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Relief from rainfall limited

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City able to divert, store only so much water

By Karl Moffatt
Las Vegas Optic

Despite recent heavy monsoon rains during the month of July, the city was not able to capture and store much of the runoff due to limitations of the water system and regulations governing use of Gallinas river water, city officials say.

“People keep coming up to me and saying, “We’re in great shape because of all that rain, que no?” said Las Vegas City Manager Tim Dodge. “Well, I’m sorry but that’s just not been the case.”

The city can capture and store only so much water due to the limitations of the water system, and even when there’s an abundance of water flowing in the river, state regulations require much of it to be passed through to downstream users.

From May to October, the city is required by the state to go to a one-week-on and one-week-off diversion schedule on the Gallinas River, explained Don Cole, the city’s water systems manager.

But that schedule does not apply if the river is running below 4 cubic feet a second (cfs) as it typically does.

Then the city is allowed to divert as much as it can, which is usually just enough to satisfy the city’s demand under current water restrictions.

However, if the river runs higher, as it does during the monsoon season, then the city must adhere to the schedule, and if it’s an “on” week, it can divert only 3 to 5 cfs, depending on the flow.

The rest is passed through to downstream users.

So any time a big rain occurs, such as on July 15, when the river rose from the previous day’s high of 3.3 cfs to 6.4 cfs, the city has to adhere to the schedule, and the very next day was off the river and required to stop drawing water for a full week.

And during those next seven days the river ran high on several days due to heavy monsoons, but the city could not capture and store any of it, due to the schedule.

“It’s complicated,” Cole notes.

To further complicate things there’s the following scenario where if the river reaches a point where it runs at or above 16 cfs, as it did for three days in late July, then all bets are off and the city can trap and store whatever water it can.

But the city’s water catchment and storage system can handle only so much flow before it’s overwhelmed, and that limit is 14 cfs. The city was able to capture only 7.2 million gallons from a big storm on July 26 that sent an average of 130,000 cfs downriver, which equals about 84 million gallons of water per second, Cole said.

The city is planning on expanding its ability to capture more water during these kinds of events,  with improvements to the system in the planning stages and funding in the budget.

Storrie Lake has not benefitted from the rains as much as one might have hoped, and the city has seen its storage there decline from about 66.5 million gallons at the end of June to about 64.8 million gallons by the end of July. Some of that is lost to evaporation computed to be about 2.5 percent, Cole said.

So the bottom line is that despite the rain, the city is still just breaking even on a routine basis and has only about 60 days of reserve supply on hand.

And while the rains have been above average in July by about an inch, the total for the year still lags behind by a couple of inches, said Jason Frazier, a meteorologist with the National Weather Service in Albuquerque.  

Las Vegas normally can expect about 16 inches of rainfall in a year, with a little more than half of that coming during the monsoon months of July, August and September, Frazier said.

The city received just over 4 inches of rain in July, about an inch more than normal and another 1.4 inches in August, but still lags behind expectations with total rainfall for the year standing at 6.39 inches, when it should be about 8.6 inches, Frazier said.

“So there’s some making up to be done with just over another month to go,” he said.

In the meantime, city officials urge residents to remain vigilant in the conservation efforts and to make use of the recycled waste water program to keep their trees and lawns green and thriving.

Note: Follow-up tests of the city water supply show the water is safe for human consumption with no contaminants having been found.