This is how rain affects Lake Mead water levels

LAS VEGAS (KLAS) —  Television and social media videos of raging floodwaters after Tropical Storm Hilary lead the public to ask, “Does all that water help?” The answer is complicated, but officials say Hilary’s direct effect on Lake Mead was “minor” and had more to do with reduced demand than anything else.

Rainfall doesn’t immediately add inches to the nation’s largest reservoir. Lake Mead is enormous, and even the storm that dumped 8-10 inches of rain at Mt. Charleston covered a much smaller area than the lake.

“We saw Mead’s water level tick up a couple tenths of an inch. Precipitation contributed to it, but releases from Lake Powell have a much greater impact on water levels,” Bronson Mack, spokesman for the Las Vegas Valley Water District, said Thursday.

The reduced demand for water during Hilary was far more significant than water gushing down the wash or pooling on roads in the Las Vegas valley, figures provided by Mack show.

“The biggest benefit of rainfall and storms is being able to reduce the amount of water used for outdoor irrigation,” Mack said. “This reduces water demands and eliminates irrigation when we already have Mother Nature doing it for us. This saves a large amount of water.”

“Sunday, Aug. 20, was the lowest day, but that’s a day when everyone turns off irrigation systems,” Mack said. “This also shows how much is used outdoors Monday through Saturday. That’s a 100-million-gallon swing from Saturday to Sunday when Saturday water use was already low.”

Mack said residents can leave sprinklers off through this week as soil moisture remains high. And monsoon moisture has been helping on that front, too. Sprinkler schedules change to three days a week in September.

Normally in summer, Mack said, the water district sees Sunday use about 130 million to 150 million gallons below Saturday use.

Lake levels

Over the past week (Thursday, Aug. 17 through Wednesday, Aug. 23), Lake Mead has risen about 9 inches (0.74 feet) to 1,064.09 feet above sea level. That’s a lot of water, adding about 18.8 billion gallons to the lake in just one week’s time. These calculations are based on daily averages posted on Reclamation’s website.

Graphics above show Lake Powell’s level on the decline (left) and Lake Mead’s increase (right).

The reason for the increase is really about the dam operations — water coming in and water going out. Lake Powell has been dropping since July 8, when the Bureau of Reclamation started letting more water flow downstream to Lake Mead. Lake Powell has released an average of 13,669 cubic feet per second (cfs) over the 7-day period, while Lake Mead released an average of 9,825 cfs. The result, of course, is that Lake Mead has been rising.

“Although every drop counts, the reality is that the rain we received from Tropical Storm Hilary and runoff into the tributaries that enter Lake Mead as well as reduced releases from Hoover Dam — due to a decrease in downstream demand — has had some minor impact on the lake’s elevation,” according to U.S. Bureau of Reclamation spokesman Doug Hendrix.

The Bureau of Reclamation cut releases from Hoover Dam by 30% due to decreased water demand in agricultural areas of Southern Arizona and Southern California, Hendrix said Thursday. If demand had not changed due to the storm, there likely would not have been any change to Lake Mead’s elevation since the local runoff was not enough to impact the reservoir significantly.

“While the amount of precipitation received from the recent storms in the lower basin and from tributary inflows helps, the greatest source of water for Lake Mead is still from snow melt and flows from the upper basin — especially from good winter snowpack from the west slope of the Rocky Mountains,” Hendrix said.

Climate questions

Dan McEvoy of the Desert Research Institute and the Western Regional Climate Center said Hilary brought a welcome break after a scorching July.

It’s been a wild weather ride this year, alternating from a wet winter for the Sierra Nevada and the Upper Colorado River Basin, then record heat in July, capped by Hilary’s impact on the Southwest U.S.

“The really big snowpack and the wet conditions … that was an anomaly, but the real surprise was that it stayed cold most of the winter just because we’ve seen so many warm winters,” McEvoy said.

McEvoy looks beyond the impact on Lake Mead to broader “human health impacts” that Hilary brought.

“Of course, July was record-setting in Vegas and around the Southwest. And so this storm, while it did bring beneficial rain and some drought relief, it brought several days of cloud cover and temperatures anywhere 10 to 20 degrees below normal,” McEvoy said. “After seeing the record-setting temperatures and heat in July, I think this was a big relief to the area just to have that extended cloud cover and cool temperatures and really reduce the heat impacts and the amount of water being used compared to if it was normal hot temperatures.”

Mack added that some water savings would come from evaporative cooling systems not having to work as hard due to the cooler temperatures, and less evaporation from pools with auto refills.

McEvoy and Mack agreed that rainfall would only have minimal impact.

McEvoy distinguishes between short-term and long-term drought.

“We have to think of the way that drought works in the West in the long term and particularly in the Colorado River Basin. We’re talking long term, we’re talking multiple decades going back to around 2000 where the water supply and the reservoir levels have been in decline. And so this individual event in the grand scheme of things is going to have very little impact on the long term drought,” McEvoy said.

The bigger picture

A needed perspective on Hilary’s deluge comes from Lorraine Garcia, communications manager for the Coachella Valley Water District (CVWD) in California. Garcia put it plainly: “It’s floodwater, not rain.” She said the immediate concern following Hilary was safety.

Raging floodwaters aren’t always an opportunity to add to water storage. Floods are dangerous, threatening lives and property.

As residents recover from Hilary’s impact along Interstate 10 in California — Palm Springs, Cathedral City and other communities in Garcia’s territory — photos show it’s a little early to be thinking about the positives of a very destructive storm. In time, conditions will get back to normal and there will be time to assess how the storm affected the region.

Canals funnel water to the Salton Sea in the Imperial Valley, Garcia said.

And if Las Vegas is fascinated by news about Lake Mead going up or down, Californians are just as tuned in to the subject of groundwater, according to Garcia. She said she fields a lot of questions about whether rain will help restore groundwater in the region.

Conservation concerns both states.

“CVWD always encourages conservation among all customers, even when the state is not in a drought emergency,” Garcia said. “During wet years, we store more imported water in the groundwater basin while continuing to work with customers to improve water efficiency and reduce water waste through conservation programs. CVWD is also investing in nonpotable and recycled water projects that further reduce demand on the basin, ensuring water for future generations.”