Changing Planet

To Understand Water, Learn the Math

This map portrays the volume of water being depleted from rivers, lakes and aquifers on a daily basis (MGD = million gallons per day). Note that some of the highest rates of water consumption are associated with regions with heavy groundwater use, including the Central Valley of California, the Ogallala/High Plains Aquifer of the Midwest, and the Snake River Aquifer in Idaho. The divisions on the map represent major water resource regions (map from EPRI, 2014).

 OK, I’ll admit to being a bit of a geek when it comes to water numbers.

I never got all that excited about algebra in school, and college calculus was a real struggle.  But water numbers fascinate me.  If you’ve read any of my earlier blogs it will be apparent that this fascination borders on obsession.

Don’t get me wrong.  I can be perfectly content sitting on the bank of a river and watching the water flow by, never thinking about the number of gallons moving downstream.

But I find that numbers can be really helpful in understanding the causes of water shortages, or how much water we are wasting, or how often you might expect a flood of a certain size.  When you gain a quantitative grasp on the volume of water falling on the land as rain or snow, the volume moving into rivers or aquifers, or the amount of water being used in cities, industries, or farms, a much fuller water picture reveals itself.

That’s why I got excited when the Electric Power Research Institute (EPRI) recently released its new report on “Evaluating Thermoelectric, Agricultural, and Municipal Water Consumption in a National Water Resources Framework.”

Bear with me.  I’ll explain why this report is important.

To really understand what is happening with the water in a particular river, lake, or aquifer, or even at the scale of an entire state or country, you need to build a “water budget” – an accounting for how much water is being supplied by rain or snow, how much is being removed from each water source, AND how much gets returned to the water source after use.  The volume that is removed and NOT returned is called “consumptive use” or simply “consumption,” as illustrated in the diagram below.  If you want to understand why or the rate at which a river, lake, or aquifer is being depleted (dried up), you absolutely need to know how much water is being consumptively used.


water withdrawals and consumption
To truly understand what is happening with the water in a river or aquifer, you need to keep track of how much water is being replenished, withdrawn, used, and returned to the original water source after use. The portion that is “consumptively used” causes depletion of the water source. Illustration from Chasing Water: Moving from Scarcity to Sustainability (Brian Richter, Island Press, 2014)

The U.S. Geological Survey has been estimating water withdrawals within each state and large river basin since 1950, updating those estimates at five-year intervals.  In 1955 the agency began also estimating consumptive use in its 5-year reports.  However, after completing its 1995 estimates, the USGS stopped estimating consumptive uses, for reasons I’ll explain in a moment.

For water geeks like me, it has been quite unnerving to go without consumptive use numbers for more than a decade.  That’s why the new EPRI report is important.  At long last, we have estimates of consumptive use, once again.

Here are some of the highlights of the EPRI report:

  • Only 23% of the total volume of water withdrawals in the U.S. is consumptively used.  The rest goes back to a water source, available for further use or to support ecosystem health.
  • Withdrawals of water used to cool electricity-generating power plants account for 41% of the total withdrawals in the U.S., but only a tiny fraction is consumed (lost to evaporation).  As a result, electricity generation accounts for less than 5% of total consumptive water use.
  •  The largest consumptive user of water is irrigated agriculture, accounting for nearly two-thirds of all water depleted from freshwater sources.
water consumption pie chart
Irrigated agriculture accounts for nearly two-thirds of all consumptive use of water in the United States, followed by public/domestic water supplies. Electricity production requires large volumes of water withdrawal for power plant cooling, but accounts for less than 5% of total consumptive use (from EPRI, 2014)

You might be wondering why a research institute focused on the electrical power industry would go to the trouble and expense of preparing such a comprehensive national water report?

“When you look at withdrawal estimates only, the electric power sector is responsible for around 40% of all withdrawals, so it’s identified as a prominent user of freshwater resources,” says Bob Goldstein of EPRI, who led the preparation of the new report.

