What is the evidence for declining mountain snowpack in western North America? Is this decline the result of natural processes, the buildup of greenhouse gases in the atmosphere, or both? What is the scale of this decline, and what are the implications for water resources now and in the future? What is the present state of water resources in the major western U.S. cities relative to their rates of growth? How resilient are these major western U.S. cities to changes in the amount, timing and rate of delivery of water to these cities?
Introduction: Dr. Antonio J. Busalacchi, Jr., Chair of the Climate Research Committee of the National Academy of Sciences, Director of the Earth System Science Interdisciplinary Center (ESSIC) and Professor of Meteorology at the University of Maryland, College Park, MD
Dr. Phillip Mote, Climate Impacts Group, University of Washington, Seattle, WA
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Dr. Soroosh Sorooshian, Professor of Engineering, University of California, Irvine, CA
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Declining Mountain Snowpack and its Effects on Streamflow and Water Resources in Western North America:
Warming trends in the western U.S. have already produced significant changes in snow-driven hydrology. In the past 50 years, dates of peak snow accumulation and of peak snowmelt runoff have shifted earlier, typically by 10-40 days, and spring snowpack has decreased in most of the West (by about 35% in the Cascade Mountain Range). More precipitation is falling as rain rather than snow. Correspondingly, river flows have declined in late spring and summer and have increased in late winter.
Using several independent approaches, scientists have separated the roles of changing temperature and precipitation in these trends, and have demonstrated that the observed changes are most likely due to a climate warming attributable to the buildup of greenhouse gases in the atmosphere. Although a causal connection between the observed changes and rising concentrations of greenhouse gases cannot yet be established with absolute certainty, it is unlikely that the declines are solely part of a natural cycle.
Continued warming is considered very likely in this region as greenhouse gases continue to accumulate in the atmosphere. The net effect of this continued warming is expected to accentuate these shifts. The reductions in summer flow are particularly problematic in parts of the West (i.e., California and the Pacific Northwest) where reservoir storage is only a few months' worth of flow. Should these trends continue as expected, it will become increasingly difficult to satisfy demands for irrigation water, summer hydropower, and in-stream flow requirements for fish.
The Challenge of Managing Scarce Water Resources in the Western U.S. in the Face of a Changing Climate:
The Western United States is dominated primarily by Arid and Semi-Arid climate conditions. Availability of adequate water supply to support the agricultural (by far the largest user), domestic, industrial, recreational and various environmental needs of the region are crucial and a matter of continuous discussion in the social, political and economic sectors. Population growth in the Southwest continues to outstrip the rest of the country, with Nevada and Arizona leading the way. Over the past 35 years, the U.S. has averaged about 1% annual population growth; Albuquerque 2%; Tucson 3%; Phoenix 4%; Las Vegas 5.5% and LA 11%. Over the same time period, shrinking household sizes have also led to more homes being built, with more landscapes and pools per capita, increasing the need for reliable water supply to meet demand.
Major urban areas tend to have access to two or more water supplies. Albuquerque has local Rio Grande water and vast aquifers; Tucson has imported Colorado River water, vast aquifers, and an effluent distribution system; Phoenix has imported Colorado River water, water from the Salt/Verde reservoir system, and groundwater. And California has what the New York Times once described as a “water system [that] might have been invented by a Soviet bureaucrat on an LSD trip.” California is so thoroughly plumbed and inter-connected that water can be traded, borrowed, leased and sold across much of the state.
The major river basins of the Southwest face numerous water challenges - surging populations, historical Native American water rights claims, endangered species habitat needs, growing demand for water-based recreation, water-intensive industrial processes, climate change and recently-discovered wide-spread pollutants. On top of this, much of the region has experienced the shock of a severe and sustained drought, with some relief only this past winter. Larger river basins are inherently more resilient to drought than smaller ones. As an example, the major reservoirs on the Colorado River can hold 60 million acre-feet of water, or four years’ average flow. Even after 6 years of deep drought, the system is at about 50% of capacity.
