance of antecedent subsurface moisture to variability in seasonal streamflow. A standardized moisture index and other modeled hydrologic data were used in regression analyses to assess the relationship between antecedent subsurface moisture and streamflow. The relationship was found to be strongest during late spring and early summer, or the annual period of greatest runoff. During the warm runoff season of mid-summer through early fall it is the relationship between streamflow and monthly basin net water (the total of rain and snowmelt minus potential evaporation summed for the whole basin) that is strongest. The addition of subsurface moisture indices as independent regression variables increased the explained variance in streamflow by 30% for the more productive basin and by 14% for the other basin. Incorporation of the subsurface moisture indices in streamflow forecasts at lead times of two to five months increased the explained variance by 3% (5-month lead) to 19% (2-month lead). (C) 2009 Elsevier Ltd. All rights reserved.