Colorado State University’s Brad Udall has been doing some really interesting thinking about how to conceptualize and communicate climate change risk to water supplies in the Colorado River Basin.
Shown above (and shared with permission) is one of Brad’s “selected averages” graphs. The horizontal lines show the average river flow value for a period of interest – the entire period of record, for example, or the 21st century. (It’s from a presentation Brad gave to the Colorado Water Conservation Board earlier this month.)
While the general pattern will likely be familiar to people working on Colorado River issues – wetter in the long run, drier in recent decades – the value being graphed may not be. It’s “unregulated inflows”, which is the number calculated by the Bureau of Reclamation and the Colorado Basin River Forecast Center to estimate the actual inflows into Lake Powell, minus the impact of upstream reservoir operations.
It’s the best number to use if you want to look at the the actual water flowing into Lake Powell, and therefore available to meet compact-driven Upper Basin commitments to deliver water downstream to the Lower Basin at Lee Ferry. It combines both natural flows, and Upper Basin consumptive uses.
The key number to look at here is the one in the blue box. Since 2000, the average unregulated inflows have been 8.38 million acre feet per year. The Upper Basin’s Lee Ferry compact delivery “obligation” (lawyers please don’t subpoena me on this point, I ) is 8.23 maf, which means that we’re right on the edge of trouble even now, in terms of the Upper Basin’s ability to meet the expected deliveries to users in the Lower Basin. Increased Upper Basin use, or decreased flow (see Brad’s 6.66maf for 2018-2021) would cross that trouble threshold.
What this means is that, based on 21st century hydrology, the Upper Basin is already caught in what Doug Kenney and colleagues dubbed “the Upper Basin climate change squeeze”.
This is what Brad and I were getting at in our May editorial in Science. Lower flow scenarios are entirely credible based on the best available climate science and the hydrology we are seeing. We need analyses – modeling runs – that consider them as we prepare for the next round of river management negotiations, so that we know what will do if Brad’s scary “6.66” is a harbinger of our future.