In nearly 1 in 10 U.S. watersheds, water use exceeds the natural water supply. Water stress worsens as colors trend from green to yellow to orange to red. Map courtesy of the Cooperative Institute for Research in Environmental Sciences, Western Water Assessment.

In more and more places around the world, water demands are bumping up against the limits of Earth’s finite water supply.

Each year seems to bring another analysis of “water stress” to help us get a fix on how dangerous our water situation is becoming, whether in a particular country or in the world as a whole.

Among the latest efforts is by a team led by researchers at the Cooperative Institute for Research in Environmental Sciences at the University of Colorado in Boulder who examined 2,103 watersheds across the United States and found that in 193 of them – nearly 1 in 10 – water use has surpassed the natural water supply.

These basins are classified as “water stressed,” and the researchers find that as climate change unfolds, this water predicament will worsen.

The team set a high bar for “stressed” water conditions. Whereas other analysts have called watersheds stressed when human water demands reach 40 percent of natural supplies – in part, because freshwater ecosystems would be severely impacted at this level – this team declares stress to exist when demands reach or exceed 100 percent of natural supplies. (See the map above, where stressed basins are colored yellow, orange or red.)

To meet the demands of their farms and cities, these regions typically rely on some combination of multi-year water storage, water imports from other basins, and groundwater pumping.

Not surprisingly, most of the stressed watersheds lie in the western part of the country, where irrigated agriculture typically accounts for more than 80 percent of total water withdrawals.

But the team, which published its results in Environmental Research Letters, ran separate stress tests for each major sector of water use – agriculture (including irrigation and livestock), thermoelectric power plant cooling, and municipal and industrial use – and this turned up some interesting results.
One is that the large volume of water needed to cool nuclear and coal-fired power plants creates pockets of water stress all across the country, including in the fairly well-watered East and Midwest.

Thermoelectric power generation demands more water than any other economic sector –some 201 billion gallons a day, or 49 percent of total US water withdrawals.  (These figures are for 2005, the latest available from the US Geological Survey.) The high volumes withdrawn to cool a single plant can push a watershed into the stressed category.

Even though plants with once-through cooling systems return most of the water they withdraw back to its source, they often kill numerous fish and other aquatic life in the process.

A report just out from the GRACE Communications Foundation found that each year the nation’s thermal power plants kill more than 2 billion fish and destroy more than half a trillion eggs and larvae of aquatic species.

In southern California and the Las Vegas area, urban demands alone are sufficient to cause water stress. Courtesy of the Cooperative Institute for Research in Environmental Sciences, Western Water Assessment 

Another important finding of the University of Colorado study is that most of southern California is classified as water stressed even when looking only at municipal and industrial water use.  Farms and power plants worsen the stress (the map goes from yellow to orange and red), but urban water demands alone exceed the natural supply.  Southern California makes up most of the difference by importing water from the northern part of the state, an increasingly contentious solution, as well as from the overtapped Colorado River.

A third crucial take-away is that we’re not paying enough attention to groundwater, a message we’ve conveyed many times in Water Currents.
Even though groundwater makes up a growing share of water use nationally and globally, we don’t measure and monitor it carefully enough to come close to getting an accurate picture of its role in our water accounts.

Due to data limitations, the model used by this research team assumes that there’s no limit on groundwater supplies – when, of course, in reality there is.  Groundwater is being depleted in many parts of the country, from the High Plains of Texas to California’s Central Valley to parts of the Southeast, to name just a few.

While the study team offers the caveat that “the role of groundwater is beyond the scope of this analysis,” this omission inhibits a true rendering of water vulnerabilities nationwide.

After all, what did Texas irrigators do during the severe droughts of recent years?  They pumped more groundwater to make up for the rainfall deficit.  In July 2012, the High Plains Water District, based in Lubbock, reported an average drop in groundwater levels in its 16-county service area of 2.56 feet (0.78 meters) – the largest annual decline recorded in the last 25 years.

As droughts become more widespread or severe, our depletion of groundwater will pick up speed.

And what are southern California cities doing to wean themselves from long-distance water imports?  To be sure, they’re investing more heavily in water conservation and reuse, but they’re also planning to pump more groundwater from local aquifers.

It’s a truism that we can’t manage what we don’t measure and monitor.  It’s time to begin tracking and managing groundwater use and supply as a matter of national security, because once the wells run dry we’ll be severely stressed indeed.

Originally published at National Geographic Newswatch