Scientists and government officials have been warning that California faces drier times in the years ahead due to a projected gap between available water supplies and growing demands for water. And in many areas, water levels in aquifers have already been declining. It’s a problem in various parts of the country, particularly in the southern half of the United States, as shown by data published this week in the journal Science.
In their article, researchers James S. Famiglietti and Matthew Rodell explain how satellite data can make possible better management of groundwater supplies on a regional scale. They note that extreme flooding and extreme drought are both occurring more frequently as the climate changes, and that most climate models predict the world’s dry regions will become drier. Meanwhile, the researchers say that ”groundwater reserves, the traditional backup for water supplies during extended periods of drought, are in decline globally.”
Famiglietti, a professor at UC Irvine and director of the UC Center for Hydrologic Modeling, and Rodell, of the Hydrological Sciences Laboratory at the NASA Goddard Space Flight Center, point out that satellite data from a joint U.S.-German mission called GRACE are providing an important big-picture view of changes in water supplies.
GRACE, which stands for the Gravity Recovery and Climate Experiment mission, was launched in 2002 and consists of two satellites that fly separately in orbit 137 miles apart. They monitor changes in Earth’s gravity field and act as a “scale in the sky” to weigh the total amounts of water, both above and below ground, and measure changes on a monthly basis.
With their article, the scientists published a map showing nationwide GRACE data from 2003 to 2012. The picture that emerges is one of big shifts, with farming regions such as California’s Central Valley and the Southern High Plains appearing as hotspots with losses in total water supplies of between 1.5 centimeters and 2.5 centimeters a year. Parts of the Southeast also show up as red areas where groundwater is being depleted, while areas such as the Upper Missouri River basin have grown wetter.
“One of the things that comes out of the image that’s with that paper is that water management is I think emerging as national scale problem, which I think argues for a national water management strategy,” Famiglietti said in a telephone interview. “We need to recognize that a lot of our water law was written a long time ago, before we had the population that we have and before we understood how the water cycle works. And so it’s time to really rethink some of this stuff because the picture that emerges from that graphic is a very complicated one.”
While scientists still don’t know how much water is stored underground in the United States, the satellite data provide a regional picture as pumping depletes aquifers in many areas, Famiglietti said. “This is really a regional problem. It’s really happening all over the southwestern U.S., and it’s happening all over Southern California, and we can quantify it.”
The researchers say that in order to maximize the value of GRACE data for water management, future missions should aim to achieve finer scales “so that smaller river basins and aquifers can be observed directly.”
GRACE is providing “the first real, continental scale measurements of changes in water, and that’s new,” said Gordon Grant, a research hydrologist with the USDA Forest Service’s Pacific Northwest Research Station in Corvallis, Oregon.
The big-picture view provided by GRACE reveals trends in the changing volumes of water in aquifers, but it’s a coarse monitoring tool that doesn’t specify causes, which in some areas include prolonged drought, Grant said. “It’s certainly suggesting that there are some large-scale climatic trends that are playing out at the continental scale.”
Scientists say that sorting out the pressures on water supplies in the nation’s “hotspots,” including parts of California, requires additional on-the-ground research.
The U.S. Geological Survey collects data from wells across the country, but there are gaps and some areas aren’t well covered.
“We don’t have a national network for groundwater as of yet,” said Kevin Dennehy, Chief of the USGS Groundwater Resources Program. The project has been included in budget proposals, and the agency has carried out a pilot study.
“The hope is that we will be able to develop this national groundwater monitoring network and to incorporate the information of others into that so we can get a better picture of the groundwater resources of the U.S.,” Dennehy said.
The challenge for officials will be finding funding in tight budget times.