3 facts of CAP efficiency: seepage and evaporation

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New Waddell Dam Lake Pleasant inlet towers

Maybe it was one of the first wonders of science you learned. Water continuously moves through our environment through processes of evaporation, transpiration, precipitation, runoff, infiltration and accumulation.

Feeling nostalgic?

We know the first step in this hydrologic cycle as evaporation.

CAP reliably delivers Colorado River water to central and southern Arizona, and in the process, some water is “lost” to seepage and evaporation. Just how much?

Perhaps less than you might think.

Water system managers refer to the water they had in their system, but don’t end up delivering to customers, as system losses. Most utilities focus significant time and resources reducing water losses through improved efficiency.

Here are three facts about CAP efficiency:

  1. Total system loss is nominal

CAP’s average annual system loss (system loss = seepage + evaporation) during the past 20 years is  approximately 4.5 percent of Colorado River diversions due to CAP’s canal design, construction, and efficient operation methods. CAP’s system includes the aqueduct and Lake Pleasant.

  1. Lake Pleasant annual loss: about 3% of annual diversions

The math is elementary: more surface area equals more evaporation, right? As of the publishing date of this article, Lake Pleasant was at 7,755 acres of surface area and 66-percent full. Lake Pleasant, CAP’s storage reservoir that provides the flexibility to balance Colorado River supply diversions and customer deliveries, has an annual loss of around 3 percent. Consequently, about two-thirds of CAP’s annual system loss occurs at the lake just northwest of Phoenix.

  1. CAP canal annual loss: about 1.5% of annual diversions

The 336-mile CAP canal is a relatively narrow passage. Along with a concrete liner, this helps explain why CAP’s average annual loss from the aqueduct is only around 25,000 acre feet or 1.5 percent of total diversions. That figure includes 9,000 acre-feet lost due to seepage. Even in a state that enjoys more peak sun hours that any in the nation, this loss is nominal.

So all told, based on our scientific calculations, evaporation and seepage accounts for approximately 4.5% of the amount of water CAP diverts from the Colorado River.

Considering CAP diverts around 1.5 million acre-feet each year in a normal, non-shortage supply year, evaporation and seepage on this system is quite minimal thanks to the efficient, engineering marvel that is the 336-mile CAP system.

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