From Tucson.com (Tony Davis):
In the gloomiest long-term forecast yet for the drought-stricken Colorado River, a new study warns that lower river basin states including Arizona may have to slash their take from the river up to 40% by the 2050s to keep reservoirs from falling too low.
Such a cut would amount to about twice as much as the three Lower Basin states — Arizona, California and Nevada — agreed to absorb under the drought contingency plan they approved in early 2019.
Overall, the study warned that managing the river sustainably will require substantially larger cuts in use by Lower Basin states than currently envisioned, along with curbs on future diversions by Upper Basin states.
While climate change’s impacts on the river have been repeatedly studied, this is the first study that seeks to pinpoint how warming temperatures would translate into reductions in water that river basin states could take over the long term.
Carrying out the study’s recommendations, under the most likely conditions of climate change, almost certainly would mean more supply curbs for the $4 billion Central Arizona Project.
The CAP is already slated to lose nearly half its total allocation under the worst case, shorter-term scenarios envisioned under the 2019 drought plan.
Tucson and Phoenix-area cities and tribes, along with Central Arizona farmers, all depend on the CAP for water for drinking or irrigation.
The study, written by 13 researchers, was posted online about a week ago, at a time the drying river is on the edge of its first major shortage.
Federal forecasters predict Lake Mead will drop low enough to require cutbacks in water deliveries to Central Arizona farmers in 2022 due to river flow declines.
But exactly how much will be cut in long-term, future water deliveries is far from settled. The seven states are about to start renegotiating guidelines under which the river has been managed since 2007. Changes to the guidelines won’t take effect until 2026.
In other forecasts, the study took a shot at longstanding plans by the four Upper Basin states — New Mexico, Colorado, Utah and Wyoming — to increase their take from the river under rights held from the 1922 Colorado River Compact.
The Upper Basin states’ forecasts of river diversions are unrealistic and would make it virtually impossible to maintain stable water supplies over an extended period, the study said.
“New demands in the era of climate change resulting in decreasing flows are the equivalent of self-inflicted wounds,” the study said.
Also, more, major Upper Basin diversions could drain both lakes Mead and Powell, dramatically reducing the amount of water available to serve people for drinking and irrigation and to generate electricity, the study said.
That would also result in the release of very warm water from Powell, compared to colder waters being released today. The Grand Canyon’s ecosystem downstream would be drastically changed, said Jack Schmidt, one of the study’s authors.
The study also warned that the current, downward trend in river flows will likely continue or worsen as temperatures keep rising.
That will lead to additional evapotranspiration — the absorption of atmospheric water supplies by plants — and aridification of the landscape, in which soils get drier and runoff keeps declining, the study said.
“Under this scenario, the basin will soon face a tipping point,” the study said.