Expert Report

Colorado River Basin Water Management: Evaluating and Adjusting to Hydroclimatic Variability (2007)

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Recent studies of past climate and streamflow conditions have broadened understanding of long-term water availability in the Colorado River, revealing many periods when streamflow was lower than at any time in the past 100 years of recorded flows. That information, along with two important trends -- a rapid increase in urban populations in the West and significant climate warming in the region -- will require that water managers prepare for possible reductions in water supplies that cannot be fully averted through traditional means. This National Research Council report assesses existing scientific information, including temperature and streamflow records, tree-ring based reconstructions, and climate model projections, and how it relates to Colorado River water supplies and demands, water management, and drought preparedness. The report concludes that successful adjustments to new conditions will entail strong and sustained cooperation among the seven Colorado River basin states. The report recommends conducting a comprehensive basinwide study of urban water practices that can be used to help improve planning for future droughts and water shortages.

Key Messages

  • Based on analysis of many recent climate model simulations, the preponderance of scientific evidence suggests that warmer future temperatures will reduce future Colorado River streamflow and water supplies. Reduced streamflow would also contribute to increasing severity, frequency, and duration of future droughts.
  • Measured values of streamflow of the Colorado River and its tributaries provide essential information for sound water management decisions. Loss of continuity in this gaged record would greatly diminish the overall value of the existing flow data set, and once such data are lost they cannot be regained.
  • Several hydroclimatic studies project that significant decreases in runoff and streamflow will accompany increasing temperatures.Other studies, however, suggest increasing future flows, highlighting the uncertainty attached to future runoff and streamflow projections.
  • Steadily rising population and urban water demands in the Colorado River region will inevitably result in increasingly costly, controversial, and unavoidable trade-off choices to be made by water managers, politicians, and their constituents. These increasing demands are also impeding the region's ability to cope with droughts and water shortages.
  • Technological and conservation options for augmenting or extending water supplies although useful and necessary in the long run will not constitute a panacea for coping with the reality that water supplies in the Colorado River basin are limited and that demand is inexorably rising.
  • The 20th century saw a trend of increasing mean temperatures across the Colorado River basin that has continued into the early 21st century. There is no evidence that this warming trend will dissipate in the coming decades; many different climate model projections point to a warmer future for the Colorado River region.
  • The interstate cooperation and initiative exhibited by the Colorado River basin states in their February 2006 letter to the Secretary of the Interior is a welcome development that will prove increasingly valuable and likely essential in coping with future droughts and growing water demands.
  • These reconstructions, along with temperature trends and projections for the region, suggest that future droughts will recur and that they may exceed the severity of droughts of historical experience, such as the drought of the late 1990s and early 2000s.
  • Tree-ring-based reconstructions of Colorado River flow indicate that extended drought episodes are a recurrent and integral feature of the basin's climate.
  • covering hundreds of years, have transformed the paradigm governing understanding of the river�s long-term behavior and mean flows. These studies affirm year-to-year variations in the gaged records.