NASA Scientists Devise New Method To Improve Groundwater Management

The new method sorts out how much underground water loss takes place from aquifers confined in clay, and how much from soil that is not confined in an aquifer. 

New Delhi: Underground water is of utmost importance for irrigating agricultural fields. NASA scientists have devised a new method which holds the promise of improving groundwater management. This is critical to both life and agriculture in dry regions. 

The new method sorts out how much underground water loss takes place from aquifers confined in clay, and how much from soil that is not confined in an aquifer. 

The study was recently published in the journal Nature Scientific Reports. 

What Is An Aquifer?

An aquifer is a body of rock or sediment that holds groundwater. There are two types of aquifers, confined and unconfined. Aquifers must be both permeable and porous and include rock types such as sandstone, conglomerate, fractured limestone, and unconsolidated sand and gravel. 

How Aquifers Confined In Clay Are Different From Underground Water Coming From Soil

Aquifers confined in clay can be drained so dry that they will not recover, while the underground water that comes from soil and is not confined in an aquifer can be replenished by a few years of normal rains, the study said.

California's Tulare Basin, which is a part of the Central Valley, was studied. The scientists found that the key to distinguishing between these underground sources of water relates to patterns of sinking and rising ground levels in this heavily irrigated agricultural region, the study said. 

Though the Central Valley makes up only one per cent of US farmland, it grows an amazing 40 per cent of the nation's table fruits, vegetables, and nuts annually. 

Since farmers augment the valley's five to 10 inches of annual rainfall with extensive groundwater pumping, such productivity is possible. More than 80 per cent of irrigation water during drought years comes from underground.

However, underground water resources are dwindling. In order to find adequate water, wells in the Tulare Basin are drilled as much as 3,500 feet. Ground water management involves monitoring whether water is being drawn from aquifers or from loose soil, known as the water table. Since there are tens of thousands of unmetered wells, using satellite data is the only practical way to monitor groundwater. The amount of water remaining underground cannot be measured. 

How NASA Scientists Devised Method To Improve Groundwater Management

Researchers at NASA's Jet Propulsion Laboratory in South California and the US Department of Energy's Lawrence Berkeley Laboratory in Northern California tried to solve this problem by combining data on water loss from NASA's Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On satellites with data on ground-level changes from a European Space Agency (ESA) Sentinel-1 satellite, the study said.

When ground is drained of water, it eventually slumps together and sinks into the spaces where water used to be. This process is called subsidence, and ground-level changes in the Tulare Basin are often related to this process.

According to the study, the Tulare Basin is subsiding drastically, the current rate being about one foot of sinkage per year.

However, from one month to the next, the ground may drop, rise, or stay the same, the researchers noted in the study. Moreover, the changes are not always related to the expected causes. 

For instance, the water table rises after a heavy rainfall. Though this is expected to make the ground level rise, it sometimes sinks instead. 

Kyra Kim, a researcher at JPL who co-authored the study, said the main question was how does one interpret the change that is happening on these shorter time scales. 

How Different Kinds Of Soils Determine Whether Ground Level Rises Or Falls

The researchers believed the changes were related to the different kinds of soils in the basin. Aquifers are confined with layers of stiff, impermeable clay, while unconfined soil is looser. 

When one pumps water from an aquifer, the clay takes some time to compress in response to the weight of land mass pressing down from above. On the other hand, unconfined soil rises or falls more quickly in response to rain or pumping.

The scientists devised a simple numerical model of these two layers of soils in the Tulare Basin, and produced a dataset of only the month-to-month variations. 

The scientists observed that all of the ground-level change can be explained by changes in aquifers, and not in the water table. 

For instance, there is little rainfall in the Central Valley in spring, because of which the water table is usually sinking. However, runoff from snow in the Sierra Nevada recharges the aquifers, which causes the ground level to rise.

If aquifers are compressing as a result of being pumped during the preceding dry season, and at the same time, rainfall is causing the water table to rise, the ground level will fall. 

The model matched the small amount of available data from Wells and GPS, and correctly reproduced the effects of weather events like heavy rainfalls in the water of 2016-17.

According to Kim, the new model can also be used in other agricultural regions. The NASA-ISRO (Indian Space Research Organisation) Synthetic Aperture Radar (NISAR) mission, which is planned for launch in 2023, will measure changes in ground level at even higher resolution than Sentinel-1.