“The general public doesn’t understand the difference between withdrawal and consumption.  They don’t understand that most of the water that the power industry withdraws is returned to the water source, so it doesn’t evaporate the water; it isn’t lost from the local system. The water is still available for someone else to use downstream.  We felt it was in everyone’s interest to clarify the difference between withdrawal and consumption.”

Fortunately, the USGS will soon be generating its own estimates of consumptive use, and those estimates will be much more accurate in the future, according to Eric Evenson, the team lead for the agency’s water program.  Evenson explains that his agency never wanted to stop estimating consumptive use, but lacked the resources to do it well.

“There was a concern about the technical rigor in our estimates of consumptive use, and a lack of consistency in methods being used across the country,” he says. ” There was a lot of variation.  We didn’t have the resources at that time to improve the technical rigor to the degree that we needed, so we had to stop estimating consumptive use.”

That funding shortfall was largely fixed with the national SECURE Water Act, passed in 2011.  The act authorized new investment of $20M per year to launch a national assessment of water availability and use. The USGS has received $6M per year for its new “WaterSmart” initiative, of which $2.5M a year is applied to water use science. “We really want to increase our understanding of site-specific water use, instead of relying only on broad-scale estimates.  This new initiative will finally enable us to do that,” says Evenson.

Evenson expects that much more accurate estimates of water consumption will become available from his agency beginning in 2017.  They have just completed an in-depth assessment of more than 1300 individual power plants across the country, documented in a report released in November 2013.  Another report detailing the plant-by-plant consumptive uses is expected out this summer.

Similarly, the USGS plans to do detailed estimates of the 57,000 public water supply systems in the country.  In collaboration with the U.S. Environmental Protection Agency, they aim by 2016 to create a database with withdrawal estimates for 7,000 surface water intakes and 117,000 groundwater wells used for public water supply.  Consumptive use estimates for public supply systems will be forthcoming following a detailed analysis of seasonal water withdrawals at the public supply facilities.

The holy grail of water quantification – accurately accounting for water use in agriculture using site-specific measurements – remains out of reach, however, at least in the near future.  “Irrigation is the 800-pound gorilla,” laments Evenson.

“There are millions of withdrawal locations, with huge volumes of consumptive use. We are researching right now the consumptive use that comes from each field of irrigated agricultural land, using remote sensing techniques and thermal infrared imagery provided by the MODIS and LANDSAT satellites. That information will be available this year. But that doesn’t tell us where the irrigation water comes from.”

Bob Goldstein of EPRI agrees that much more needs to be done to pin down the water use in agriculture.  “There’s a lot of uncertainty in the accounting for water use in agriculture,” says Goldstein.

He points out that the most recent USGS estimates suggest that 128,000 MGD of water is withdrawn each year for agriculture, but the Farm & Ranch Irrigation Survey conducted by the U.S. Department of Agriculture estimates that only 81,000 MGD was applied on farms.  That suggests that as much as 47,000 MGD – equal to the amount that gets consumed on the farms themselves – may be getting lost before getting to the farm, perhaps through leaking canals or pipes or by evaporating en route.

Evenson adds that nobody really knows where all of that water goes, whether it flows back into a river, into an aquifer, diverted for another use, or evaporates to the atmosphere.

“Coming up with a site-specific irrigation database may take a very long time,” admits Evenson.   “I haven’t even tried to calculate how long that might take for the whole country.”

Brian Richter’s new book on Chasing Water: Moving from Scarcity to Sustainability will be available from Island Press in June 2014.

