River Downer

(Editor’s note: This is the second part of a three-part series. In Part 1, Oro Valley Water Utility Director Peter Abraham discussed: What triggers a supply shortage? How much will the Tier 1 supply shortage be and who will it affect? (Nov. 24). Here in Part 2, Abraham answers the questions: How did we end up in a supply shortage situation to begin with? At what point does the water surface elevation at Lake Mead stabilize?)



n the early years of the 20th century, the Southwest had dreams of big growth, and that growth was tied to taking water from the Colorado River that meandered through the mountains, canyons and plains of the Southwest. 

In the early 1920s, policy makers of the seven basin states (Arizona, California, Colorado, Nevada, New Mexico, Utah and Wyoming) and the federal government developed a way to share the water resources provided by the Colorado River.  The result of this work became known as the Colorado River Compact of 1922.

Dividing up the water first meant measuring the water.

The policy makers of the time had about 20 years of stream gauge data. This data was used to divide the waters of the Colorado River among the seven basin states. Unfortunately, current historical climatology science has revealed to us that the original 20 year data that was the basis of dividing up the Colorado River resources happened to be an unusually wet period for the West. Because of this, the Colorado River had been over-allocated from the beginning. 

It is estimated that the Colorado River is over-allocated by approximately 1.2 million acre-feet per year. (An acre-foot is equal to 325,851 gallons.)

This over-allocation has only recently become a problem because of the region’s 20-plus year drought. In addition, those users with an allocation of Colorado River water now have the infrastructure in place to take delivery of their Colorado River water allocation. As a result, the water surface elevation of Lake Mead has been dropping steadily for nearly 20 years. As discussed last week, it is the water surface elevations at Lake Mead that determines the availability of Colorado River water to the Lower Basin States (Arizona, California and Nevada).

In an effort to close the gap on the over-allocation and mitigate the declining water surface elevations of Lake Mead, the U.S. Bureau of Reclamation approved the implementation of the 2019 Lower Basin Drought Contingency Plan (LBDCP). This plan requires the lower basin states to take reductions in the delivery of Colorado River water at specific water surface elevations of Lake Mead, known as tiers, to stabilize the water surface elevation.

As mentioned in last week’s article, the U.S. Bureau of Reclamation is predicting the water surface elevation of Lake Mead will fall low enough to trigger a Tier 1 shortage declaration. A reduction of Colorado River water deliveries will begin in January 2022. 

A Tier 1 shortage reduces the demand on the Colorado River by 713,000 acre-feet per year. While this helps slow further declines of the water surface elevation of Lake Mead, it is not enough to offset the current over-allocation. If the demands on the Colorado River water system is left unchanged and the drought conditions continue, water professionals expect the water surface elevation of Lake Mead to continue to drop and a Tier 2 water shortage be declared.

A Tier 2 shortage reduces the demand on the Colorado River by 1,288,000 acre-feet per year. It is at this point that we expect to see the closing of the over-allocation gap and the water surface elevations of Lake Mead stabilize.

A Tier 2 shortage reduces Arizona’s 2.8 million acre-feet per year allocation of Colorado River water by 640,000 acre-feet per year, all of which comes from the Central Arizona Project (CAP). A 640,000 acre-feet per year reduction in CAP eliminates all the CAP water that was available for agriculture. Higher priority CAP water users like Oro Valley would see no reduction in deliveries during a Tier 2 shortage.

If the Tier 2 shortage is not enough to mitigate the declining water surface elevation of Lake Mead, a Tier 3 shortage could be declared. Next week, I will present Part 3 and explain how a Tier 3 shortage declaration affects Arizona and Oro Valley as well as what is being done to mitigate potential CAP supply shortages to municipalities like Oro Valley. Stay tuned!


Peter Abraham is director of the Oro Valley Water Utility.

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