Twin Satellites Measure Water Storage Change in Colorado River

Researchers seek to apply GRACE’s big-picture data to region, subregion
A perennial quest of hydrologists is determining water availability for water resource planning and management purposes. If hydrologists, for example, could better predict Colorado River flow, the job of dividing its waters among seven states and a foreign country, often a troublesome and contentions issue, might go more smoothly.
The Gravity Recovery and Climate Experiment is providing hydrologists a new tool that some day might help them predict the availability of water supplies: a pair of satellites is sending back data measuring water storage over large areas of the globe. The next step is to find ways to apply the data on a smaller scale. Researchers, including some in Arizona, are working at this task.
GRACE data is the result of team work performed by twin satellites measuring changes in the Earth’s gravity field. In March 2002, the two satellites were launched in the same orbit about 700 kilometers above the Earth’s surface, with one following the other at a distance of about a few hundred kilometers.
Radio signals emitted from the second satellite are transmitted back from the first, with the signal’s travel time a measure of the distance between the two satellites. This distance is a vital indicator of the land mass below, whether it is solid and dense or whether it has changed its density due to water storage.
The lead satellite will experience a stronger gravitational pull when flying over a dense land mass because gravity is related to mass. This will cause the satellite to travel faster increasing its distance from the second satellite which is not yet feeling the effects of the land mass. When the first satellite flies over land of lesser density, it will travel slower decreasing the distance between the two satellites.
By tracking the distances over time between the two satellites, which is constantly changing as they circle the Earth, scientists are able to reproduce the changes in the gravitational field from one month to the next. Such changes are mainly related to water storage, including water stored in rivers, lakes and reservoirs or as snow and ice in floodplains or as subsurface water in soils and aquifers.
Not only can researchers figure out the amount of water stored, but GRACE’s five-year data record enables them to estimate water storage variations over time in more than 50 river basins, including the Colorado River Basin. They can also distinguish between long-term trends and seasonal variations. The basins cover most of the Earth’s land surface.
Peter Troch, University of Arizona hydrologist, is using GRACE data to infer water storage changes in the Colorado River basin. He is attempting to interpret differences between the upper and lower basin to get the big picture. The lower Colorado drainage basin includes most of the state of Arizona. The entire Colorado River Basin covers 637,000 kilometers. Troch explains that a fundamental hydrological equation is the water balance of a river basin. On one side of the equation is precipitation, the rain or snow that falls in the basin; this is input that increases storage. Part of the precipitation will evaporate; some will runoff and discharge into the river. This is output. Depending upon whether input or output is greater determines water storage in a basin during a particular month, whether it increased or decreased.

Troch explains the present difficulty of measuring input and output at the large scale of the Colorado River basin, an area that includes seven states. He says on-the-ground rain gauges do not do the job. “Think of the snow that falls on the top of Rocky Mountains where there are no stations. We don’t measure that. The same is true for evapotransporation. It is done only at a few points in the landscape.

“With GRACE you have a direct measurement of the storage changes; therefore you can better estimate what is going on in the basin in terms of the fluxes.”

Using gravity to determine water storage changes is not a new technique. Don Pool of the Tucson office of the U.S. Geological Survey has conducted in situ gravity monitoring since the 1980s in various areas of the state, including Tucson and Avra Valley. Absolute gravity is regularly measured at various locations that are part of a monitoring network. Gravity changes that occur are then related to storage changes in the subsurface. This allows hydrologists to focus at the local level. Pool says GRACE researchers have expressed an interest in comparing the ground-based measurements with GRACE data.

GRACE is capable of measuring differences within a centimeter of water stored on the land surface or the subsurface. This accuracy, however, applies only to large areas, 100,000 to one million square kilometers. GRACE’s specialty is the big picture, its accuracy greatly diminishing at smaller scales. Troch says, “What GRACE is capable of doing is measure over large regions the changes of storage of a vertical column of water from groundwater to the surface at scales of, say, the Colorado River Basin or the state of Arizona.”
This greatly limits GRACE’s application. Hydrologists generally focus on the regional or subregional, not so much on the global. For example, GRACE would not be very helpful to hydrologists concerned with the flow of the San Pedro or Verde rivers. A priority task of GRACE researchers is to work out applications on a smaller scale where it would be of more direct use to hydrologists.

Troch work’s is breaking ground in this direction. He is studying the different components that make up changes in water storage in a catchment area. Using hydrological modeling he has been able to interpret GRACE’s signals to partition incoming rainfall, whether released into the atmosphere through evaptransporation or stored in the soil and the aquifer.

He says, “If we take that work further we would probably be able to say something about aquifer storage, but at larger scales. We can’t zoom in, for example, to the Tucson basin or see what happens in the Phoenix area.”

The question then remains: Can GRACE data be used to assist hydrologists in their quest to determine available water supplies? Will Bureau of Reclamation officials be able to use the data to better allocate Colorado River water. GRACE would be making a major contribution to that effort if its data enabled Reclamation to improve estimates of water availability over the next six months to a year.

Troch says, “We are still a long way from [establishing] a direct link between GRACE-derived estimates of storage change and something the Bureau could use for planning. ... The next step is to see if we can take the signal and disaggregate it into its different fluxes and hopefully also its different components, like surface storage, soil water storage, groundwater storage, snow; all these different things need to be figured out.”
“It will take some years to convince first ourselves and then stakeholders that they can actually start using it. But it is a matter of time; ten years ago no one paid attention to information such as El Niño and La Niña. Now the scientific evidence of links between those signals and water availability has grown.”

Part of Troch’s research is funded by the University of Arizona’s Water Sustainability Program Technology and Research Initiative Funds.