Scientists from NASA are using satellite data to develop a method of monitoring underground water loss.
Groundwater, which is found in aquifers below the surface of the Earth, is one of the world's most important natural resources and a decline in groundwater levels is a serious problem. In a bid to tackle this issue a joint project between NASA's Jet Propulsion Laboratory and the US Department of Energy's Lawrence Berkeley Laboratory has developed a model to monitor the rate and type of water loss underground.
"We're heading toward a really beautiful marriage between remote sensing and numerical models to bring everything together."
While California's Central Valley makes up only 1% of farmland in the US - it grows around 40% of the nation's table fruits, vegetables, and nuts. This is only possible because farmers supplement the valley's 125-250mm of annual rainfall with extensive groundwater pumping.
In drought years, more than 80% of irrigation water comes from underground, however, after decades of pumping, underground water resources are dwindling. Ground-level changes in this region are often related to water loss, because when ground is drained of water, it eventually slumps together and sinks into the spaces where water used to be—a process called subsidence.
The team 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. NASA's method was able to distinguish much underground water loss comes from aquifers confined in clay - which can be drained so dry that they will not recover - and how much comes from soil not confined in an aquifer, that can be replenished by a few years of normal rains.
"The main question was, how do we interpret the change that's happening on these shorter time scales: Is it just a blip, or is it important?" said Kyra Kim, co-author of the paper, which appeared in Nature Scientific Reports.
Kim and her colleagues believed the changes were related to the different kinds of soils in the basin. Aquifers are confined by layers of stiff, impermeable clay, while unconfined soil is looser.
When water is pumped from an aquifer, the clay takes a while to compress in response to the weight of land mass pressing down from above. Unconfined soil, on the other hand, rises or falls more quickly in response to rain or pumping.
The researchers created a simple numerical model of these two layers of soils in the Tulare Basin. By removing the long-term subsidence trend from the ground-level-change data, they produced a dataset of only the month-to-month variations. Their model revealed that on this time scale, virtually all of the ground-level change can be explained by changes in aquifers, not in the water table.
For example, in spring, there's little rainfall in the Central Valley, so the water table is usually sinks. But runoff from snow in the Sierra Nevada recharges the aquifers - causing the ground level to rise. When rainfall causes the water table to rise, if the aquifers are compressing at the same time from being pumped during the preceding dry season, the ground level will fall.
Reproducing weather effects
The model correctly reproduced the effects of weather events like heavy rainfalls in the winter of 2016-17. It also matched the small amount of available data from wells and GPS.
The research team believe the new model could be used in other agricultural regions where groundwater needs to be better monitored.
With a planned launch in 2023, the NASA-ISRO (Indian Space Research Organization) Synthetic Aperture Radar (NISAR) mission will measure changes in ground level at even higher resolution than Sentinel-1. Researchers will be able to combine NISAR's dataset with data from GRACE Follow-On in this model for the benefit of agriculture around the globe. "We're heading toward a really beautiful marriage between remote sensing and numerical models to bring everything together," Kim said.
Researchers observed California's Tulare Basin in the Central Valley, and combined data on water loss from the US-European Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On satellites and a European Space Agency (ESA) Sentinel-1 satellite.