And while these water systems have not yet collapsed, taps have not gone dry, nor has there been a great deal of conflict between water users, the systems are nonetheless, stressed. Regional aquifer reservoirs have provided much needed resiliency but at a premium since these reservoirs took centuries and millennia to fill and have been drawn down significantly in less than a century. In short, these aquifers are valuable but precarious resources due to over-pumping.
The challenge of managing western water resources will be far greater if the uncertainties associated with climatic change and variability are seriously factored in. It is worth noting that of the total amount of annual rainfall in the Southwest, over 90% (as high as 98% for most of the region) of it evaporates back to the atmosphere. This leaves a very small fraction which goes to recharge the aquifers and fill the surface reservoirs. Even though the recent drought in the region has been labeled as extreme, it is relatively minor in the context of long term historical records. As for long term future climate scenarios, climate models differ in their regional predictions. Some models predict a wetter future while others predict dryer conditions for the region. It is therefore imperative that water management in the west be viewed in a multi-criteria framework. A long term strategy with only population growth scenarios will not be sufficient. To ensure resiliency, it is critical to also consider the uncertainties associated with climate variability and change. Water managers will need new scientific understandings and near-real time hydrologic data provided by the development and application of new technologies.
Dr. Philip Mote is a research scientist with the Climate Impacts Group at the University of Washington, is State Climatologist for the State of Washington, and is an Affiliate Professor of atmospheric sciences at the University of Washington in Seattle. He also conducts research describing variability and change of climate in the Pacific Northwest and its connections to natural resources, particularly snow, streamflow, and forest fires; and on the lower and middle parts of the tropical atmosphere. Dr. Mote has published over 50 peer-reviewed research articles in various scientific journals and has also served as editor on a recent book on climate modeling. Dr. Mote has also provided testimony on climate change before the Senate Commerce Committee.
Dr. Mote Served as a Research Fellow at the Department of Meteorology, University of Edinburgh, Edinburgh, Scotland (1994-1996); was a research scientist with NorthWest Research Associates in Bellevue, WA (1996-1998); and served as Co-Director of the NATO Advanced Study Institute on Numerical Modeling of the Global Atmosphere in Italy (1998), and as Director of an international workshop on regional integrated assessments in Italy (2002). He holds a Bachelor of Arts degree in physics from Harvard University (1987), and a Ph.D. in Atmospheric Sciences from the University of Washington (1994), Seattle, WA.
Dr. Soroosh Sorooshian is the UCI Distinguished Professor of Civil and Environmental Engineering, and Earth System Science and Director of the Center for Hydrometeorology and Remote Sensing (CHRS), at the Henry Samueli School of Engineering, University of California-Irvine. His major interests are hydrometeorology and hydroclimate modeling, remote-sensing applications, and water resources management in semi-arid regions. Prior to Arriving at UC-Irvine, Dr. Sorooshian was in the Department of Hydrology and Water Resources at the University of Arizona in Tucson, serving as Department Head from 1989-1996, and Regents Professor.
Among a host of accomplishments too long to list here, Dr. Sorooshian is a member of the National Academy of Engineering (NAE); Corresponding Member, International Academy of Astronautics (IAA); Fellow, American Association for the Advancement of Science (AAAS); Fellow, American Geophysical Union (AGU); Fellow, American Meteorological Society (AMS); and Fellow, International Water Resources Association (IWRA). He is author or co-author of over 100 peer-reviewed publications in professional scientific journals, and author of two scientific books. Dr. Sorooshian has also testified before various U.S. Senate and House Committees, and is this year’s recipient of NASA’s Distinguished Public Service Award.
Dr. Sorooshian received a B.S. degree in Mechanical Engineering, California Polytechnic State University (1971); an M.S. degree in Operations Research, UCLA (1973); and a Ph.D. in Systems Engineering (Water Resources and Hydrologic Systems Analysis), UCLA (1978).ward.
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