Scenario A, worst-case:
Climate change is at the worst end of consideration, at 6 weeks peak, 50 percent combined flow and ET impacts. Weighted average demand growth rate is 1.2 percent, resulting in a 2065 treated water demand of approximately 6,320 acre-feet. And flow adjustment factors are at their expected values. Credit: City of Aspen/Headwaters
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Scenario B, no-growth
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Scenario C, intervention
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ASPEN – Whether Aspen needs to build a reservoir to meet water demands in 2065 may depend in part on whether it wants to keep irrigating its municipal golf course during an apocalyptic drought.
According to a water attorney and an economist working for the city on a risk analysis of future water shortages, Aspen may find itself unable to meet domestic water demands — including both indoor and outdoor water use — anywhere from two out of 25 years in an optimistic scenario to 19 out of 25 years in a worst-case scenario.
The most optimistic scenario can be achieved, in theory, if the city limits outdoor watering by its customers and also stops diverting water from Castle Creek to irrigate the 148-acre municipal golf course and other nearby open space.
Outdoor water use accounts for about 60 percent of current demand for city water.
The members of the Aspen City Council took a sip of such concepts Monday at a work session on the results of a water demand study.
Aspen Mayor Steve Skadron said he expects the council to now spend “several months” grappling with the city’s future water needs as part of an exercise to identify alternatives to maintaining conditional water rights for two large reservoirs on upper Castle and Maroon creeks.
Climate wildcard
Monday night, George Oamek, an economist with Headwaters Corp., presented three scenarios from a risk analysis he’s been developing for the city.
He told the council that his model is packed with uncertainties, mainly around the severity of climate change, but also around the amount of flow in Castle and Maroon creeks and the future demand from Aspen’s water customers.
“We’ve got just a tremendous amount of variability in the existing information that gets translated into our analysis,” Oamek said.
“There is so much uncertainty,” concurred council member Ann Mullins.
“Climate change is everything,” Oamek said. “And it’s the thing we know the least about.”
Oamek also said his model includes a 1-in-100 chance that the factors will line up to cause havoc, which he said is a common risk assumption for municipal water providers and in floodplain mapping with its concept of a “100-year-flood.”
“Frankly, water planners are risk adverse,” he said.
In Oamek’s “worst-case” scenario, runoff would come six weeks earlier in the spring and there would be half as much water flowing in Castle and Maroon creeks, the city’s primary sources of water.
Water rights portfolio
The city owns two large senior diversion rights on Castle Creek tied to the historic Castle Creek-Midland Flume. The city has an 1892 decree allowing it to divert 60 cfs. On top of that, it has another right from 1892 for 100 cfs, giving it the ability to divert 160 cfs from Castle Creek.
The city’s streamwide diversion dam is just downstream of Midnight Mine Road and the water is sent via a pipeline to the city’s water treatment plant on a knoll above Aspen Valley Hospital.
Water from Maroon Creek is also sent via pipeline to the treatment plant and the associated 10 acre-foot Leonard M. Thomas Reservoir, which serves as a forebay to the treatment plant, holding water from both Castle and Maroon creeks.
The city owns a 3.4 cfs diversion right on Maroon Creek with an 1893 decree date and another 65 cfs diversion right with a 1949 decree date that, notably, includes an 1892 appropriation date. The city’s streamwide diversion dam on Maroon Creek is located at the T-Lazy-7 Ranch.
The water rights from Castle and Maroon creeks give the city a portfolio of “paper” rights adding up to 228.4 cfs, which is a much larger amount than the city runs through its water treatment plant, even in dry, high-demand, years.
According to a water availability study from Wilson Water adopted by the city in June 2016 as a planning document, the city in the last big drought year of 2012 brought between 2.38 and 9.4 cfs of water into its water treatment plan from Thomas Reservoir. The peak intake of 9.4 cfs was in June.
The city’s pipeline from the Castle Creek diversion limits the amount of water that can be sent from Castle Creek to the treatment plant to 25 cfs and the pipeline from Maroon Creek can move up to 27 cfs.
The city’s diversion rights are separate from its two conditional water storage rights higher on Castle and Maroon creeks.
