It sounded too good to be true โ an official forecast that 2016 water use in Arizona, California and Nevada will be the lowest since 1992.
That forecast from the U.S. Bureau of Reclamation was too good to be true โ by the bureauโs own admission. It was widely reported recently as a sign of major progress toward conservation. But what the bureau calls its more accurate forecast, while still showing progress, is significantly higher, predicting water use in the states will be its lowest in 11 years โ not 24.
For those three Lower Colorado River Basin states, the bureau predicts their 2016 total use will be 7.29 million acre-feet of water. Thatโs enough to serve about twice that many homes with water for a year.
The forecast comes from the bureauโs monthly study that looks up to 24 months ahead to predict how high Lake Mead and other reservoirs will be in a given month.
Itโs still good news, the bureau said โ about 200,000 acre-feet less water out of Lake Mead this year than the states have the right to take. Itโs also about 150,000 acre-feet less than the states used in 2015.
Thatโs due to conservation efforts by the three states, bureau officials say. Water officials across the West say the states need to conserve more to keep Lake Mead from dropping so low as to require severe cutbacks in water deliveries, including to Arizonaโs $4 billion Central Arizona Project that delivers Tucsonโs drinking water. The lake already has fallen more than 100 feet since 1999 due to drought and a chronic โstructural deficitโ in which use exceeds supplies even in years of normal runoff.
The other bureau forecast, updated daily, has predicted since late July that the three states will use 6.9 million to almost 7 million acre-feet this year. Thatโs around 500,000 acre-feet less than they have the right to take.
The last year the three states used less than 7 million was 1992 โ the final year of George H.W. Bushโs presidency. This lower forecast made news in the Las Vegas Review Journal and on the water news website Circle of Blue as a sign of how much progress the three states are making in conservation.
In a water-oriented blog, Albuquerque-based author John Fleck wrote that the 6.9 million forecast illustrated a central point of his new book, โWater is for Fighting Overโ: That โwhen people have less water, they use less water โ that we have the ability to significantly reduce our water use in the arid West and still thrive, and that we are in fact already making significant progress.โ
But when asked by the Star why the bureau predicted such a dramatic decrease, bureau officials said recently that this forecast is likely to be wrong.
The discrepancy stems from the fact that the agencyโs two forecasts have different purposes and rely on different assumptions.
The higher-use forecast comes from a study the bureau uses to predict whether it will have to cut water deliveries to states the following year. That study relies on computer models to project future reservoir conditions and potential dam operations. Those models rely on a range of factors, including existing reservoir conditions and forecasts of how much water will flow into the reservoirs and how much the states will take out of them.
The other forecast helps the bureauโs staff and water users monitor projected water diversions and use to avoid โoverruns,โ or the diversion of more water than theyโre allowed to take, said Paul Matuska, water accounting and verifications manager in the bureauโs Boulder City, Nevada, office. Itโs based on the amount of water that users order โ and that the bureau approves โ to be diverted from the river to farms and cities in the three states.
But while the bureau considers the 24-month study forecast more accurate, itโs not planning to abandon the daily forecasts. The two forecasts differ slightly during the year, but theyโll dovetail by yearโs end, said Rose Davis, a bureau spokeswoman.
โThe two forecasts begin with the same assumptions of water use for the year, but vary in how the changes in water use are recorded or observed,โ Davis said.
Some Lower Basin users are interested in both sets of data and understand their complexities, she said, although the information can seem confusing at times.
