The conventional simplification of the Colorado River Compact’s water allocation scheme is that it set aside 7.5 million acre feet of water use for the “exclusive beneficial consumptive use” of the states of the Lower Basin – Nevada, Arizona, and California.
In the 21st century, the official accounting shows the Lower Basin states using an average of 7.4 million acre feet per year (the blue line in the graph above), so we’re good, right?
When the states of the Upper Basin gathered in the late 1940s to negotiate the Upper Basin Compact, they recognized that if you’re going to build a reservoir to enable the consumptive use of water, you need to account for the evaporative losses from that reservoir. Article V of the Upper Basin Compact goes into some detail about how to account for “all losses of water occurring from or as the result of the storage of water in reservoirs” built by the states and the federal government.
But, as Eric Kuhn reminded us in a post yesterday, we don’t have a corresponding Lower Basin Compact. Instead, we have the 1963 Supreme Court decision in Arizona v. California, and the subsequent implementing decrees, to set the parameters for the water’s allocation. The court didn’t require an accounting of evaporation, only the water actually released from the reservoir. Here’s Eric:
I have no doubt that the 1922 compact negotiators considered this evaporation a man-made beneficial consumptive use to be covered by the (water) apportioned to the Lower Basin, but it is not covered in the Supreme Court’s decision.
Not everyone agrees about the compact negotiators’ intent. See for example Jason Robison and Larry MacDonnell in their review of AZ v. CA. But Robison and MacDonnell also note that the subsequent failure to fully wrestle with the question of reservoir evaporation and related questions like Lower Basin tributary use – the sort of things that negotiating a Lower Basin Compact might have adressed – has left a bit of a mess.
Legal nuances aside, the reality is that in the absence of a Lower Basin Compact to come to a negotiated agreement on questions like this, we’re left with significant uncertainties on this and other questions. And by my math, if you count reservoir evaporation, the Lower Basin states have been using an average of 8.4 million acre feet a year in the 21st century (the pinky-orangey line above). On this point Kuhn, Robison, and MacDonnell agree – that’s a problem.