Those rights, as currently decreed with a 1971 date, are for storing 4,567 acre-feet of water just below the confluence of East and West Maroon creeks in the Maroon Creek Reservoir, and for storing 9,062 acre-feet of water in the Castle Creek Reservoir two miles below Ashcroft.
The combined storage capacity of the potential reservoirs, as currently decreed, is 13,629 acre-feet. The reservoirs, notably, would be located above the city’s two downstream diversion dams.
And both the city’s diversion rights and its conditional storage rights are separate from rights it owns in three irrigation ditches on Castle Creek, downstream from its diversion dam. The headgates for the three ditches on Castle Creek are near the Marolt housing complex.
The city calculates the instream flow at a location below the headgate of the Marolt Ditch, as it is the lowest of the three ditches.
Supply down, demand up
In his presentation to the City Council, Oamek said his worst-case scenario assumes that demand for treated water would be 6,320 acre-feet of water a year, up from about 3,500 acre-feet today.
The assumption includes a negligible 0.4 percent growth rate in the permanent population in Aspen’s water service area, and a 2 percent growth rate for the part-time population and commercial sector.
That assumption does seem to run counter to Aspen’s past ability, and plans, to lower water demands while the population rises, which may be why the three scenarios also include a no-growth-in-demand scenario, where demand is held flat at current levels, regardless of potential population growth.
For example, a 2014 water efficiency plan from Element Consulting and WaterDM projects the city will, by 2035, “reduce treated demand by about 583 AF — an overall 14 percent reduction in demand.”
And the city has been making solid progress on reducing water demand. In 2012, city staffers told the council the city had reduced water consumption by “over two-thirds over the last 19 years.”
But the water efficiency plan does raise a cautionary note about the city’s lack of storage.
“On an annual basis, the dry year yield of the City’s water rights appears to be more than sufficient to meet current and forecast future demands,” the plan says. “However, the city does not have storage to regulate the timing of supply to match demands, and therefore is vulnerable to peak demand shortfalls in dry years when physical streamflow conditions are limited, or in emergencies such as a fire or landslide when one or more particular water supply sources may become unavailable.”
Setting aside the downward demand trend, the most draconian scenario developed by Oamek assumes a near doubling of demand in a much hotter and drier world.
And it shows the city might not be able to meet all municipal water demands — including both indoor and outdoor use — in 19 of 25 years.
“There are frequent shortages for Aspen’s potable supply during that period,” Oamek said.
In 15 of those years, water shortages could be greater than 100 acre-feet of water.
In four of those years, water shortages could be greater than 1,000 acre-feet.
And in one of those years — think the drought year of 1977 — shortages could be greater than 2,000 acre-feet.
“Over 1,000 acre-feet … that would definitely cause some hardship,” Oamek said. “A lot of these shortages, they are not occurring during the irrigation season, or during the summer where you might be able to reduce outdoor use, or work some deals with the irrigators.
“The shortages are occurring kind of in the shoulder season, occurring in late summer, early fall, and also during the winter. And those shortages may be a little harder to mitigate through the utilization of outdoor sources.”
No outdoor watering
The picture gets brighter in Oamek’s “intervention scenario,” the least demanding of the three scenarios.
Runoff would still come six weeks earlier, and there would still be half as much water flowing down Castle and Maroon creeks.
But demand for city water is projected at 2,280 acre-feet a year, as the scenario assumes the city will curtail the use of treated waters for outdoor purposes during a drought.
“During times of shortages, we set outdoor usages to zero,” Oamek told the council, explaining that would drop annual demand in the model to about 2,200 acre-feet, down from 3,500 acre-feet.
In that scenario, there might be 14 years out of 25 when there are water shortages, but only in five of those years would the shortages be over 100 acre-feet, and none would produce shortages over 1,000 acre-feet.
No ditch water
Cindy Covell, the city’s water attorney with Alperstein and Covell, then told the council she asked Oamek — the day of the council work session — to run another scenario where the city also stopped diverting water it controls into three irrigation ditches on lower Castle Creek, downstream of the city’s diversion dam to its treatment plant.
“I was thinking, if you were going to run a scenario that involved no outdoor irrigation – you’re telling your customers they can’t water their lawns – you probably are going to have a hard time taking irrigation water down those ditches and irrigating your golf courses and your parks,” Covell said.
