#ColoradoRiver: @USBR runs the numbers for next winter #aridification

Credit: USBR

Here’s the release from US Bureau of Reclamation:

In addition to the April 2018 24-Month Study based on the Most Probable inflow scenario, Reclamation conducted model runs to determine a possible range of reservoir elevations under Probable Minimum and Probable Maximum inflow scenarios. The Probable Minimum inflow scenario reflects a dry hydrologic condition which statistically would be exceeded 90% of the time. The Most Probable inflow scenario reflects a median hydrologic condition which statistically would be exceeded 50% of the time. The Probable Maximum inflow scenario reflects a wet hydrologic condition which statistically would be exceeded 10% of the time. There is approximately an 80% probability that a future elevation will fall inside the range of the minimum and maximum inflow scenarios. There are possible inflow scenarios that would result in reservoir elevations falling outside the ranges indicated in these reports.

The projected Lake Mead elevations resulting from these three inflow scenarios are summarized in a graph located at the following link: https://www.usbr.gov/lc/region/g4000/24mo/2018/April-Chart.pdf.

The water year 2018 unregulated inflow into Lake Powell under the April Probable Minimum inflow scenario is 4.32 maf, or 40 percent of average. Consistent with the Interim Guidelines, the Most Probable 24-Month Study set an April adjustment to balancing releases at Glen Canyon Dam for the remainder of water year 2018. With the Probable Minimum inflow forecast, the Probable Minimum 24-Month Study results in a projected annual release volume from Glen Canyon Dam of 9.00 maf in water year 2018 and 8.81 maf in water year 2019.

From InkStain (John Fleck):

A new Bureau of Reclamation analysis puts some numbers to the fear – a credible risk that Lake Mead could drop to elevation 1,062 by the end of 2019, just 20 short months away.

This nice chart put together by the Metropolitan Water District of Southern California, part of Met’s Water Supply Conditions Report (pdf), nicely illustrates what’s been going on in recent years:

Credit: Metropolitan Water District of Southern California

There’s a point my friend and book-writing partner Eric Kuhn has been making that shows up nicely in this graph. We’ve had four consecutive decent years. From 2014 to 2017, we have’t been in “drought” (whatever that word even means any more). That string of relatively good years (or at least “not bad years”?) has enabled the 9 million acre foot per year releases that has so exercised the interbasin conflict between the Central Arizona Project and other basin water users. 9 million acre feet per year – well above the Law of the River-mandated 8.23 million acre foot release from Lake Powell – has bought time for negotiations over new management rules to reduce everyone’s demand on the system. But even with those big releases – the Upper Basin from 2014 to this year has delivered 2.3 million acre feet more than the Law of the River requires – Lake Mead has dropped 10 feet.

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