The researchers traced the subterranean locations where water is being pumped for irrigation, intertwined patterns of sinking and rising land.
Scientists have created a new method that holds promise for improving groundwater management – vital to both life and agriculture in arid regions. This method measures how much groundwater is lost from aquifers in soil that can be drained so much that they cannot recover, and how much comes from soil that is not confined to an aquifer, which some can be completed after years. normal rain.
The research team studied the Tulare Basin in California, part of the Central Valley. The team found that the key to distinguishing between these underground sources of water is related to the pattern of sinking and rising ground levels in this heavily irrigated agricultural area.
The Central Valley makes up only 1% of US farmland, yet it annually grows an astounding 40% of the country’s table fruits, vegetables, and nuts. Such productivity is only possible because farmers increase the valley’s annual rainfall by 5 to 10 inches (12 to 25 cm) with extensive groundwater pumping. In drought years, more than 80% of irrigation water comes from underground.
After decades of pumping, underground water resources are depleting. Wells in the Tulare Basin must now be deepened to 3,500 feet (more than 1,000 m) to find enough water. There’s no way to measure how much water remains underground, but managers need to make the most of what they have. This involves monitoring whether water is being drawn from aquifers or loose soil, known as the water table. There are thousands of unmetered wells in this large area, the only practical way to do this is to use satellite data.
A research team from NASA’s Jet Propulsion Laboratory in Southern California and the US Department of Energy’s Lawrence Berkeley Laboratory in Northern California devised a method that would do exactly that. They attacked the 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 the ESA (European Space Agency) Sentinel-1 satellite . Ground level changes in this area are often related to water loss because when water drains into the ground, it eventually erodes together and sinks in places where water used to be – a process known as subsidence. is called.
The Tulare Basin is shrinking considerably: the current rate is about a foot (0.3 m) of subsidence per year. But from one month to the next, the ground may fall, rise or remain the same. What’s more, these changes are not always in line with the expected reasons. For example, after heavy rainfall, the water level rises. It seems obvious that this would also raise ground level, but it sometimes sinks instead.
The researchers thought that these mysterious short-term changes may be the key to determining the sources of the water pumped. “The main question was how do we interpret change on these short time scales: is it just a blip, or is it significant?” said Kyra Kim, a postdoctoral fellow at JPL and co-author of the paper, which appeared in Scientific Reports.
clay vs sand
Kim and his colleagues believed that the changes were related to the different types of soil in the basin. Aquifers are limited by layers of hard, impervious soil, while unrefined soils are loose. When water is pumped from an aquifer, it takes some time for the soil to compress in response to the weight of the land mass pressing down from above. On the other hand, unrefined soils rise or fall more quickly in response to rain or pumping.
The researchers created a simple numerical model of these two layers of soil in the Tulare Basin. By removing long-term subsidence trends from ground-level-change data, they produced a dataset of only month-to-month variations. Their model showed that on this time scale, almost all ground level changes could be explained not by changes in water levels, but by changes in aquifers.
For example, in the spring, there is less rainfall in the central valley, so the water level is usually sinking. But runoff from the ice in the Sierra Nevada is recharging the aquifers, and causing ground levels to rise. When the water level is rising due to rainfall, if the aquifers are compacted by being pumped at the same time during the preceding dry season, the ground level will drop. The model correctly reproduced the effects of weather events such as heavy rainfall in the winter of 2016-17. It was also matched by the small amount of data available from wells and GPS.
Kim pointed out that the new model could be repurposed to represent other agricultural sectors where better monitoring of groundwater use is needed. With a planned launch in 2023, The NASA-ISRO (Indian Space Research Organisation) Synthetic Aperture Radar (NISAR) mission will measure ground level changes at a higher resolution than Sentinel-1. Researchers will be able to combine datasets from NISAR with data from the GRACE follow-on in this model to benefit agriculture around the world. “We’re moving towards a really beautiful marriage between remote sensing and numerical models to bring everything together,” Kim said.