“If we fail to ensure that the levels of these water sources return to previous values, the soil will sink, potentially by tens of centimeters a year, for decades. But if they do, the situation will improve almost immediately, “said Matthew Lees, lead author of the new study. His research was published on June 2 in the journal Water Resources Research.
The study examined the effects of the drought, which is getting worse in this state from year to year. The changing climatic conditions there are recording warmer and warmer weather and a lack of precipitation. The first four months of 2022 marked the driest start in California since 1895. Reservoir levels are so low that, for the second year in a row, a number of irrigation sites from the federal network of reservoirs and canals are ready for no water this year. they don’t get. It is approximately three million acres of agricultural land.
Groping in the dark
“It is crucial to better understand the underlying mechanisms that govern the relationship between groundwater and surface deformation,” said Rosemary Knight, another study author.
The problem in California is that there is little water on the surface. But farmers need it to irrigate crops – and they need it most during the growing season, when it is least. So when farmers do not find water in the tanks, they get it from the ground, they mine the ground’s reserves.
As the water disappears from the ground, the soil layers settle, becoming more concentrated and denser – and the surface sinks.
The Central Valley produces about a quarter of US food, so it’s key to America’s food self-sufficiency and food price stability. At the same time, most of California’s underground water reservoirs are located here, which are identified as critically endangered by excessive pumping under the groundbreaking California Act of 2014. This law requires that by 2040, groundwater abstraction and reserves be equalized at selected locations.
The earth is not solid
“The seemingly motionless solid surface we live on is actually dynamic and moving and constantly changing. These changes affect our lives much more than most of us realize, ”Lees said. “The problem of land subsidence is a great example.”
The new research simulated 65 years of landslides near the town of Hanford in the San Joaquin Valley, which forms about the southern half of the vast Central Valley. The simulation was based on a combination of satellite data on land subsidence, records from private wells and water level measurements from the 1950s. The result is a physical model of the subsoil of the area, including layers of sediments and clays, which compact like a squeezed sponge when the water level drops.
The results suggest that the soil layers throughout the valley will compact, and thus sink for decades to centuries after the water level stabilizes. “This is a very important finding,” the authors write in the study. Sustainable management plans for this area have so far assumed that the mere stabilization of groundwater will prevent land subsidence. “But this assumption is wrong,” Knight said. The decline will only slow down if stocks in groundwater reservoirs from precipitation or controlled water replenishment projects start to exceed consumption.
The new modeling also shows that compaction in the deepest layer of the groundwater system is responsible for more than 90 percent of the declines in the study area over the last twenty years. “If it is essential to prevent soil subsidence, then it is much better to draw water from a shallow zone than from a deep one,” Lees added.
Although the study focuses on a relatively small area, the authors argue that the results are likely to be representative of all declining areas throughout the San Joaquin Valley and that their methodology is so versatile that it can be used to understand this phenomenon in similar systems throughout the world.