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Drought or glut? Health of water resources depends on how deep you look

Fascinating new views of Earth popped up over the holidays, drawing on advanced satellite sensing to visualize the state of groundwater resources around the globe.

The picture is not pretty in the world’s drier regions, where aquifers are being drawn down faster than they can recharge. But in Minnesota and much of the north-central U.S., the situation looks pretty good.

The map above compares current levels of groundwater storage to levels going back to 1948, with the results ranked in 11 color-coded tiers.

In areas marked with maroon — like the zones of severe long-term drought in Texas, Oklahoma and elsewhere — groundwater has been measured at levels seen only 2 percent of the time over that 63-year record.

At the other end of the spectrum, areas marked in darkest blue — like most of Minnesota’s western half — groundwater storage is at levels seen for at least 96 percent of that period, and areas in lighter shades of blue were at or above the 70 percent mark.

This is quite a different picture of hydrological health than one might gather from, say, the latest drought map for the United States. It shows every county in Minnesota experiencing some degree of drought:

The U.S. Drought Monitor is a partnership between the National Drought Mitigation Center, United States Department of Agriculture, and National Oceanic and Atmospheric Administration. Map courtesy of NDMC-UNL.

More in a moment about why that’s so. But first let’s look at the new images that made their public debut last month, and how the mapping team at the GRACE project came up with them.

Twin satellites, tugged by Earth
GRACE is an acronym for the Gravity Recovery and Climate Experiment, a collaboration of the U.S. and German space agencies. It uses twin satellites, launched in 2002, to make detailed measurements of how Earth’s gravitational field changes from place to place, and time to time, with a special focus on causes and effects related to climate.

The satellites trail each other in orbit, about 135 miles apart, held in place by the pull of Earth’s mass. But that mass is not uniform: when the lead satellite passes over an especially massive feature, like a mountain, gravity’s pull increases and the satellite speeds up, temporarily lengthening its distance from its trailing twin.

The same thing happens as groundwater levels rise and fall, increasing and decreasing gravitational pull on the orbiters. According to ScienceNews, the satellites can register a one-centimeter change in groundwater levels — but only over an area the size of Illinois.

Last month, reporting the project’s findings to the American Geophysical Union, the GRACE team’s Jay Famiglietti said that nine years of data show groundwater "being depleted at a rapid clip in virtually all of the major aquifers in the world’s arid and semiarid regions."

According to the ScienceNews article and another in the New York Times, notable declines have been detected in North Africa, northern India, northeastern China, southern Argentina, western Australia — and, a bit closer to home, in California’s Central Valley.

That last finding has brought some pushback from California water officials concerned about its implications for the state’s agriculture industries and for its already complicated struggles over water allocation.

A work in progress
Matt Rodell, a GRACE team hydrologist who works for NASA, told me by email that the project’s groundwater-measurement products are still being refined and improved (we didn’t specifically discuss the California flap).

He also explained why GRACE is unable to show with certainty how much water is actually still in the ground and available for use, compared to historic levels.

Most of the data used in GRACE’s modeling — precipitation, temperature, humidity and other climate factors — go back a long way. But for groundwater pumping, historical records are sketchier.

GRACE’s own observations since 2002 enable the team to gauge groundwater depletion over the last nine years, but major drawdowns before then wouldn’t necessarily be reflected.

Different tools for different depths
The "coarseness" of GRACE’s large-area measurements is at the opposite end of the spectrum from those used to produce the U.S. Drought Monitor maps, first unveiled in 1999.

Some 40 to 60 types of indicators contribute to the Drought Monitor, including highly localized measurements of soil moisture, stream flows and water-table levels as registered in wells. Its authors use mathematical modeling, too, but also acknowledge that preparing the map requires some subjective judgments — a blend of art and science, as Mark Svoboda puts it.

Svoboda, a founding author of the Drought Monitor, said that one way of thinking about the difference between his map and GRACE’s is that his measurements are more concerned with conditions near the surface, and the satellite is peering much deeper to gauge groundwater storage.

(The satellite data also yield other maps of moisture in the top two centimeters of soil and in the "root zone" — to a depth of one meter — and these look quite different from the GRACE map above; in Minnesota, the blue areas shrink and the red and yellow areas expand. But the overall picture remains rosier than the Drought Monitor's.)

It's also the case that all of these maps are, like opinion polls, "snapshots in time" — and the conditions adding up to drought can change rapidly. At the end of September, the Drought Monitor for Minnesota had only about half the state experiencing some degree of drought. And the portion experiencing "severe" or "extreme" drought has grown more than fivefold since then.

'Holy grail' of moisture measurement
Because most earth-bound measurements of soil moisture are taken at or near the surface, GRACE data hold the immediate promise of improving the Drought Monitor’s assessment of moisture in the root zone — what Brian Wardlow, a staffer there, called "the holy grail" of information most needed by farmers, forecasters and policymakers.

Svoboda told me he sees the GRACE project as more of a collaborator than competitor with his program, and its findings more often complementary than contradictory. Indeed, he has been comparing unpublished GRACE maps with his own and has been impressed by how the "general patterns have been eerily similar over the past two years."

Over time, he thinks the new satellite data will help the Drought Monitor improve its renderings — and that, in turn, ground-based data gathered by his program will help the GRACE team "ground-truth" its methods for turning little gravitational tugs into panoramas of our planet’s hydrologic health.

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