Oamek ran a calculation — not a full model run — and said curtailing irrigation drove the number of years with indoor water shortages down to just two years out of 25, and in only one of those years was the shortage greater than 100 acre-feet.
In 2012, the city diverted up to a total of 20.5 cfs into the three Castle Creek irrigation ditches, with the highest diversion rate in June, according to the Wilson Water study.
The city has diversion rights in the Holden Ditch of 25.9 cfs with a 1952 decree, in the Marolt Ditch of 13.6 cfs with a 1934 decree, and in the Si Johnson Ditch of 2.55 cfs with a 1936 decree, according to an agreement with the Colorado Water Conservation Board. That adds up to a “paper” portfolio of 42.05 cfs worth of irrigation rights.
“To some extent you already have a bucket of water, which is the downstream irrigation ditches,” said water attorney Paul Noto, who represents three clients opposing the city’s conditional storage rights in water court, and was asked to comment at the work session by the mayor.
“Tonight we talked about what’s the worst-case scenario, [and] might I suggest that we look at priority irrigation under those ditches,” Noto said. “So perhaps we say, at the golf course we want to keep our fairways and greens green, but maybe we don’t irrigate the rough if the streamflows are below x.”
Noto also pointed out to the council that almost all of their municipal water comes from Castle Creek, and that the water in Maroon Creek is now primarily diverted to power the city’s small hydropower plant on the banks of Maroon Creek.
Maintaining instream flows
Maintaining instream flows is a challenge in each of the scenarios presented, as there are dry years when it’s hard for the city to reach its goal of leaving enough water in Castle and Maroon creeks to maintain the environmental flows while also meeting all municipal water demands.
“Worst-case, maximum growth, there is a lot of damage to the instream flows,” Oamek said, noting the annual instream-flow shortages were over 10,000 acre-feet in the worst year in the model.
The city has a policy of maintaining minimum, or instream, environmental flows in Castle and Maroon Creeks.
The Colorado Water Conservation Board holds an instream flow right in Castle Creek for 12 cfs and a right in Maroon Creek for 14 cfs. The state defines that level of flow as the amount of water needed to protect the environment “to a reasonable degree.”
Both of the CWCB’s instream flow rights are junior to the city’s senior diversion rights on Maroon and Castle creeks.
The city, based on the recommendation of a consulting biologist, recently increased its minimum flow target on Castle Creek to 13.3 cfs. As such, the combined minimum instream flow level in Castle and Maroon creeks that the city seeks to maintain is 27.3 cfs.
The city’s policy of voluntarily honoring the state’s junior instream flow rights is centered on a 1997 agreement with the CWCB to protect 12 cfs of flow on Castle Creek. The agreement does not technically extend to Maroon Creek, although the city’s stated policy does.
However, the agreement with the state also includes a provision that allows the city to exempt itself from the policy during periods of “extraordinary drought,” which are not defined.
The provision gives the city latitude to meet its municipal demands and “invade,” as Oamek put it, the junior minimum instream flow rights held by the state, as necessary.
Need a bucket
Staff in the city’s Water Department continue to point out to the City Council that Aspen likely needs some amount of water storage in the future.
“In our integrated water supply system, there are alternatives to storage than can help mitigate our shortages, things like re-use, conservation, ag transfers … but even though these combined can minimize the shortages, storage is still needed because of timing issues,” Margaret Medellin, a utilities portfolio manager for the city told the council Monday night. “To really make these other mechanisms work, we still need a bucket to be able to augment and … re-time the water.”
That message has gotten the attention of Mayor Skadron.
“As we proceed, my goal would be to ensure a sufficient water supply for future generations and to ensure that their options are open,” he said.
He also asked during the meeting, “Does a scenario exist in municipal water planning where storage is not needed beyond just what nature provides?”
“Historically, that’s how Aspen has operated,” Medellin replied. “Aspen has very little storage and has historically operated as a direct-flow water provider. And in areas maybe that are wetter, back East, it is not as problematic.
“And I think the concern is as we are starting to see runoff happening earlier, the demand being extended and happening later into the system … [and] what has worked historically for Aspen, we aren’t convinced is going to work for the next 50 years.
“Even though that it is something that other communities can do, and something that Aspen has done, as we are looking into our models going forward, we’re not convinced that it’s something that is sustainable here,” Medellin said.
