This is the third part of the Water Woes Series which describes how the Drought Conservation Plan (DCP) Shortages are implemented.
Lake Powell - Lake Mead Balancing During Tier 0 and Tier 1 Shortages
The volume of water in Lake Powell, the storage lake behind the Glen Canyon Dam in the Upper Basin of the Colorado River, is balanced with the volume in Lake Mead, the storage lake behind Hoover Dam in the Lower Basin of the of Colorado, via planned, controlled releases of water from Powell to Mead under the direction of the US Bureau of Reclamation (Reclamation).
The Bureau uses the lake levels to balance storage while assuring agreed-upon deliveries to those with Colorado River water rights in the US and Mexico. Lake Mead and Lake Powell are approximately equal in size. It is desirable operationally and politically to keep both lakes at equal capacity. So if Lake Powell has water available, and Lake Mead is low, Mead will be brought up to the same percentage of capacity as Powell.
On January 19, 2021, Lake Powell was at 3,579 ft, putting it in an operational tier of "Upper Elevation Balancing," and Lake Mead was at 1,085 ft, putting it in an operational tier of "Drought Contingency Plan (DCP) Tier 0." The figure below shows a high-level summary of how the balancing is done, and what the status was with respect to Lake Mead deliveries to Nevada, California, and Arizona as of the lake levels in January, 2021.
Note: "MAF" means millions of acre-ft where 1 acre-ft = 325,851 gallons.
The range of 7-9 MAF in the Upper Elevation Balancing Tier for Lake Powell means that throughout the year, releases in that range will be made to Lake Mead, depending on its level. The details of the exact release from Powell corresponding to various levels of Mead are omitted from the figure, but suffice it to say that when Lake Mead is "sufficiently high," then only 7 MAF will be released from Powell. On the other hand, if Lake Mead is "sufficiently low," then 9 MAF will be released from Powell, because Mead needs it more.
Lower Basin DCP Contributions To Lake Mead
The DCP Tier 0 status of Lake Mead means that deliveries to California, Nevada, and Arizona will be only 7.3 MAF instead of 7.5 MAF, a difference of 0.2 MAF, which equates to 200,000 (200 KAF). The figure below shows that the 200 KAF cut will be shared by Arizona (192 KAF), and Nevada (8 KAF). California will take no cuts. Mexico will take a 41 KAF cut due to a different agreement from the Lower Basin DCP (LBDCP).
Arizona's Tier 0 cut, which has been in force since 2019, comes from Other Excess and Agricultural Pool in that order as shown in the priority diagram below within the dotted magenta line. Arizona's Tier 1 cuts, currently projected for the next calendar year, are shown within the red dashed line. Note that Indian and M&I are not directly impacted until Tier 2 cuts which may come in 2023 and beyond.
For a wider perspective and more regional detail, a good recent (4/23/2021) article was written by Ian James at azcentral called "As a hotter, drier climate grips the Colorado River, water risks grow across the Southwest."
For a more local perspective, with details, see Tony Davis's 4/16/2021 azcentral article "First mandatory cutback of CAP water now likely in 2022." The source data for this article is the latest (4/15/2021) 24-month projection of Lake Mead and Lake Powell levels by the US Bureau of Reclamation.