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Presumed effects of global warming on Water Sustainability

In July, 2010, the National Resources Defense Council (NRDC) released a report summarizing the presumed effects of global warming on water sustainability in the United States. NRDC worked with Tetra Tech, a consulting engineering firm specializing in waterways, water/wastewater management and environmental services.

Together, the NRDC and Tetra Tech analyzed current water usage and population trends then coupled that data with models of global warming that predict climate shifts for the United States. What they found may be cause for concern; over 1,100 counties in the United States may be at risk of water under-supply or sustainability by 2050.

The report developed a new water supply sustainability index based on six risk criteria:

  • Projected water demand as a share of available precipitation
  • Groundwater use as a share of projected available precipitation
  • Susceptibility to drought
  • Projected increase in freshwater withdrawals
  • Projected increase in summer water deficit

The risk to water sustainability for counties was broken down into four regimes; low, moderate, high and extreme. Counties which met two of the criteria were classified as “moderate,” while those meeting three of the criteria were classified as “high,” and those meeting four or more were classified as “extreme.” Counties meeting less than two criteria were considered to have low risk to water sustainability.

After applying the climate change model, two potential phenomena were revealed that could directly affect water availability. The first was shifting precipitation patterns the second was an increase in potential evapotranspiration (evapotranspiration is essentially the net loss of water caused by evaporation and vegetative transpiration, combined). These forces work together to imit the total available precipitation that can become groundwater. What the model showed was that some areas could see their overall effective precipitation reduced by as much as 6 inches per year – a fairly significant reduction in available groundwater, especially if the trend is prolonged.

The map below shows the risk levels for counties in the United States if the assumptions in the model for climate change come to pass. In addition, there is a second map that shows the risk of water sustainability in the absence of the climate change drivers. It comes as no surprise that the desert Southwest and the Great Plains show a high number of counties on both maps at high or extreme risk; what may be a surprise, though is the number of at-risk cities and metropolitan areas in the 2050 map created without any climate change effects.



Among others, Columbus, Detroit, Philadelphia, Washington D.C. are all predicted to have significant water sustainability issues by 2050. In addition, a significant portion of Florida and areas near the Mississippi River are also predicted to have issues. These issues are mainly a result of population increases in already under-stress areas. It would seem that regardless the direction that climate change takes our planet, these cities and regions will likely face water availability issues in the not-too-distant future. This report should serve as a reminder to everyone that we should constantly evaluate our resources, our environment and our changing population to find the right balance of water supply and conservation to meet the challenges we will face tomorrow.