A Shark in Fresh Water

Image: Lake Powell lost nearly half of its storage capacity during 2000-2018. Plentiful snowmelt runoff this past spring did little to regain that capacity. Today the reservoir is only 53% full.             Photo courtesy of US Bureau of Reclamation.

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It’s a bit hard to believe now, but back in 2012 there wasn’t a lot of talk about fresh water in the global dialogues on climate change. Yes, there was a lot of talk about sea-level rise, the impacts of warming temperatures on the livability of cities, the need to protect forests, and that sort of thing. But the concerns held by many freshwater scientists and water managers – such as fears of worsening freshwater scarcity in coming decades – were getting very little attention.

Anticipating further freshwater neglect at the upcoming 18th Convention of the Parties (COP-18) to be held in November 2012 in Doha, Qatar, the National Water Commission of Mexico (CONAGUA) hosted a preemptive water and climate conference in Mexico City to elevate freshwater concerns on the global agenda. I’ll never forget the words of CONAGUA’s director general in opening the conference:

“Climate change is the shark in our future and water is its teeth.”

There are few places in the world feeling the bite of those teeth more painfully than the Colorado River basin of the American West.

In recent blogs I’ve offered my thoughts and opinions about the growing threat of depleted river flows for highly endangered fish like the Colorado pikeminnow, the ways in which changes in land and water management in California can change rainfall in the Colorado River basin, and the opportunity to resolve water scarcity by paying farmers to reduce consumptive water use.

But during a recent interview, when asked how much reduction in consumptive use would be needed to stabilize the water level of Lake Powell, I was embarrassed to admit that I didn’t exactly know! In all the papers, reports, blogs and books I’ve read about the Colorado River, I have not come across a simple definitive answer to this simple question.

So I cooked up my own estimates. Or more precisely I’ve generated multiple answers, based on varying assumptions.

I created a simple water budget model, using estimates of reservoir inflows and evaporation from the US Bureau of Reclamation, and reservoir outflows measured by the US Geological Survey at Lee Ferry, Arizona, located just downstream of Lake Powell.  The water budget of the Colorado River at Lake Powell looks like this.

What the graph above does not show is what was happening in Lake Powell itself. When releases from the reservoir (blue bars) — combined with evaporation from the reservoir itself (yellow bars) — is greater than the inflow of water into the reservoir, the volume in the reservoir is depleted.

Lake Powell lost nearly 600,000 acre-feet per year, on average, during 2000-2018. 

That’s the green dotted line in the graph below. About one-third of the reservoir depletion can be attributed to lower river inflows caused by warmer temperatures and increased evaporation in the river basin, according to climate scientists Brad Udall and Jonathan Overpeck (2017).

Image: The balance of historical inflows and outflows at Lake Powell on the Colorado River from 2000-2018 reveals an average deficit of ~600,000 acre-feet per year.

As with a personal checking account, the only way to resolve an overdraft is to stop spending so much. Or in the case of a river basin, to stop consuming so much water (green bars in upper graph). That means that if you had wanted to maintain stable reservoir levels over the past 19 years, you would have needed to reduce consumptive use by 600,000 acre-feet, on average, per year. That’s more than all of the water that is diverted out of the Colorado River’s headwaters to Denver and other Front Range cities each year.

The longer we wait to begin aggressive demand management in the Upper Basin of the Colorado River, the further we’ll fall behind in the water budget, and the deeper the crisis we’ll be in. Further declines in Lake Powell’s water storage could lead to many frightening risks, including the possibility that trans-mountain water deliveries to Colorado’s Front Range cities could be substantially curtailed; that hydropower production at Lake Powell (Glen Canyon Dam) would drop precipitously; and eventually, the ability to release water from the reservoir to downstream states would no longer be physically possible when the reservoir level drops below release outlets.

The above statistics and the historical balances shown in the graph above (blue bars and green dots) are based on reservoir and river flow data from 2000-2018. It looks very bad, with half of those years showing big deficits, but it’s likely to get much worse.  If you believe the climate forecasts by Udall and  Overpeck – and I certainly do believe them – the rate of decline in Lake Powell storage levels can be expected to accelerate in coming decades, assuming future decades of water use resemble the past two decades, and existing requirements remain in place for releasing an average of 8.23 million acre-feet each year from Lake Powell as mandated by the Colorado River Compact to meet water needs in the Lower Basin and Mexico.

Udall and Overpeck project further declines in Colorado River flows of at least -20% by 2050 and -35% by 2100. Assuming the mid-century rate of declining flows, we should be doing everything we can to reduce consumptive use in the Upper Basin by about 1.5 million acre feet each year. By century’s end, we’d need to be reducing consumptive use by 2.75 million acre-feet annually.

Some in the water management community and political leadership of the Upper Basin states still aren’t convinced that a demand management program to reduce consumptive water use is needed. I simply cannot understand why they would want to continue to risk potentially catastrophic disaster by waiting any longer.

As Anne Castle and John Fleck put it so poignantly in their recent assessment of water risks in the Upper Basin:

“Now is the time to think about our risks, and our insurance options.”

“Hope is not insurance.”

There are sharks in these waters. Treading water is no way to respond.

 

(special thanks to John Fleck for help in finding data)