After three very dry years, California’s reservoirs will be full again this spring. That beats the alternative, but what will it mean for water supplies over the next few years?

We’ve been here before. Much of the state relies on water captured in the main Central Valley Project (CVP) and State Water Project (SWP) reservoirs for its water supply. In spring 2019, those reservoirs were full, with about 9 million acre-feet (maf) of water stored in Shasta, Oroville, and Folsom. But less than two years later, in early winter 2021, those same reservoirs were at dangerously low levels, leading to a drought emergency.

So, what happened last time, and will it happen again?

Speculation abounded in 2021 about why reservoirs were low. Some claimed that the full reservoirs in 2019 should have gotten the state through a seven-year drought, but the water was “wasted to the sea” for the environment. Others claimed that those full reservoirs had plenty of water for ecosystems, but they were drained to send water to “billionaire farmers.”

Here are the facts: while the reservoirs can be full in the spring, they are not allowed to be full in the fall, when they are drawn down to make room for flood control. Furthermore, not all the water in a reservoir is usable. There is about 1.2 maf of “dead storage” in those three reservoirs (including 0.8 maf in Oroville alone)—that is, water at the bottom that cannot be accessed. That leaves at most about 5.5 maf in usable storage going into the winter rainy season.

In years with low precipitation, there’s a large, built-in demand for this storage that is rarely acknowledged: keeping the Delta fresh. Throughout the year, a lot of flow is needed to flush out ocean water and keep the Delta fresh enough for agricultural, municipal, and industrial uses within the Delta and in regions that use Delta water exports. In a year classified as “dry,” it takes about 4.2 maf per year of outflow from the Delta and into San Francisco Bay just to keep the Delta’s water fresh enough to use. In a “critically dry” year, when water quality standards are reduced, it takes 3.6 maf of flow.

So unless runoff from winter and spring storms brings in more water than the volume used upstream and within the Delta, the 5.5 maf of usable storage is simply not enough to keep the Delta fresh—let alone meet other downstream demands for water exports and fish protection—in just two consecutive dry years.

This storage constraint was on full display in 2020 and 2021. In 2020, storm runoff was low, and the reservoirs ended the water year with just 2.7 maf in usable storage. In 2021, runoff was lower still—in fact, total runoff equaled the water used upstream and in the Delta. (Delta farms alone use about 1.8 maf per year. In 2021, upstream uses were 7.3 maf and total runoff was 9.1 maf.) With no runoff to spare, keeping the Delta fresh required a heavy drawdown of the water remaining in the reservoirs.

The timing of runoff puts further pressures on reservoirs when it’s dry. A lot of runoff comes in the winter when upstream and in-Delta use is low; by summer—which is also irrigation season—runoff is often inadequate to meet user demands and Delta outflow needs, and reservoirs start to play a larger role. In 2021 (and again in 2022), both runoff and reservoir water were so scarce that Delta outflow requirements were relaxed and a barrier was placed in the Delta to keep the water fresh. Those actions saved about 0.4 maf of water in 2021.

While some claimed the relaxations were made to allow more exports, exports were actually severely cut back—most farms that use Delta exports got nothing or close to it, and municipal deliveries were at minimal amounts needed for health and safety.

So the bottom line is this: in 2020 and 2021, the reservoirs were not drained to waste water to the sea, nor to supply billionaire farmers. They were principally used, together with limited amounts of runoff, to manage Delta salinity (and keep that water fresh enough to use) in a very dry and difficult period.

Now, as in 2019, we find ourselves with full reservoirs and a lot of optimism. But if 2024 and 2025 are dry like 2020 and 2021, this scenario will repeat itself.

As in 2019 (and prior wet years like 2011 and 2017), conditions are now so wet that the CVP and SWP are running out of places to store water, and they are starting to reduce exports below authorized levels. To get out of this rut, we need to manage wet period flows better, putting more water into the ground and into offstream storage quickly so we can get through the long, dry periods that we know are coming. This means continuing to advance investments in storage, and improving the processes that enable us to make timely decisions to capture more water from large storms.