Editor's Note: This article covers the last two sessions of the Sixth Annual Natural History of the Gila Symposium.

By Mary Alice Murphy

The presentation phase of the Sixth Natural History of the Gila Symposium was held Thursday and Friday, Feb. 25-26 at Western New Mexico University. The last presentations included four on the Arizona Water Settlements Act of 2004 and its planning and processes.

Norman Gaume, engineer and former Interstate Stream Commission director, gave the third presentation titled: Scientific Simulation of the Water Yield of the New Mexico Unit of the Central Arizona Project.

"I have a different take from the previous speaker," Gaume began. "I have done a detailed analysis of the yield that can be expected from a diversion. A graphic in the Reclamation value study paints a clear picture of the difficulties that lie ahead."

He said the AWSA is the fourth incarnation of the Colorado River Basin Project, proposed in 1968 with authorization to build a structure on the Gila River. "It would divert junior rights water that would otherwise go to San Carlos Reservoir and it would be replaced by water from Lake Havasu."

Gaume said rarely would there be enough water in the Gila River to fulfill the AWSA allocation.

"As of today, there is no physical incarnation of a diversion," he said. "What is the water yield? What is the risk? It's an incredible stretch to say that best available science has been used. Off stream storage will be required."

He said he hired Peter Coha, "the best Excel and graphics guy I know," and Jim Brainerd, a retired hydrologist and author of the Sandia Labs Model, which was never formally released, to analyze the diversion yield that would be legally available. "That's different from usable, because of water evaporation and inefficiencies of the system."

Gaume said junior water rights are rarely available on an over-appropriated river system. John Fleck in his blog said on Feb. 18. 2016: "It 'sucks' to be junior..."

"The ISC model by Sandia Labs was abandoned because it did not show enough water available," Gaume alleged.

He showed several charts showing mean and median flows with 79 daily points for 79 years. "Because of bypass requirements of the Consumptive Use and Forbearance Act (an addendum to the AWSA), the mean has a little bit of water available, but the median none."

Gaume said the water available consisted of snowmelt and a little bit during the monsoon season. Eighty percent of the water available occurs between January and April. "With climate change, the prediction is that snowmelt will be gone, so that's a huge uncertainty. Just because there's a lot of water in the Gila doesn't mean it is legally divertible."

With a mean flow of 12,600 acre-feet, and a median of 3,900 acre-feet, "more than half the years, there is little to no water available. Brainerd's reservoir storage volume simulated usable water of 5,400 acre-feet a year. That's a far cry from 14,000 acre-feet."

He gave some insights on the record back to 1937. "In the 1980s and 1900s, we had the wettest years out of the past 1,000 years. That biases the record and the distribution is skewed. Water is rarely available, and none is available during drought. The mean is less than the magnitude of the junior water right. It requires efficient storage, which will only happen if the reservoir is lined. That's expensive."

Gaume said he thought the yield of 5,400 acre-feet a year is "an unachievable maximum. There will be required mitigation for species. How much water is there for whom? At what cost and paid for by whom?"

He also questioned the best available science and said: "No, it was the best paid advocates. What makes ISC's dedication to this junior Gila/AWSA water rational?"

Craig Roepke, Gila Project manager, said he was not going to try to tell participants the yield, "because we don't have the necessary parameters. We didn't publish our results because we would do them only on concept, with one exception. We hired nine professional engineers and tasked them to estimate the yield at that time. They said between 7,000 acre-feet a year and 9,000 acre-feet a year, if we had a big reservoir."

"No one is intending to do a 64,000 acre-foot reservoir nor a two-and-a-half mile tunnel," Roepke said.

"We tried to create something that the public could fool around with," he continued. "The model contains the parameters necessary to illustrate impacts. The validity of the model and parameter values is easily verified. The ISC used much of the model by The Nature Conservancy and the TNC drought calculators.

"Norm calls this a diversion yield model," Roepke said. "The models by Norm and ours are not that different. We linked our diversion model with what we call a yield model. You can take our model and modify it. We do not refer to mean or median, but it's a calculation of historical data. We linked possible climate change with the actual historical data."

He said Bill Evans Lake has a measured seepage of 38 percent. "If we line the reservoir, it will cut seepage. A lot of people are concerned that flows in March are critical."

"Someone said you can change the model until you get the 'right answer.'" Roepke said. "That's exactly right. This is a tool that is entirely transparent. You can look at every cell to see the calculation."

He explained the Consumptive Use and Forbearance Agreement. "The consumption happens in New Mexico. The forbearance is by those downstream. They forbear from calling for senior water rights, if we stay within the parameters of the CUFA."

Helen Sobien, ISC engineer, gave thefinal presentation. "These are the municipal water conservation projects the ISC voted to fund in December 2015."

She gave the questions to be addressed:
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