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We developed a water budget runoff model for the Salt and Verde River basins of central Arizona and used the outputs of 6 global climate models (GCMs) to estimate runoff in the future under assorted emissions scenarios developed by the Intergovernmental Panel on Climate Change (IPCC). We used a statistical downscaling routine to refine the GCM outputs for the 2 basins, and we found that all model-scenario combinations simulate a mean temperature rise in the study area of between 2.4 and 5.6°C, using year 2050 greenhouse gas concentrations. Mean changes in precipitation vary substantially among the models and scenarios, and, as a result, changes in runoff vary from 50 to 127% of historical levels. Assuming equal probabilities associated with each scenario and model run, the overall results suggest that runoff from the Salt and Verde will have an approximately 85% chance of being less strong, the certainty of which is related to consensus on warming in the study area. The large variability among predictions of precipitation trends introduces substantial uncertainty. KEY WORDS: Dryland runoff · Climate change · Water budgetResale or republication not permitted without written consent of the publisher Clim Res 35: 227-239, 2008 change are fully known. And third, if future climate conditions differ substantially from historical conditions, then the political institutions that have allocated water so effectively in the past will need to be reconsidered and revised. Real reductions in water supply, or even the possibility of reductions, will force decision makers to confront previously taboo subjects such as the efficacy of decades-old water pacts and regulations, pricing schemes that rise rapidly after basic needs are met, restrictions on landscaping treatments, and density and growth management.During the last 25 yr there have been at least 6 major studies of the potential impacts of future climate change on runoff within the Colorado River Basin, and all suggest a trend to less runoff in the future. Revelle & Waggoner (1983) used the simple approach of applying temperature and precipitation scenarios for the Upper Basin to a statistical regression equation relating the variables to runoff. Nash & Gleick (1991, 1993 used temperature and precipitation scenarios and more sophisticated GCM output under doubled atmospheric concentrations of CO 2 to run a National Weather Service hydrology model for the Upper Basin. Christensen et al. (2004) used National Center for Atmospheric Research (NCAR) GCM output from a 'business as usual' (BAU) gas emissions scenario as input to the variable infiltration capacity (VIC) hydrological model across the entire Colorado River Basin. Hoerling & Eisched (2007) applied multiple GCM outputs under BAU emissions to the calculation of the Palmer drought severity index (PDSI), which was related to runoff in the upper Colorado Basin using statistical regression. Most recently, Christensen & Lettenmaier (2006) used multiple GCM outputs for 2 emissions scenarios to alter temperatur...

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The southwestern USA is subject annually to an inflow of atmospheric moisture in association with the North American monsoon, which is a summertime shift in the atmospheric circulation stretching from the Caribbean Sea, to Mexico, and into the southwestern USA. There are currently no regional criteria used to define the temporal aspect of the annual monsoon season in the southwestern USA, and only a single local definition. A regionalized definition of the annual timing of the monsoon season would seem to be a greater representation of the synoptic-scale effects of the monsoon rather than individual definitions at point locations across the region. The research presented here outlines a method for defining the annual onset and demise of the regional monsoon season of the southwestern USA, and assesses the results through comparisons with a method currently used at one location within the region and with atmospheric composites from historic data.Using hourly dew-point temperature data at five surface stations and daily precipitation data for 193 stations across the southwestern USA for June through to October for a 52 year period, daily humidity and precipitation threshold values indicative of monsoon moisture are established. The first and last occurrences of both humidity and precipitation sustained for a synoptic period of 3 days mark the annual onset and demise of the monsoon season, whereas all days in between that meet the humidity and precipitation thresholds are categorized as 'monsoon days'. Results show good agreement with sample historic data for the sole local definition within the region, and atmospheric composites indicate a likelihood of accurate representation of monsoon onset and demise across the region. The significance of accurate onset and demise dates is apparent in the intimate relationship between seasonal precipitation across the region and the length of the monsoon season. Based upon the procedure, a historic monsoon season database was constructed for the southwestern USA for use in climatological, meteorological, and case-study analyses.

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