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Lately, the media outlets have been filled with reports and editorials about the continued drought in the Southwest States and the imminent Colorado River water shortage. We will explore and answer each of the following questions in a three-part series, with the focus being on Arizona, Oro Valley and our position with respect to Colorado River water availability.

This week, we look at: What triggers a supply shortage? How much will the Tier 1 supply shortage be and who will it affect?

In Part 2 next week, we will examine: 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?

In Part 3, in two weeks, we will explore: What is being done to mitigate the supply shortage? What does our future use of Colorado River water look like?

Availability of Colorado River water to Arizona, California and Nevada (Lower Basin) is determined by the water surface elevation of Lake Mead at Hoover Dam. Reductions of Colorado River water to users is predetermined according to the U.S. Bureau of Reclamations Drought Contingency Plan (DCP). This plan calls for increasing restrictions (curtailments) of Colorado River water deliveries to users as the water surface elevation in Lake Mead declines. There are predetermined trigger points based on water surface elevations that are referred to as tiers.

In August, the U.S. Bureau of Reclamation published the results of its modeling simulations which predicted the water surface elevation of Lake Mead at Hoover Dam would fall below an elevation of 1,075 feet in January 2022, triggering a Tier 1 shortage. Because of this, a reduction of Colorado River water deliveries will begin in January 2022.

A Tier 1 shortage results in an estimated 30% reduction to Colorado River supplies delivered by the Central Arizona Project (CAP) to water users in Maricopa, Pinal and Pima Counties. 

CAP has a longstanding priority system that governs who receives Colorado River water in times of shortage. Central Arizona agricultural users have a lower CAP priority and therefore will feel the impacts of any Tier 1 shortage reductions first. Municipalities and tribes have a higher CAP priority and would feel the impacts of shortages last. Because of this priority system, agricultural users will face cuts under a Tier 1 shortage, but municipalities like Oro Valley, will not face CAP water delivery reductions under Tier 1.

Under the current DCP municipalities, like Oro Valley, would not see any reductions in CAP water deliveries until a Tier 3 shortage was declared. This would happen if the water surface elevation of Lake Mead at Hoover Dam were to fall below an elevation of 1,025 feet.

The infographic shows the relationship between the water surface elevation of Lake Mead, a Tier 1 shortage and which users are affected.

Next week we will present Part Two of this series and explain how we ended up in a supply shortage as well as explain where we believe the water surface elevation at Lake Mead will stabilize. Stay tuned!


Peter Abraham is the Water Utility Director for the Town of Oro Valley.

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