Lake Powell: How much water do we have left?
Anybody who has gone camping in the desert for more than a day has asked the same questions that John Currier, the chief engineer at the Colorado River Water Conservation District, has been obsessing about the past 18 months.
How much water do we have left?
How much water have we been using?
How much water will we have if our friends join us and they don’t bring water?
And while many campers ask these questions standing over a 5-gallon plastic jug, for Currier, the water-storage vessel he’s concerned about, Lake Powell, holds 24 million acre-feet of water.
But the giant reservoir, formed by Glen Canyon Dam, is only 37% full.
And a lot of water is still being released from the reservoir, more demands on the water are expected, and the water supply above the reservoir, in the sprawling Colorado River system, is expected to decrease.
So Currier, along with John Carron of Hydros Consulting in Boulder, has been asking questions familiar to all campers, but asking them on a much larger scale. And with a lot more at stake.
How much water in Western Slope rivers is currently being depleted, or consumed, mainly through irrigation and transmountain diversions?
How much more water is likely to be consumed on the Western Slope, and the upper basin states of Utah, Wyoming and New Mexico?
If more water is consumed on the Western Slope and the upper basin, what does that do to the risk of Lake Powell falling below 3,525 feet above sea level? That level is beneath the intakes to the dam’s hydropower plant, aka minimum power pool.
To try to get the answers, Hydros has developed a water model for the river district’s “risk study” that uses information from two other hydraulic models: one used by Colorado called StateMod, which includes detailed information about water rights and use in Colorado; and the other used by the Bureau of Reclamation called Colorado River Simulation System, which provides a regional look at the river system.
“To the best of my knowledge, I don’t think anybody has ever practically linked StateMod with CRSS, so I think the work that Hydros is doing here is out in front of anything anybody has gotten done,” Currier said. “And they are just now really getting into the guts, the interesting stuff, of the study.”
Detailed results from the risk study are slated to be shared June 20 in Grand Junction at a regional meeting of Western Slope water users and providers.
STUDYING THE OPTIONS
To handle the supply side of the scenarios, Hydros is using the recorded hydrology from 1988 to 2015, a period that was drier than even the most severe climate-change models show. As such, it’s called the “stress test” hydrology.
To model potential future depletions, Hydros has taken guidance from a series of programmatic biological opinions, or PBOs, done in various river basins as part of managing endangered fish populations.
The study is focused on the five major river basins on the Western Slope that contribute water to the Colorado River system above Glen Canyon Dam: Yampa, White, Colorado, Gunnison and San Juan.
With supply-and-demand assumptions in hand, Currier said the model can be asked a question on many people’s minds in Colorado: How might consumptive use of water be curtailed or reduced on either a mandatory or voluntary basis in order to maintain targeted elevations at Lake Powell, such as minimum power pool at 3,525 feet?
Minimum power pool makes a good target elevation for the model, because not only is the produced electricity valuable, but the elevation level also serves as a good proxy for staying in compliance with the 1922 Colorado River Compact.
If Lake Powell stays above minimum power pool, there is almost zero chance the compact will be violated, Currier told the river district board.
Colorado also is studying curtailment options using its own methodologies, but unlike the River District, it is not releasing its findings due to concerns of potential litigation.
The Front Range Water Council, an ad-hoc group of the largest water providers between Fort Collins and Pueblo, is also conducting studies that ask questions similar to those being asked by the state’s curtailment study and the river district’s risks study, according to Currier.
The river district’s model is exploring two ways a potential mandatory curtailment in Colorado could be implemented, or administered, by the Division of Water Resources.
The first way is based on the priority system in Colorado of first in time, first in right.
Say the state, in order to not violate the compact, set a goal of sending 100,000 acre-feet of water a year to Lake Powell from the Western Slope, water that otherwise would have been used or consumed.
And say the state began curtailing water rights, starting with the most junior rights, and proceeded down the list of rights, by date, until it reached rights that carry a date prior to Nov. 24, 1922, when the compact was signed.
Such pre-compact water rights are exempt from its terms.
How far down the list would the state have to curtail to put 100,000 acre-feet in Lake Powell?
And which junior rights, in each the five basins, would be curtailed first?
For example, almost all of the 600,000 acre-feet of water diverted through transmountain diversions was developed after 1922, and so the Front Range cities and farmers relying on that water are vulnerable to a compact call.
Knowing how a mandatory curtailment, administered in priority, rolls out “would really be useful for a lot of users,” Currier said.
Another way to potentially administer a curtailment is to do it on a pro-rata basis
For example, of all of the post-compact depletions occurring in Colorado, 70% are happening in the Colorado River basin proper, which includes flows above Grand Junction.
Currier said, for example, that a preliminary model run shows if the state wanted to curtail 300,000 acre-feet of post-compact water today, do so on a pro-rata basis among the Western Slope basins, the Colorado basin would have to come up with 69% of the water. And the White River basin would have to come up with just 1% of the water.
Today across the Western Slope, an annual average of 2.6 million acre-feet is being depleted, or consumed, according to StateMod. And the risk study estimates an average annual increase in depletions of 287,000 acre-feet.
The Colorado River basin, above Grand Junction, accounts for 1.2 million acre-feet of those depletions, the Gunnison for 575,000, the San Juan for 500,000, the Yampa for 197,000 and the White for 62,000.
The estimated 287,000 of total future average depletions on the Western Slope represents an 11 percent increase in water use, Currier said.
If that 11% increase is applied to the current use in the other upper basin states, it means another 390,000 acre-feet of water could be depleted in the future above Lake Powell.
Which leads to the posing of a series of question to the Hydros model.
Let’s say an additional 390,000 acre-feet of water is developed in the upper basin, the dry stress-test hydrology is applied over 25 years, and the upper basin reservoir re-operations, recently approved by Congress as part of a drought contingency planning program, are not yet in effect. What, then, happens to Lake Powell?
Well, this scenario shows there is a 17% chance that Lake Powell will fall below 3,525 feet, or minimum power pool. The risk study calls this the “baseline, future” scenario.
Now, let’s say that the new 390,000 acre-feet of depletions are made, but the drought contingency planning measures are applied, including releasing water from three big upper basin reservoirs.
This scenario, called “DCP, future,” cuts the risk level at Lake Powell to 10 percent.
Now, let’s say that no new water is developed, or consumed, but the DCP measures are not yet in place.
That scenario, “baseline, current,” cuts the risk to about 5%.
And finally, assume that no new water is developed, but the DCP water conservation and supply measures are in place.
The risk drops to about 3%, in the “DCP, current” scenario.
“You get down to maybe a 3% chance that you’re going to drop below 3,525,” Currier said.
Given the 3% risk factor, should the upper basin also shore that number up by adding 500,000 acre-feet of water into a new demand-management pool?
If demand management — difficult and expensive to implement — is going to provide only a small pillow against minimum power pool, is it worth doing?
If it helps answer the question, Currier said the 500,000 acre-foot demand-management pool at Powell amounts to 8 feet of additional elevation, once the reservoir has dropped to 3,525 feet.
“We’re not talking a huge pillow here to save us, with 500,000 acre-feet,” Currier said.
But he noted that trying to fill that pool could still yield benefits.
First, it could show the lower basin states that the upper basin states can actually use less water, and securely get it to Lake Powell — which might lead the lower basin states to agree to an even larger demand-management pool.
Also, it could help water users in Colorado figure out how to use less water on a voluntary basis.
If they do that, they might be able to camp out a little longer with the water they have.
Aspen Journalism covers rivers and water in collaboration with The Aspen Times and other Swift Communications newspapers. More at http://www.aspenjournalism.org.
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