Brian Richter has been a global leader in water science and conservation for more than 25 years. He is the Chief Scientist for the Global Water Program of The Nature Conservancy, an international conservation organization, where he promotes sustainable water use and management with governments, corporations, and local communities. He is also the President of Sustainable Waters, a global water education organization. Brian has consulted on more than 120 water projects worldwide. He serves as a water advisor to some of the world’s largest corporations, investment banks, and the United Nations, and has testified before the U.S. Congress on multiple occasions. He also teaches a course on Water Sustainability at the University of Virginia. Brian has developed numerous scientific tools and methods to support river protection and restoration efforts, including the Indicators of Hydrologic Alteration software that is being used by water managers and scientists worldwide. Brian was featured in a BBC documentary with David Attenborough on “How Many People Can Live on Planet Earth?” He has published many scientific papers on the importance of ecologically sustainable water management in international science journals, and co-authored a book with Sandra Postel entitled Rivers for Life: Managing Water for People and Nature (Island Press, 2003). His new book, Chasing Water: A Guide for Moving from Scarcity to Sustainability, was published by Island Press in June 2014.
  • Bruce Potter

    OK, if I want to find out about the water budget for my personal watershed on the Maryland portion of the Western Shore of the Chesapeake Bay, how do I find out? I’m a bit worried because the area has grown fast in the past decade or two and a relatively large portion of the population uses ground water wells.

    If USGS says it had an unreliable methodology for its large watersheds, what confidence do I have that Maryland or Anne Arundel County have an adequate methodology, applied with the proper rigor at an appropriate cost, to maintain the sustainability and quality of my watershed’s groundwater?

    • Terrific question, Bruce. Given that this is a rapidly growing area, I’d be surprised if some agency or university hasn’t developed a pretty decent water budget for your aquifer. I would start by asking the Maryland District office of the USGS. If they haven’t prepared a water budget themselves, someone there will likely know (ask for their groundwater hydrologist when you call). If you hit a dead-end there, I would next try someone in the state’s water resources agency. When water budgets are prepared for a specific watershed or aquifer, they tend to be more accurate than water budget estimates that are generated using national or global models and datasets.

  • Mary Beth

    This is a fascinating topic especially to a Californian in 2015. We are told that 80% of our water use is agricultural, that we never should have planted crops in the desert (we supply a large portion of the vegetables in the Us), and that the worst offenders are almonds and alfalfa. I have read that some crops return a large percent of their water to the aquifer. What are the characteristics of a crop that make it use more water? Or does one have the measure each crop individually? We don’t currently have the power to tell any farmer what to plant, but we could tax certain crops that use water that is not returned to the aquifer.
    I hope that you are still monitoring you comments, as it has been a year since you wrote this. Has the information from the satellites supplied anything helpful or useful?

    Thank you.

    • Mary Beth, there are two factors that influence how much water will be returned to a water source after irrigation water is applied to a crop. The first is the irrigation method; drip irrigation, by applying smaller volumes of water directly to the plants, will result in very little water being returned to an aquifer or river because the water is used very efficiently by the crop. When using flood irrigation, a lot more water is applied, but some of that water usually returns to an aquifer or river. The key here is to understand “consumptive use,” i.e., the amount of water that gets “lost” or evaporated in the irrigation process, as my blog explains. The second factor is related to the plant/crop itself. Some crops require a lot more water to support their growth. That means that even when applying water using drip irrigation, you have to apply more water to certain crops to sustain their growth. The bottom line: we should try to use efficient irrigation practices like drip irrigation wherever possible to reduce evaporation losses, and we should try to grow crops that generate the greatest revenue ($$) per unit of water consumed, and provide the greatest number of jobs. The caveat to that is that we also need to ensure that we are able to grow all essential crops in some place!

About the Blog

Researchers, conservationists, and others share stories, insights and ideas about Our Changing Planet, Wildlife & Wild Spaces, and The Human Journey. More than 50,000 comments have been added to 10,000 posts. Explore the list alongside to dive deeper into some of the most popular categories of the National Geographic Society’s conversation platform Voices.

Opinions are those of the blogger and/or the blogger’s organization, and not necessarily those of the National Geographic Society. Posters of blogs and comments are required to observe National Geographic’s community rules and other terms of service.

Voices director: David Braun (

Social Media