May 2012, Issue 60: Editors’ Notes

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How much difference does one very wet or dry year make? In terms of planning for future shortages, not much.

We know that variability is a constant in the West’s hydrologic cycle; the high highs and low lows we experience relegate runoff averages to mere statistical abstractions. The last 2 years, for example, have been a roller coaster in the Colorado River Basin: following last year’s near-record-high flows, the winter of 2011–2012 has been one of the driest on record.

Compare Reclamation’s April 1 runoff forecasts for 2011 and 2012. In early 2011, snowpack was approaching 300 percent in some parts of the Upper Basin and runoff predictions were 120 percent of normal. By the end of summer, actual runoff was close to 140 percent of average, surpassing these predictions. By the end of 2011, water levels in Lakes Mead and Powell had both risen about 50 feet from the lowest points in the previous year.

flow-forecasts_2011-2012

Just a year later, the situation is very different. Snow water content is now about 40 percent of average in the Upper Basin. Reclamation is predicting that April–July runoff will be less than 50 percent of average and that much of last year’s gains in Lake Powell will be gone by December. After this year’s runoff trickles in, the levels in both Lakes Mead and Powell in April 2013 are projected to be only about 15 feet higher than the lowest elevations of 2011.

So how do these extremes affect shortage planning? Apparently, not much. After the prolonged drought of 2000–2010, one extremely wet year only pushed shortage predictions back by a few years. In 2011, CAP was preparing for shortages as early as 2013; in a recent interview with the Arizona Daily Star, Tom McCann, CAP’s Assistant General Manager of Operations, Planning, and Engineering, predicted 2017 or 2018 as the earliest chance for shortage.

Given Reclamation’s latest projections, one very wet or very dry year has limited impact on reservoir levels in a system as large as the Colorado. It takes a long time to deplete and refill these reservoirs. Although the high runoff of 2010–2011 gave us a few extra years, Arizona will still need to continue planning efforts for recovery and other shortage-related projects.

Juliet M. McKenna, MS, PGTaylor D. Shipman, MSMichele Robertson, PG