By George B. Kauffman
According to a study by Jiangfeng Wei, Research Engineering Scientist Associate IV, in the School of Geological Sciences in Austin, Texas; Qinjian Jin, Postdoctoral Research Associate at the Massachusetts Institute of Technology in Cambridge, Massachusetts; Zong-Liang Yang, a professor in the Jackson School of Geological Sciences in Austin, Texas; and Paul Dirmeyer, a professor at George Mason University in Fairfax, Virginia, published in Geophysical Research Letters, a journal of the American Geophysical Union, on June 30, California droughts are primarily controlled by wind rather than by the amount of evaporated moisture in the air.
The research increases our understanding of how the water cycle is related to extreme events and could eventually help in predicting droughts and floods. Wei said, “Ocean evaporation provides moisture for California precipitation but is not the reason for droughts there, although the ocean evaporation is slightly lower during droughts,” Wei said.
The scientists analyzed 30-year data sets recording precipitation, ocean evaporation, surface wind speed, and atmospheric pressure on and near the west coast of the United States, all factors that influence the water cycle in California. Wei admitted that one of the difficulties of studying the water cycle is that the water sources for precipitation cannot be directly observed so the team also used a mathematical moisture-tracking method and high-resolution model simulations.
The team’s analysis showed that moisture evaporated from the Pacific Ocean is the major source for California precipitation, but the amount of water evaporated did not strongly influence precipitation in California, except in cases of very heavy flooding. This is because the amount of water evaporated from this ocean region does not change much year by year and did not cause rain to occur more or less often. Instead, the scientists found that disturbances in atmospheric circulation, the large-scale movement of air, have the most effect on drought.
Most of California has been in a severe drought since 2011, but a strong El Niño in the winter of 2015 helped diminish the drought. The present drought is caused by a high-pressure system that disturbs the atmospheric circulation. The development of the high-pressure system is related to a sea surface temperature pattern in the Pacific Ocean. Although this is a very rare event, the probability of this kind of high-pressure system is likely increasing with global warming. Yang said that the research could help predict droughts and floods by improving scientific understanding of the intricate factors that influence rainfall the most.
According to Yang, “The topic is extremely timely as current and future climate change would mean more changes in extreme events such as droughts and floods. Understanding this asymmetric contribution of ocean evaporation to drought and flooding in California will ultimately help us make better predictions.”
To combat the drought California is pumping billions of gallons of water from the ground, but this depletes natural water reserves faster than they can be replenished. A recent controversial study published in the Proceedings of the National Academy of Sciences suggests that the state may hold more water in very deep aquifers than previously thought. The study is the first to measure California’s groundwater below 300 meters. Satellite images can reveal changes in an aquifer’s water storage but not the total volume of water that it contains. Direct samples, on the other hand, are costly and difficult to obtain.
Therefore the scientists turned to public records. They analyzed salinity data from 360 oil and gas fields as well as the region’s geology to extrapolate the volume of water in aquifers beneath our Central Valley down to 5,000 meters. They found that the aquifers might hold three or four times the amount of fresh water than previously thought. However, as much as 35 percent of this could be vulnerable to oil and gas development, and much of this water is so deep that it is not practical to extract.
For additional information see Jiangfeng Wei et al., “Role of ocean evaporation in California droughts and floods,” Geophysical Research Letters (2016). DOI: 10.1002/2016GL069386 and http://phys.org/news/2016-07-california-droughts moisture.html#jCp.
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George B. Kauffman, Ph.D., chemistry professor emeritus at Fresno State and Guggenheim Fellow, is a recipient of the American Chemical Society’s George C. Pimentel Award in Chemical Education, the Helen M. Free Award for Public Outreach and the Award for Research at an Undergraduate Institution, and numerous domestic and international honors. In 2002 and 2011, he was appointed a Fellow of the American Association for the Advancement of Science and the American Chemical Society, respectively.
