The best way to meet Colorado’s growing water demand and still protect irrigation water rights is probably a combination of increased surface storage and underground, or aquifer storage. But even that combination won’t bridge the gap between water demand and supply.
That’s the good news and the bad news from the recently-completed South Platte Storage Study Final Report, released Dec. 15. The report was written by Stantec, a Canada-based design, engineering and construction firm, and Leonard Rice Engineering of Denver.
The study, authorized by the Colorado General Assembly in House Bill 16-1256, looked at the stretch of the South Platte River between Kersey and the Nebraska state line in an attempt to find water storage to fill a crippling water gap that is just 12 years away. According to the 2015 Colorado Water Plan, by 2030 the need for water in Colorado will exceed supplies by 560,000 acre feet, or 182 billion gallons per year, and most of that is here in the South Platte River Basin.
Experts already have said that water conservation alone won’t bridge the gap as thirsty Front Range cities continue to grow; even legislators have made it clear that they want to see proposals for storage as much as for conservation.
But, as with everything else having to do with water, finding and then using that storage is going to be complicated.
According to the SPSS report, it’s estimated that the South Platte carries almost 300,000 acre feet of water per year out of Colorado in excess of the amount needed to satisfy the South Platte River Compact with Nebraska. There are, however, a lot of “buts” that need to be attached to that broad statement.
For one thing, that’s not an average, that’s what the study authors called an “annual median.” That’s the middle number between the largest and smallest amounts that are lost; median, or “mean,” often is used instead of average because it’s a more accurate estimate of something over time.
Actual losses over a 20-year period between 1996 and 2015 varied from a paltry 10,000 acre feet to a whopping 1.9 million acre feet. It’s important to note that stream flows during that time frame included one of the largest floods in the state’s history and a follow-up flood that did nearly as much damage in the lower reaches of the river, as well as a period of extended drought.
The report also says that considerably more water is available at the Julesburg end of the reach than at the Kersey end, primarily because of return flows from irrigation. That indicates the need for a large reservoir to capture water before it leaves the state…
That may not be easy; according to the report, massive amounts of water would have to be diverted.
“Large diversion and conveyance structures would be needed to capture and convey water from the river to off-channel storage,” the study says. “At the Balzac gage near the middle of the SPSS study area, a diversion capacity of 550 (cubic feet per second) would be needed to capture 85 percent of the available water.”
That’s as much as some of the largest diversion structures now on the river. The North Sterling Inlet Canal, for instance, was taking around 520 cfs before cold weather and icing required it to be scaled back. Prewitt Reservoir Inlet can divert as much as 600 cfs when the water’s available.
It’s important to note the phrase “85 percent of available water.” Elsewhere in its recommendations section the report states that capturing all of the excess water is simply not feasible, and that’s not just during flood conditions.
“No feasible storage concepts or reasonable combinations of concepts are capable of putting all the available flow in the lower South Platte River to beneficial use,” the report says. “Therefore as a general principle, more storage will always be ‘better’ in this region in terms of maximizing available supply for basin water users.”
Still, finding and optimizing storage is a must if there is to be any hope of providing enough water to go around. The report, naturally, recommends a combination of storage methods, and even suggests that a cooperative effort of upper basin and lower basin storage concepts would be more efficient and store more water than a major “on-stem” reservoir. On the other hand, the on-stem option would be easier to build and yield more water quickly; it also faces possibly insurmountable permitting requirements.
No water storage concept is without good-versus-bad arguments. Aquifer or “underground” storage is complicated to manage but cheaper to create, and it can be easily ramped u over time. Storage options are grater in the lower basin but they’re further from where the water will actually be needed. Underground storage is great for agricultural use but the water would have to be extensively treated for municipal and industrial use.
The study even raised some new questions and left unanswered some old ones. For instance, abandoned gravel pits weren’t even included in the project, and the SPSS authors recommend further study of that option. They also recommend further studies of the South Platte above Kersey and of the Cache la Poudre basin.
Ultimately, the study’s authors say, the SPSS is a “starting point” and further investigation of any of the storage methods or sites would be needed.
“The work in the SPSS is a starting point for more specific alternative investigations,” the study says, “but substantial additional analysis will be required to test the feasibility of specific storage options based on points of diversion, intake systems, and methods of operating to meet demands.”