Blue Mesa Reservoir water bank study #ColoradoRiver

Aspinall Modeling Memo coverviaarkansasbasinroundtable122014
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From The Pueblo Chieftain (Chris Woodka):

Another piece of the Colorado River shortage puzzle has been put in place with the completion of a Blue Mesa Reservoir water bank study. The study was a joint effort by the Arkansas Basin Roundtable, Gunnison Basin Roundtable and Colorado Water Conservation Board. It looked at whether water could be stored in Blue Mesa Reservoir near Gunnison to be released during a drought when Colorado might owe water to downstream states.

“There are benefits to the environment during low-flow periods,” said Mark McCloskey, of CDM-Smith, consultants for the study, as he explained the study to the Arkansas Basin Roundtable this week.

Under the 1922 Colorado River Compact, upper basin states (Colorado, New Mexico, Utah and Wyoming) are required to deliver 75 million acre-feet to Lake Powell under a 10-year rolling average. If that fails to happen, downstream states (Arizona, California and Nevada) could issue a call on the river. Colorado’s share is 51.25 percent of the deficit.

Another 1.5 million acre-feet annually must be delivered to Mexico.

While there has never been a shortfall of deliveries, there are indications from tree-ring studies that decades-long dry spells are possible.

The study used the worst-case scenarios from the Bureau of Reclamation’s Colorado River Basin study — high demand in very dry years — to develop models of optimum timing and levels of storage in a water bank in Blue Mesa. It projected water that would be needed if levels fell to 80-98 percent of minimum levels. The study also determined how much water would be lost to evaporation or to stream banks along the way to Lake Powell.

Replacement water likely would be purchased by Front Range or statewide interests from ranchers, and it’s not known how those purchases would affect high-altitude hay meadows, McCloskey acknowledged.

It’s important to the Front Range, because a call on the Colorado River could mean curtailment of diversions across the Continental Divide.

A curtailment could mean less water for Pueblo, the Fryingpan- Arkansas Project, Colorado Springs, Denver, Aurora and the Colorado-Big Thompson Project.

All of those have water rights that were established after the 1922 compact.

The study showed the optimum time to store water would begin when deliveries fell to 85 million acre-feet in a 10-year period. The optimum amount to keep in storage would be about 300,000 acre-feet. Some benefit was also seen in deficit irrigation below Blue Mesa in dry years to preserve river flows.

The compact was drawn up by the states and approved by Congress because down­stream development was already occurring in Arizona and California. While it was known that drought impacts the basin, most thought the average flows in the 1920s could be used as a yardstick.

The flows at that time actually were higher than they have been in the ensuing decades. Record low flows were recorded during the 2000s.

More Colorado River Basin coverage here.

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