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Recently the Southwest has experienced a spate of dryness, which presents a challenge to the sustainability of current water use by human and natural systems in the region. In the Colorado River Basin, the early 21st century drought has been the most extreme in over a century of Colorado River flows, and might occur in any given century with probability of only 60%. However, hydrological model runs from downscaled Intergovernmental Panel on Climate Change Fourth Assessment climate change simulations suggest that the region is likely to become drier and experience more severe droughts than this. In the latter half of the 21st century the models produced considerably greater drought activity, particularly in the Colorado River Basin, as judged from soil moisture anomalies and other hydrological measures. As in the historical record, most of the simulated extreme droughts build up and persist over many years. Durations of depleted soil moisture over the historical record ranged from 4 to 10 years, but in the 21st century simulations, some of the dry events persisted for 12 years or more. Summers during the observed early 21st century drought were remarkably warm, a feature also evident in many simulated droughts of the 21st century. These severe future droughts are aggravated by enhanced, globally warmed temperatures that reduce spring snowpack and late spring and summer soil moisture. As the climate continues to warm and soil moisture deficits accumulate beyond historical levels, the model simulations suggest that sustaining water supplies in parts of the Southwest will be a challenge.climate change | regional modeling | sustainability | water resources P ersistent dry conditions have generally prevailed in the Southwest during the early years of the 21st century (1), after wetter than normal conditions in the preceding years. Such droughts have substantial impacts on the humans, animals, and plants inhabiting the Southwest, and call into question whether we can sustain the water resources that we have come to depend upon in the 20th century. This study uses high resolution (1∕8°× 1∕8°) hydrological model simulations, driven by observed and downscaled global climate model meteorological fields, to investigate the region's droughts. Our goals are to place the 21st century drought into the context of the 20th century, and determine how Southwest drought is likely to change from its 20th century patterns in the future.The Southwest's hydrology is marked by strong variability on seasonal to multiannual time scales, reflecting its sensitivity to fluctuations in large scale atmospheric circulation patterns. Preinstrumental paleoclimate records indicate that periods of extreme dryness have occurred sporadically during the last millennium (2, 3), so the 21st century drought is far from unprecedented. Some of the most prominent of these prehistoric droughts occurred in the midst of anomalously warm conditions, perhaps in similar fashion to the recent early 21st century drought. A protracted period of such dry conditions is...

Sixteen global general circulation models were used to develop probabilistic projections of temperature (T) and precipitation (P) changes over California by the 2060s. The global models were downscaled with two statistical techniques and three nested dynamical regional climate models, although not all global models were downscaled with all techniques. Both monthly and daily timescale changes in T and P are addressed, the latter being important for a range of applications in energy use, water management, and agriculture. The T changes tend to agree more across downscaling techniques than the P changes. Year-to-year natural internal climate variability is roughly of similar magnitude to the projected T changes. In the monthly average, July temperatures shift enough that that the hottest July found in any simulation over the historical period becomes a modestly cool July in the future period. Januarys as cold as any found in the historical period are still found in the 2060s, but the median and maximum monthly average temperatures increase notably. Annual and seasonal P changes are small compared to interannual or intermodel variability. However, the annual change is composed of seasonally varying changes that are themselves much larger, but tend to cancel in the annual mean. Winters show modestly wetter conditions in the North of the state, while spring and autumn show less precipitation. The dynamical downscaling techniques project increasing precipitation in the Southeastern part of the state, which is influenced by the North American monsoon, a feature that is not captured by the statistical downscaling.

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