Lower #ColoradoRiver reservoir evaporation the focus of new analysis from the Southern #Nevada Water Authority — KUNC #COriver #aridification #CRWUA2022

The drought’s bathtub ring of Lake Mead at Hoover Dam May 2022. Photo: Don Barrett CC-BY-NC-ND-2.0

Click the link to read the article on the KUNC website (Luke Runyon). Here’s an excerpt:

An analysis compiled by the Southern Nevada Water Authority estimates the total amount of water lost in the river’s lower reaches. If implemented in its current form, the proposal would translate to significant cutbacks for users in Nevada, Arizona and California. The agency’s staff presented the analysis to representatives from the seven U.S. states that rely on the beleaguered Colorado River for drinking and irrigation water supply. Federal officials were also present at the Manhattan Beach, California meeting held in the third week of October. Farmers and cities in the river’s Lower Basin states of California, Arizona, and Nevada have never had to fully account for the amount of water lost to evaporation, or to leaky infrastructure, also called transit losses. About 1.5 million acre-feet of water is lost to evaporation and other losses each year, according to the Southern Nevada Water Authority analysis. That’s more water than the state of Utah uses from the river annually…

September 21, 1923, 9:00 a.m. — Colorado River at Lees Ferry. From right bank on line with Klohr’s house and gage house. Old “Dugway” or inclined gage shows to left of gage house. Gage height 11.05′, discharge 27,000 cfs. Lens 16, time =1/25, camera supported. Photo by G.C. Stevens of the USGS. Source: 1921-1937 Surface Water Records File, Colorado R. @ Lees Ferry, Laguna Niguel Federal Records Center, Accession No. 57-78-0006, Box 2 of 2 , Location No. MB053635.

The analysis examines where water loss occurs downstream of Lee’s Ferry in northern Arizona to the northern boundary of the U.S.-Mexico border. Both the U.S. and Mexico rely on the river. The analysis divides the river into five reaches, and includes the large reservoirs in the Lower Basin — Lake Mead, Lake Mohave and Lake Havasu. The analysis then calculates which states and which users within each state could be cut back to account for the overall basin-wide loss. Users upstream, like the Southern Nevada Water Authority, carry a lesser burden than those downstream, as users upstream are not reliant on downstream infrastructure and reservoirs to deliver their water supplies. Those users further downstream on the river, like California’s Imperial Irrigation District, would face the highest volume of potential cutbacks, factoring in their placement on the river and their volume of overall use, according to this analysis. There is no set standard to account for these losses, [Colby] Pellegrino said, and this initial analysis is meant to get the conversation started as a potential model for how to divvy up the cuts among users…

Using the Southern Nevada Water Authority’s methods, the river’s big users could be staring down significant cuts to their supplies to account for evaporative and transit loss. To achieve the total savings of 1.5 million acre-feet per year, the analysis assigns cutbacks of 509,508 acre-feet on the Imperial Irrigation District, 190,474 acre-feet on the Central Arizona Project system, and 110,464 acre-feet to the Metropolitan Water District of Southern California, with the rest being contributed by dozens of other smaller users. Mexico, which is able to store some of its river water in American reservoirs because of binational agreements, is by treaty not required to share in transit losses. But if the country were to share in additional reductions related to evaporation and transit loss, the country’s total could be 333,040 acre-feet per year when considering its total uses and its placement as the river’s final user, according to the analysis.

Upper Colorado River basins. (The border of Wyoming and Colorado is mislabeled.) (U.S. BOR)

Accounting for evaporation has become a rallying cry from users in the river’s Upper Basin states of Colorado, Wyoming, Utah and New Mexico and a tension point in ongoing negotiations. Those states already use a system to track losses and are charged for them in their basin-wide accounting. Upper Basin water managers say the current system is unfair.

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