California officials plan for a dry 2025 with grim water supply guesswork

Each December there’s a new version of an old guessing game about how much water will be provided to agricultural and municipal users in the year ahead.

Federal and state water agencies post initial, and usually very low, estimates based on the current condition of reservoirs, soil conditions that affect runoff, and assumptions of rain and snow during the winter and spring.

Over the next few months, the estimates are upgraded as firmer precipitation data accumulates, often — but not always — increasing.

For 2022, as drought gripped California, the state Department of Water Resources initially projected zero water deliveries, later raised them to 15% of the contracted supplies, but finally delivered just 5%.

One year later, however, the department initially promised 5%, but after a very wet winter finally delivered 100%. This year began with a 10% estimate of state water supply and ended up with 40%.

The huge swings in initial allocations and final deliveries are an obvious headache for the 29 local and regional water purveyors supplied by the state water system, serving some 29 million people. Do the public water agencies impose strict conservation on their customers in years with low initial projections, try to obtain supplemental supplies, take a chance that eventual deliveries will be higher, or all of the above?

The annual game resumed this week, when the Department of Water Resources announced an initial 2025 estimate of just 5%.

“Based on long-range forecasts and the possibility of a La Niña year, the State Water Project is planning for a dry 2025 punctuated by extreme storms like we’ve seen in late November,” department director Karla Nemeth said in a statement. “We need to prepare for any scenario, and this early in the season we need to take a conservative approach to managing our water supply. Our wettest months of the season are still to come.

“What we do know is that we started the water year following record heat this summer and in early October that parched the landscape. We must account for dry soils in our State Water Project allocation planning and our runoff forecasts for the spring.”

That’s a fairly grim scenario that may already be outdated because it was made without counting the heavy rains and snows that hit the state in late November.

“These storms will be taken into account along with other variables for future allocation updates. Prior to these storms, the start of the water year had been dry and warm,” the department said. It’s also noteworthy that after the spate of storms, California’s weather has returned to dry and warm.

Another factor in the guessing game is the status of reservoirs, not only the state’s Lake Oroville, but the multiple storage projects managed by the federal government, such as Lake Shasta, and those owned by cities and irrigation districts.

At the moment, the state’s reservoirs are generally above 100% of historic averages after a couple of relatively wet winters, which indicate that ultimate water deliveries will be higher than the low initial estimates, although how much higher is uncertain. Shasta is at 113% and Oroville at 109%.

The annual guessing game would be more accurate if the state had done what it should have done decades ago — developed more storage capacity, either in reservoirs or aquifers, that could be filled in wet years and cushion the impact of drought.

A couple of storage projects are underway, Sites Reservoir on the west side of the Sacramento Valley and an enlargement of the San Luis Reservoir in the Pacheco Pass west of Merced.

Much more is needed as climate change affects the precipitation cycle.