Reclamation’s latest predictions show the chance of shortage on the Colorado River increasing but remaining less than 50 percent in 2016.
Reclamation’s April 2014 projections predict the likelihood of Colorado River shortage based on various modeled inflow scenarios. These projections show Lake Mead dropping to 1,069 feet in June 2015. However, a declaration is triggered only if the lake is predicted to drop below the official shortage elevation of 1,075 feet at the end of the year.
Lake Mead’s elevation is regulated by releases upstream from Lake Powell, which are ultimately determined by annual runoff (inflow). Among the inflow scenarios modeled by Reclamation are “most probable” and “probable maximum.” The most probable scenario reflects a median hydrologic condition that statistically would be exceeded 50 percent of the time. Under this scenario, the predicted elevation for Lake Mead in December 2015 is 1078.8 feet — 3.8 feet higher than the shortage trigger.
The difference between the most probable and the probable maximum inflow for Lake Powell — 2.7 MAF in 2014 (11.1 vs. 13.8 MAF) — may seem significant; however, this volume only amounts to a difference of about 1.4 feet (1084.7 vs. 1086.1 feet msl) of lake elevation at year end. This relatively muted response demonstrates that we need several back-to-back years of above-average runoff to make a meaningful dent in dwindling reservoir elevations and stave off the near-term possibility of shortage.
Curiously, the 1,075-foot marker is not labeled on Reclamation’s graph. Standard issue from the agency’s monthly “24-Month Study” updates, these graphs have previously shown this elevation marker. The increasing likelihood of shortage is a critical message for Arizonans — one that should be broadcast, not buried, by Reclamation and state water managers. Let’s hope it was just an oversight and we are not burying our head in the sand.