NATIONAL RESOURCES DEFENSE COUNCIL
Climate change will have a significant impact on the sustainability of water supplies in the coming decades. A new analysis, performed by consulting firm Tetra Tech for the Natural Resources Defense Council (NRDC), examined the effects of global warming on water supply and demand in the contiguous United States. The study found that more than 1,100 counties— one-third of all counties in the lower 48—will face higher risks of water shortages by mid-century as the result of global warming. More than 400 of these counties will face extremely high risks of water shortages.
The analysis by Tetra Tech—a highly-respected consulting firm used by the federal government, electric utility and other industries, finds that some states have an extreme or high risk to water sustainability, or are likely to see limitations on water availability as demand exceeds supply by 2050. These areas include parts of Arizona, Arkansas, California, Colorado, Florida, Idaho, Kansas, Mississippi, Montana, Nebraska, Nevada, New Mexico, Oklahoma, and Texas. In particular, in the Great Plains and Southwest United States, water sustainability is at extreme risk.
The report concludes that climate change will greatly increase the risk that water supplies will not be able to keep pace with withdrawals in many areas of the United States. This conclusion has significant implications for future water management and climate change adaptation planning efforts. The pressure on public officials and water users, such as farmers, to manage demand and supply will be greatest in the areas facing these higher risks.
To be clear, the report was not intended to predict where water shortages will occur, but rather where they are more likely to occur. The goal of this analysis was to estimate future renewable water supply compared with water demand, assuming a business-as-usual scenario of growth in demand for electricity production and domestic use, both largely driven by population growth, with other demands remaining at their present levels. In addition to providing the most current analysis of these impacts, this report is also the most comprehensive quantitative assessment of water supply and demand under future climate scenarios performed to date.
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Changes in Precipitation Two of the principal reasons for the projected water constraints are shifts in precipitation and potential evapotranspiration (PET). Evapotranspiration is the sum of evaporative loss of water from the ground surface and transpiration losses through vegetation. PET is a calculated metric used to represent evapotranspirative losses under idealized conditions, where a full water supply is available for evapotranspiration. Together, changes in precipitation and potential evapotranspiration have significant effects on available precipitation, estimated as water falling either as rain or snow that would not be consumed by the potential evapotranspiration. Climate models project decreases in precipitation in many regions of the U.S., including areas that may currently be described as water short. Moreover, based on their projected temperature increases, potential evapotranspiration is also computed to increase. Generally, anticipated increases in PET range from 4 to 5 inches per year. Southern areas, however, may anticipate PET increases of as much as 5 to 6 inches per year.
The combination of decreased precipitation in some regions and increased PET in most regions indicates that many areas will face decreases in overall available precipitation, affecting their water supplies. Projected available precipitation will vary greatly depending on geographic region. It is expected to average less than 2 inches per year for many areas in the West, but more than 15 inches in the Northeast, Northwest, and South Atlantic regions. Projected decreases in available precipitation from historical records will also vary: most areas will experience only slight decreases—drops of less than 2.5 inches per year between 2005 and 2050—with some regions, such as east Texas, the Lower Mississippi Basin, California Central Valley, and the Southeastern U.S., experiencing decreases of more than 5 inches per year by 2050. (For maps detailing this individual impact, please see the full Tetra Tech report at http:// rd.tetratech.com/climatechange/projects/nrdc_climate.asp)
Changes in Water Withdrawals Increases in water withdrawals, from groundwater, lakes, rivers, streams, and manmade structures like dams, are also a primary reason for increasing vulnerability. The three categories of water use with the greatest demand are agricultural use, power plant cooling, and domestic use. Under the business-as-usual scenario assumed in this analysis, total water demand is projected to increase by as much as 12.3 percent between 2000 and 2050. Demand for municipal use and electric cooling is projected to grow along with the U.S. population, while use for irrigation, livestock, aquaculture and mining is assumed to remain at the same levels as 2005, as withdrawals in those sectors have remained relatively flat in the last two decades.
The analysis found that total freshwater withdrawals in 2030 and 2050 are anticipated to be significant in the major agricultural and urban areas throughout the nation. For this analysis, the volumes of freshwater withdrawal at the county level were normalized to the county area and represented in inches per year, such that they could be compared with estimates of available precipitation. Total freshwater withdrawals in 2030 and 2050 will be between 0.2 to 0.5 inches per year, with some areas in the West showing water demand of 1 to 5 inches. Some areas in California, Texas and the Lower Mississippi River basin show projected water demand of more than 10 inches per year. These withdrawals will differ significantly from 2005 levels, with anticipated decreases in much of the Great Plains and increases in some areas in the South, Southeast, and Western regions of the U.S. Overall, water demand in most regions is projected to increase 0.1 inches per year, with a few areas showing growth in demand of up to 3 inches per year. Expressed as a percentage, water withdrawal will grow by 25 percent in many areas of the U.S. including the arid Arizona/New Mexico area, the populated areas in the South Atlantic region, Florida, the Mississippi River basin, and Washington DC and surrounding regions.
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The Future of Water Management for Sustainability
This analysis shows that climate change will have significant impacts on water supplies throughout the country in the coming decades, with over 1,100 counties facing greater risks of water shortages due to the effects of climate change. While water management and climate change adaptation plans will be essential to lessen the impacts, they cannot be expected to counter the effects of a warming climate. One reason is that the changes may simply outrun the potential for alternatives such as modifying withdrawals, increasing water use efficiency, increased water recycling, enhancing groundwater recharge, rainwater harvesting and inter-basin or inter-county transfers to make up for water deficits. The widespread nature of the risk of water shortages may also limit the effectiveness of local solutions—such as acquiring more water from a neighboring county or basin—since many other localities will be trying to get control of the same resource. Further, the pressure on water supplies will not cease in 2050. If climate warming continues to increase, we can expect the risks of water shortages to increase with it. There is no way to truly manage the risks exposed by this report other than taking the steps necessary to slow down and reverse the warming trend. Doing so requires Congressional action and global leadership.
Download full fact sheet: Climate Change, Water, and Risk
View other report materials (NRDC.org)
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Tags: National Resources Defense Council, NRDC, Tetra Tech, Water
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