How California Reservoir Water Levels Changed After Hilary

Many of California's reservoir water levels are above their historical average only days after Tropical Storm Hilary battered the state with heavy rainfall.

The tropical storm landed in California on Sunday after hitting the Mexican peninsula of Baja California over the weekend. The storm caused substantial flooding in Los Angeles before moving further inland, where overwhelming flooding hit the desert city of Palm Springs. Death Valley National Park—which usually receives about two inches of rain per year—closed Sunday due to flooding.

Some drought-stricken areas such as Lake Mead in Nevada and Arizona benefitted from the onslaught of rain, but many of California's reservoirs' water levels were already above their historical averages after a wet winter and 14 atmospheric rivers—a long, narrow band of water vapor—dumped massive amounts of rain on the state.

In past years, Tropical Storm Hilary's rainfall predictions would be welcome news, especially as California suffered from a years-long drought. However, last winter already supplemented many of the state's parched lakes and reservoirs and eased the state's devastating drought.

AccuWeather chief meteorologist Jonathan Porter previously told Newsweek that few, if any, lakes in the area need help from the rain. A map of California's largest reservoirs reiterated that fact, with most of the reservoirs already operating at above-average levels before the storm.

The map, which was compiled by the California Department of Water Resources, showed that Tropical Storm Hilary didn't add much to the reservoirs' water levels. Southern California was hit with the brunt of the storm, and despite the full reservoirs in that region, water levels have mostly remained stable or declined.

"A lot of the reservoirs are pretty full as it is," AccuWeather Senior Meteorologist Tom Kines told Newsweek. "When they're really low and you get a lot of rain, they tend to rise pretty quickly."

Castaic Lake, located 45 miles north of downtown Los Angeles, is operating at 93 percent full, which is 110 percent of its historical average. The chart shows that the lake has leveled off since its severe rise in the spring, and water levels have since started a slow decline.

Cachuma Lake in Santa Barbara County tells a similar story. The lake is operating at 137 percent of its historical average, according to the data, but the lake has since leveled off since its steep increase in the spring. Kines said that although Cachuma Lake is in southern California, it didn't receive as much rain during the storm as other southern California reservoirs.

However, Diamond Valley Lake in Hemet, California, is operating at 106 percent of its historical average and water levels are still rising. The lake is at 79 percent of its total capacity.

A graph from the California Department of Water Resources shows that Diamond Valley Lake's rise in the spring months wasn't as steep as other reservoirs in the state. The lake has steadily been rising ever since, although it didn't appear that Tropical Storm Hilary had a substantial impact on the lake's water levels.

Kines said rain still impacted the reservoirs, although not as severely as if the water levels had been low.

He added that it could take several days for Tropical Storm Hilary's influence to be reflected in the water level charts, given the rain that falls at higher elevations and then makes its way to the reservoirs.

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