Southwest braces for water cutbacks as #drought deepens along the #ColoradoRiver — #Arizona Republic #COriver #aridification #DCP

From The Arizona Republic (Ian James):

Unrelenting drought and years of rising temperatures due to climate change are pushing the long-overallocated Colorado River into new territory, setting the stage for the largest mandatory water cutbacks to date.

Lake Mead, the biggest reservoir on the river, has declined dramatically over the past two decades and now stands at just 40% of its full capacity. This summer, it’s projected to fall to the lowest levels since it was filled in the 1930s following the construction of Hoover Dam.

The reservoir near Las Vegas is approaching a threshold that is expected to trigger a first-ever shortage declaration by the federal government for next year, leading to substantial cuts in water deliveries to Arizona, Nevada and Mexico.

Arizona is in line for the biggest reductions under a 2019 agreement that aims to reduce the risks of Lake Mead falling to critical lows.

The river has been slipping closer to a shortage for years, and the drought has deepened over the past year, shrinking the flow of streams that feed the river in its headwaters in the Rocky Mountains. The soils across the watershed remain parched and will soak up some of the melting snow this spring and summer. The amount of water that flows into Lake Powell at the Utah-Arizona state line over the next four months is projected to be only about 45% of the long-term average and among the lowest totals in years.

April 1, 2021 streamflow forecast Colorado Basin River Forecast Center.

Legend for streamflow forecast.

With the reservoirs continuing to drop, the expected cuts next year will reduce the Central Arizona Project’s water supply by nearly a third…

Managers of Arizona’s water agencies say they have detailed plans in place to deal with the reductions in water supplies over the next five years, even if the drought continues to worsen. These initial steps to cope with shortages are playing out while the seven states that depend on the river prepare for difficult talks on post-2026 rules, negotiating a plan for adapting to a river that’s yielding less as the watershed grows progressively warmer with climate change…

Officials who manage Arizona’s 336-mile Central Arizona Project Canal, which runs from Lake Havasu to Tucson, have known since plans were first drawn up for the system that they hold the lowest priority and could face cuts in a shortage…

Representatives of the seven states in the Colorado River Basin signed the set of agreements known as the Drought Contingency Plan nearly three years ago in a ceremony at Hoover Dam. Under one of the agreements, Arizona and Nevada agreed to take the first cuts to help prop up the level of Lake Mead, while California would participate at lower shortage levels if the reservoir continues to fall.

Under a separate deal, Mexico agreed to help by leaving some of its water in Lake Mead.

The deals lay out shortage tiers based on Mead’s levels. The federal government’s latest projections show the lake level will sit below the threshold elevation of 1,075 feet at the beginning ofnext year, triggering what’s called a Tier One shortage.

For Arizona, that means a cut of 512,000 acre-feet or about a third of the CAP’s supply…

The Colorado River’s flow has shrunk during one of the driest 22-year periods in centuries. Scientists say the West is experiencing a megadrought and one that’s worsened by humanity’s heating of the planet.

The drought over the past year has hit especially hard in the Colorado River watershed. Last spring and summer, months of extreme heat combined with the lack of monsoon rains baked the soils dry and shrank the amount of runoff, sapping the river and its tributaries.

This winter, the storms that rolled across the Rockies brought some snow, but not nearly enough to brighten the picture. The snowpack in the Upper Colorado River Basin now stands at 75% of the median for this time of year…

The upshot, as climate researcher Jeff Lukas puts it, is that “the exceptionally low soil moisture will turn a blah snowpack into a terrible runoff year.”

The effects will ripple downstream to Lake Powell and Lake Mead, which hold supplies for cities, farming districts and tribes across the Southwest.

The country’s two largest reservoirs are both headed for record lows. The last time Lake Mead reached a record low level was in 2016. The latest projections from the federal Bureau of Reclamation show Mead could fall below that mark as soon as July. Lake Powell is now just 36% full, and estimates show it could decline to a record low around March 2022.