Atmospheric rivers won’t end California’s drought

Even after six atmospheric river driven storms, a majority of the state is still in a drought that began three years ago. Why rain alone doesn’t solve dry conditions has much to do with what happens to that rain once it falls and how climate change is disrupting that cycle.

In the 2021–22 rainy season, bomb cyclones from the Pacific brought an end to the wildfire season with their moisture rich pressure systems. Snowpack along the eastern parts of the state rose to higher than average levels. Then January 2022 came, and that was the end of the rain. What water remained dried up rapidly in a spring heat wave, intensifying the drought.

With three more atmospheric rivers on the horizon for California, the question is whether 2023 will face a similar dry spell.

Research into climate’s role in the extreme wet and dry conditions last season is part of the work by Daniel McEvoy at the Desert Research Institute in Nevada. A recent study showed how climate change not only reduces snowpacks, but how extreme temperatures in the spring intensify the drought heading into the summer months.

McEvoy puts it this way: “Let’s say this wet year continues all the way to March and we have a 200 percent average snowpack … that might feel like the drought is as almost over — but that’s not always the case simply because we’re having warmer summers that drying things out much faster.” Drought is a long-term trend California faces. Should we think about it in the same ways we think about Colorado River’s historic drought? That is a question worth considering, says McEvoy.

Usually snowpack means bankable water sources for nearby counties that depend on it in the warmer parts of the year. But when the land is persistently dry, runoff from melting ice and snow goes to dampening the soil first before trickling down to reservoirs. Sudden high heat evaporates the snow too, moving it away from the area through the atmosphere.

California’s water supply in periods of drought relies heavily on stored water in reservoirs, underground, and across state lines. Not all rainwater makes it, though, to collection.

Jeanine Jones, the interstate resources manager at the California Department of Water Resources, explains that “on average in California, we get about 200 million acre feet a year of precipitation. But that 200 million acre feet only amounts to 70-something million acre feet of runoff on average.” Jones recognizes that we almost never get an average year, but the idea is that the precipitation is lost to evaporation, plants and soil.

In an average year, the residents and businesses use up roughly a quarter of reservoir water in the dry months. Since the beginning of the 2020 drought, overall reservoir levels have declined by a third. Recent rainfalls improved the situation somewhat, but the rain comes in at such a rapid rate that reservoirs small and large must release water to mitigate flooding. Rain that is not making it to capture in the first place is behind the flooding rivers and towns along the Pacific coast.

It matters too where these atmospheric river storms are “aimed,” says Alison Bridger, department chair of meteorology and climate science at San José State University. In an email, Bridger says she has been watching Lake Shasta’s levels gain, but they are still sitting below historical averages. “The storms we’ve experienced since Boxing Day have mostly been aimed at ‘central’ California … What would be good is if the next few storms were more focused on northern CA.” It would be nice if the state’s largest reservoir got some of these flooding rain waters.

As with snowpack, we won’t know how much these gains will improve water resources until March or April, when water demand begins drawing from local reservoirs again. In previous years, large reservoirs like Lake Shasta and Oroville Dam see a 30 percent draw on their levels. The rains in these past few weeks improved levels to 44 percent and 49 percent of their total capacity respectively.

Jones says, “if you think about what happened last year, we were in great shape at the start of the new year … and then everything completely shut down for the rest of the season … But we’re certainly not out of the woods yet by any means.” If the summer months behave like previous ones, Californians will be right back where they started: in a water deficit.

Marty Ralph, director of the Center for Western Weather and Water Extremes, notes that Oroville and Shasta reservoirs are still below their average for this time of year, but he is encouraged by the snowpack along the Sierras. “It’s a different story, of course, if [the snow] shuts down for the rest of the winter,” he says. California’s water management system is designed with awareness that snow in the mountains is a part of supply acts as a natural reservoir.

Depending on one extreme weather event of heavy rain and inundation to counteract the shorter rain season, warmer springs and longer wildfire seasons is not the solution the multifaceted ways the southwest faces climate change driven megadroughts. Persistent drought, whether briefly paused by atmospheric rivers, is a function of a world now shaped by climate change.

“What happens between now and the summer is very important,” Ralph says.

While improving water capture is certainly an adaptation that’s possible, it is a human-centered solution to man-made problems. One dimension of the California drought is how much of this runoff can be stored for residents and agriculture. The other is how much is left for the land.