A US Geological Survey (USGS) report released last month, “Estimated Use of Water in the United States in 2010,” surprised many of us in the water research community. According to the report, about 355 billion gallons of water per day were withdrawn for use during 2010, which represented a 13 percent decrease relative to 2005 withdrawals and the lowest level of water withdrawals estimated by the USGS since before 1970. Between 2005 and 2010, thermoelectric power and irrigation, the two largest water users, reduced their withdrawals by 20 percent and 9 percent, respectively. (This is the most recent report in a series that has been published by the USGS since 1950 and represents the longest compilation record of water use data by a US federal agency.)
Based on this finding that water withdrawals have significantly decreased since the last report was released, one might think that our water scarcity woes are over. Some news articles have characterized this report as proof that the US has already reached “peak water” (see, for example, this article). However, it is important to note that the USGS and water researchers in general have a very specific definition for the word “withdrawals.” The report defines withdrawals as “the total amount of water removed from the water source for a particular use, regardless of how much of that total is consumptively used or returned to the hydrologic system for future use.” Consumptive use, in turn, is defined as water removed that is not returned to the hydrologic system, at least for a period of time. As such, consumptive use includes the amount of water transpired by crops during plant growth, water that evaporates from the soil surface of crop land, and water consumed by livestock or humans. With respect to thermoelectric power, consumptive use would include water evaporated or incorporated into byproducts as a result of the production of electricity, but not any water that is returned to rivers and streams after being used for cooling within a power plant.
Quantifying consumption may be as, if not more, important than quantifying withdrawals because consumption actually precludes the subsequent or downstream withdrawal of water for another use. Unfortunately, estimates of consumptive use were discontinued by the USGS after 1995 due to resource and data constraints. So what do we know about recent trends in water consumption in the US? Another recent USGS report, titled “Withdrawal and Consumption of Water by Thermoelectric Power Plants in the United States, 2010,” uses information from the US Department of Energy to show that water consumption by thermoelectric plants decreased by 34 percent between 2005 and 2010. This decrease occurred at the same time that net electrical generation for water-using thermoelectric plants increased by 6.4 percent, so this is good news on the consumption front.
However, much less is known about the other major use of water: irrigation. According to the US Department of Agriculture, irrigation in the United States accounts for approximately 80 percent of the nation's consumptive water use and over 90 percent in many western states. Furthermore, recent research suggests that water conservation measures in agriculture may actually increase consumption because efficient irrigation technologies reduce return flows and limits aquifer recharge (see, for example, this study and this study). It is quite possible that water consumption has decreased together with withdrawals, but this is not something that can be concluded from the USGS report.
At the same time, research efforts are underway to close this knowledge gap. For example, researchers at the Desert Research Institute in Nevada are developing methods to use remotely sensed data to estimate actual historical agricultural water use. Consumptive use measures from these projects can complement existing USGS estimates on withdrawals so as to provide a more complete characterization of the regions that are at risk of water scarcity and excessive water use.
Finally, it is important to recognize that the reductions in water withdrawals identified in the USGS report are outcomes of a variety of efforts to introduce policies to regulate water withdrawals and encourage water efficiency in all sectors of the economy. Instead of taking these latest withdrawal estimates as an indication to ease up on water management activities, we should interpret them as evidence that our policies are starting to work and that further analysis is needed to ensure that these policies continue to protect our water resources in a cost effective manner.