According to the latest U.S. Drought Monitor (July 11), less than 32 percent of the state faces drought conditions and only 1 percent of the state is experiencing “severe” drought. Nowhere in the state are we experiencing “exceptional” or “extreme” drought. Governor Jerry Brown ended the drought state of emergency in most of California on April 7.
A year ago, more than 90 percent of the state was in some form of drought. The drought’s end comes thanks in large part to so-called atmospheric rivers (AR) — warm weather systems that flow east from Hawaii and the western Pacific. They carry huge amounts of moisture and provide the majority of California’s water.
Put simply, an atmospheric river is a thin, but long plume of moisture in the atmosphere that stretches from the tropics or subtropics into higher latitudes. The term was first used in a 1994 research paper and is now widely used by meteorologists.
A more recent study published in August 2015 by the University of Reading and University of Iowa questions whether the term "atmospheric river" is misleading. The study argues that the AR may not be so much a moisture pipeline from the tropics or subtropics, as previous conceptual models, but rather simply a "footprint" of moisture ahead of the cold front. One type of AR you've probably heard of is the "Pineapple Express," a moisture band lined up from near Hawaii to the U.S. West Coast ahead of a strong Pacific cold front at least once or twice each fall, winter or spring.
Regardless of how they're formed or maintained, these atmospheric rivers contain an incredible amount of moisture. According to a 1998 MIT study, it was estimated that the moisture flux of an AR is roughly the same as that of the Amazon River, about 176,000 tons of water per second.
What are the benefits of studying Atmospheric Rivers?
The community of flood control, water supply and reservoir operators of the West Coast states see ARs as a key phenomenon to understand, monitor and predict as they work to mitigate the risks of major flood events, while maintaining adequate water supply. The frequency and strength of AR events in a given region over the course of a typical west-coast wet season greatly influences the fate of droughts, floods, and many key human endeavors and ecosystems. Better coupling of climate forecasts with seasonal weather forecasts of ARs can improve water management decisions. Long-term monitoring using satellite measurements will enable society to be more resilient to storms and droughts, while protecting our critical ecosystems.
Predicting the Weather
Despite great scientific advances in the field, meteorologists still cannot make accurate long-range weather predictions. That’s why no one predicted this winter’s Great Deluge. But the big picture is becoming increasingly well understood. Because climate change increases the amount of water vapor in the atmosphere, it increases the likelihood that atmospheric rivers will be wetter and turn into rain, not snow, when they make landfall. There is also some evidence, though not yet conclusive, that climate change will increase the frequency of atmospheric rivers. So as we continue to heat the planet, California can look forward to being inundated by warmer and wetter storms, as well as the possibility of more of them.
But if we’re likely to get more rain in the future, why do experts also keep saying that we have to worry about chronic drought? Laura Feinstein, a senior research associate at the Pacific Institute, explains, “It’s a question of ‘How do you define drought?’ We live on the surface of the earth, so we tend to be sensitive to how much water there is at the surface, but there is a groundwater drought going on.” For decades before the drought, we drew more water out of the ground, especially in the Central Valley, than we replaced—and in the past five years, farmers in that region have pumped so much groundwater that levels have dropped precipitously. “Even though we’re getting so much rain this season, our groundwater is not even close to recovering from these past five years of over-drafting,” Feinstein says. “The water comes up in the wet years, but never as much as it went down in the preceding years. The overall march is downwards.” Feinstein points out that as of February 7, there were 1,185 verified active well outages in California—meaning that thousands of people, mostly in Tulare County, south of Fresno, are getting nothing when they open their taps and have to drink shipped-in water.
The other reason for concern, most experts agree on, is our threatened snowpack. California’s water system requires both rain and snow in order to function properly. We get 30 percent of our water from snowpack. But despite this winter’s near-record dumping in the High Sierra, the snow part of the equation is inexorably shrinking, both because warmer storms drop more rain and less snow and because warmer temperatures cause the snowpack to melt sooner. “Snowmelt is what carries us through our long, hot, dry summers,” Feinstein says. “When the snow melts faster, we’re going to have a hard time making it through our dry periods.” She notes that the state actually had an average winter snowpack in 2015-16, but that by late spring it was far below average. That pattern is expected to continue.
To stretch our water supply, Feinstein says, we need to both replenish our groundwater and reduce demand. Creating new floodplains in the Central Valley and capturing urban water runoff by creating permeable strips next to roads would add to the supply. A Pacific Institute study found that capturing water runoff in the Bay and L.A. areas alone could increase California’s water supply by as much as 630,000 acre-feet each year, enough to meet Los Angeles’s annual needs. Increasing our groundwater storage would be six times cheaper than building new reservoirs and would also bank a lot more water: The capacity of California’s 515 groundwater basins, estimated to be as high as 1.3 billion acre-feet, dwarfs the 50-million-acre-feet capacity of our reservoirs. On the demand side, installing low-flow toilets and using gray water would have a significant impact.
Understanding that we should recharge our groundwater banks and operating them conjunctively with surface water reservoirs is critical to our water future. But doing so may prove difficult. California’s reservoir system is set up to work with a colder climate than the one that now exists. We have an inadequate delivery system to take water from across California and deliver it to underground aquifers, and we don’t currently have adequate laws controlling who pumps groundwater. (Gov. Brown has signed a three-bill package, the Sustainable Groundwater Management Act, that when implemented will address both of these concerns.) California is awash in water right now. But it needs to move quickly to bank a large portion of its’ precious resource in the safest place there is: the ground. If it doesn’t, the benefits of wet winters like this year’s will be rapidly sucked dry.