Projections on inflow into Lake Powell in the coming “water season”: they’ve looked better

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Projections on inflow into Lake Powell in the coming “water season”: they’ve looked better
August 10, 2018

The Colorado Basin River Forecast Center has released its August- projections for unregulated runoff into Lake Powell for Water Year 2019, and, in keeping with recent drought-inspired trends, the outlook isn’t pretty.

The Forecast Center -- which provides Colorado River data to the federal Bureau of Reclamation for its crucial August 24-Month Study -- foresees an inflow of 8.1 million acre-feet of runoff into Lake Powell. If accurate, that “most probable” inflow represents about 75 percent of the 30-year average of inflow into the great Colorado River system reservoir.

That estimate of 8.1 million acre-feet is down 100,000 acre-feet from the Center’s estimate of a month ago when its modeling projected 8.2 million acre-feet.

The skill level on such longer-range projections is generally low, given the lengthy time period involved and the wide number of variables.

Not the least of those variables is the uncertainty of the region’s upcoming snowpack-season and resulting runoff. The Forecast Center attempts to incorporate a range of possibilities for the coming season’s climatic conditions.

Predictably, snowpack is a big factor in those range of possibilities. But soil-moisture conditions also factor in heavily. And, soil-moisture conditions going into the approaching snowpack season are expected to be drier than normal.

In addition to its “most probable” projection, the Center’s modeling projects a “minimum probable” inflow of 4.8 million acre-feet (44 percent of average), and a “maximum probable” projection of 15.6 million acre-feet (144 percent of average).

A Water Year runs from October 1 of each year to September 31 of the following year.

The unregulated runoff modeling projections come on the eve of the Bureau of Reclamation’s much-anticipated August 2018 24-Month Study.

The August projections are used by the Bureau and the Lower Basin States, among other things, to determine whether Lake Mead may fall to levels that could trigger a shortage declaration.

Last month’s 24-Month Study projected Lake Mead’s end-of-2018 elevation to be just above 1,077 feet.  While the troubled reservoir’s water levels have inched downward toward the shortage-triggering level – that being an elevation below 1,075 feet – previous modeling results indicate that Lake Mead should remain above that critical level for Water Year 2019.

The modeling, which employs simulations using the Colorado River Simulation System, or CRSS, is updated and maintained on a continuous basis by the Bureau of Reclamation’s Upper and Lower Colorado Regions.

Conservation efforts have had a great impact on keeping Lake Mead from falling into a “Tier 1” shortage.

In 2017 alone, over seven feet of elevation was added to bolster Lake Mead by its water suppliers and users in the Lower Basin States.