Colorado River study predicts even bigger water cuts. That's not why it's so intriguing

A new Colorado River study predicts we may need to make even deeper cuts to keep our reservoirs from tanking over the long haul.

But the dire conclusions within the study aren’t what make it so intriguing.

It’s how the group arrived at them.

The Future of the Colorado River project, an effort based out of Utah State University, has produced six white papers to evaluate new approaches to water management along the river.

And, most notably, it is using the Colorado River Simulation System (CRSS), the same modeling tool the Bureau of Reclamation uses to develop its long-term water availability forecasts for the basin.

Arizona also is using this model to help evaluate various scenarios as it and other basin states begin renegotiating how the river will be managed for the next 20 years.

This isn't even the worst-case scenario

The most recent Future of the Colorado River analysis uses many of the same assumptions that Arizona is using in its forthcoming analysis, including more recent drought conditions, tree-ring data from more historical drought periods and climate change predictions.

The model runs also are based on two relatively middle-of-the-road futures – that the Colorado River basin doesn’t get any drier than it is now, or that it only gets moderately hotter and drier. These are hardly worst-case scenarios.

The group found that the cuts we agreed to in the Drought Contingency Plan alone will not be enough to sustain lakes Mead and Powell. If the upper basin grows as projected, water levels will continue to decrease over 20 years until they reach “dead pool” levels – the point at which no water leaves the reservoirs.

That doesn’t mean both lakes are doomed. The study points out that a cap on consumptive use in the upper basin and additional cuts in the lower basin (of which Arizona is a member) could help sustain the reservoirs.

But that conclusion comes with important caveats. The group argues, for example, that it’s more instructive to calculate storage at both reservoirs as if it were one total system – not the two somewhat independent lakes that operate today. It also contends that the official estimate of how much water the upper basin may use in the future is vastly overestimated.

Upper, lower basins will have to cut use

But the implications are no less controversial.

To avoid an even messier “compact call,” which would jeopardize the water rights of many users in the upper basin to provide water guaranteed to the lower basin in the Colorado River Compact, the upper basin may have to essentially agree not to grow its use over time, even though it had long assumed that it could.

Likewise, the lower basin would mostly likely not be able to live with the Tier 1 or even Tier 2 shortages that are growing increasingly likely over the next two years. Metro Phoenix cities may have to face annual Tier 3 shortages – the worst cuts we could face under the Drought Contingency Plan – or perhaps something even more drastic simply to keep the reservoirs steady.

How much depends on which combination of upper basin caps and lower basin cuts might play out under various hydrologies and operational assumptions – it’s sort of like a choose-your-own-adventure book where the outcomes range from “jeez, that’s not so great” to “oh dear Lord, no.”

Why these models are so valuable

But that’s why this sort of analysis is so valuable. We can see how the impacts of a complex web of actions (or inactions) can affect how much water there is to go around.

It also can help put to rest some of the points that have divided us, such as whether we should fill Lake Mead or Lake Powell first as supplies recede. The Future of the Colorado River analysis found that while there are environmental pros and cons to each approach, neither would markedly minimize evaporation losses or bolster the security of our supply.

Which brings me back to the Arizona effort, which also is using this CRSS model.

Few expect the modeling team to tackle nearly as controversial ideas as the Future of the Colorado River team has. There are far more politics involved in the state’s effort, given the number of disparate internal interests it must keep on board and the minefield of negotiations it will soon face with other states (trust me, they’re closely watching our process).

But the model should give us a lot more state-specific data about how deep cuts could go and who could be most impacted by them under various climate, use and management scenarios.

And ultimately, that should produce a much better-informed debate.

Reach Allhands at joanna.allhands@arizonarepublic.com. On Twitter: @joannaallhands.

If you love this content (or love to hate it – hey, I won't judge), why not subscribe to get more?