Lower Colorado Basin water savings not as big as I thought

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So I need to correct something that I wrote a month ago.

Tony Davis took a deep dive into the Bureau of Reclamation’s data and concluded in a story published this morning (I think accurately) that water conservation savings in the Lower Colorado River Basin will not be as large as I and others have been reporting:

It sounded too good to be true — 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 — by the bureau’s 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 — not 24.

The difference lies in the fact that there are two different water use accounting systems one can look at – the formal “forecast”, which is based on official water orders at the time the forecast is made (pdf here) and the operational plans included in the Bureau’s “24-month study“, which comes out monthly and projects water accounting balances in the reservoirs and among major users out for the next 24 months.

MWD forecast of Colorado River water use

MWD forecast of Colorado River water use

In the 24-month study, you can see that the big difference between the two is the Metropolitan Water District of Southern California. In the “forecast”, Met’s listed as taking 766,000 acre feet this year, but in the 24-month it looks like for planning purposes the Bureau expects Met to take more like a million acre feet. You can see the evolution of Met’s expected water use in this graph from the forecast report. This is the forecast for annual water use as it changes over the course of the year.

CAP forecast water use

CAP forecast water use

On the flip side of this, Arizona’s use of water from the Central Arizona Project has consistently been below the projections used in the original forecast, the result of aggressive conservation efforts in that state. (The y-axis scaling in these graphs makes comparison not exactly easy, looking closely at the numbers.)

So the bottom line: Lower Colorado River water use is currently likely to be the lowest since the second Bush administration, not the first one. Still progress, but not as much as I had hoped.

Thanks to Tony Davis for looking more deeply into this.