Measuring in the region between the surface and underground aquifers hasn't been easy or accurate. Now a seismic approach that uses human generated sound as a source -- traffic noise.
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In the wake of the 2019 magnitude 7.2 earthquake in Ridgecrest, California, Zhan set up a DAS array on a nearby cable to measure aftershocks. In collaboration with Fu, the team quickly realized that the array could also be used to measure how everyday underground vibrations change depending on soil water content. Over five years, the team collected data and created models to illustrate how moisture in the vadose zone varies over time. They found that during the historic drought in California from 2019 to 2022, moisture in the vadose zone decreased significantly at a rate of 0.25 meters per year, exceeding the mean average precipitation.
"From the top 20 meters of soil in the Ridgecrest region, we can extrapolate to the entire Mojave desert," says Yan Yang, a graduate student in geophysics and co-first author of the study. "Our rough estimation is that every year, the Mojave vadose zone loses an amount of water equivalent to the Hoover Dam. Over the drought years of 2019 through 2022, the vadose zone has been drier and drier."
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