However, a 2016 water supply report done by Wilson Water and adopted by the City Council in June of 2016 as a planning document, painted a different picture and found that no storage was necessary – even after factoring in available climate change projections.
The Wilson Water study found that “the results of this analysis indicate the city can always provide sufficient potable and raw water supplies under these modeled demand and hydrology scenarios.
“Existing water supply infrastructure and water rights portfolio developed and managed by the City do not appear to be limiting factors in this evaluation.
“However, during drought periods, physical water supplies may limit the city from satisfying desired ISF (instream flow) bypasses. These modeled ISF deficits are forecasted to occur during drought periods in only the climate scenarios with very low late summer and winter streamflow conditions.
“Most ISF deficits occur at a frequency of 5% of the time or 1 out of 20 years. The predicted average daily ISF deficits are relatively small and can be managed utilizing the existing water supply tools the city has in place and/or is actively developing,” the Wilson Water study said.
And recently completed water-efficiency plans for Basalt, Carbondale, and Glenwood Springs found that those three nearby cities have adequate water supplies for the future without significant storage “buckets.”
Shortage into storage
Medellin also told the council one of the next steps is to convert the shortage numbers from the Headwaters risk analysis to potential storage numbers.
“It’s not a one-to-one conversion,” Medellin said, noting that the storage figure is always larger than the shortage number.
She said a “reservoir operations model” will be used to “apply a reservoir efficiency factor to account for losses and reservoir integrity.”
“What that means is all of the water that we put into a reservoir, we’re not going to get that back out,” she said. “So there is a factor that is commonly added to account for that. Then the next step is going to be to determine what volume of conditional storage rights we need to satisfy that requirement.”
Covell, the city’s water attorney, further explained the process.
“You look at your streamflows, and you say, how often can I fill up this reservoir?” she said. “And on the eastern slope, you might not be able to fill it up more than once every five years. So you say, if I need to be sure I’ve got to have 100 acre-feet of water in storage, I might have to fill up 600 acre-feet, because when I need that 100 acre-feet some of the water will have evaporated and I won’t be able to top it off again because my water right won’t be available.
“And that’s a, maybe, overly simplistic example, but when you’re trying to figure how much storage capacity you need, you have to figure out when you are going to be able to put water in, how much of it’s going to evaporate, and when you’re going to need to take it back out,” Covell said.
Medellin added that the engineering firm Deere and Ault of Longmont is now making calculations for both in-situ and surface water reservoirs, and the storage needs will be based on the representative period of years in the model being used by Headwaters.
Settlement talks
It’s not clear yet how the scenarios presented Monday may change the city’s negotiating position with the 10 parties opposing its ongoing efforts in water court to maintain conditional water storage rights for reservoirs on Castle and Maroon creeks.
Last week, the city announced it now intends to transfer its water rights from Castle and Maroon creeks to two potential reservoir sites in Woody Creek, including on land it now has under contract near the Elam gravel pit, and the gravel pit itself.
“The impetus for the purchase is to seek a way to transfer decreed storage rights to locations other than the decreed locations on Castle Creek and Maroon Creek,” the city said in a July 19 press release titled “Aspen City Council to Purchase Land for Possible Alternate Site for Water Storage.”
“Since 1965, the city has held decreed water storage rights at sites in Maroon and Castle Creek Valleys but the nature of these pristine locations has made it a priority for the city to first seek other ways to address potential water shortages and to seek alternate locations for water storage,” the city stated.
And Skadron was quoted in the release as saying “securing Aspen’s water future is an essential task of today’s city council. It is council’s responsibility to look out for the welfare, safety, and health of the community and we take that very seriously. In addition, our commitment to protecting our environment is also a priority and this land purchase is a way to both protect the community and preserve Castle and Maroon valley wild lands.”
The city also said in the press release about the Woody Creek options that “other alternatives for water storage are still being explored, including in-situ reservoirs at the Aspen Golf Course, Cozy Point Ranch, the portion of the city-owned Maroon Creek golf course, and other upper valley locations.”
A second settlement conference with the opposing parties in the two water court cases is set for Aug. 2 and a status conference with the water court referee is Aug. 8.
Editor’s note: Aspen Journalism is collaborating with The Aspen Times on coverage of rivers and water. The Times published a version of this story on July 25, 2017.