Recharging more water from large winter and spring storms into California’s underground aquifers is a powerful tool for reducing groundwater overdraft, improving drought preparedness, and limiting flood risk to downstream communities. The banner wet year of 2023 saw considerable progress on this front. The state facilitated the recharge of flood flows through new executive orders and legislation (SB 122), and numerous local programs got more water into the ground. In the water-scarce San Joaquin Valley, our recent survey found that recharge had increased by 17% since 2017, another very wet year.
As we enter another storm season, stakeholders are focused on understanding a key question: how to build on this success, while ensuring that enough water stays in rivers to protect the environment and the rights of downstream water users? Like much else in California water, the answer is somewhat complicated. In this explainer, we take a closer look at some of the issues at stake in the Central Valley—California’s main source of water for recharge—and we highlight how partnerships could make the most of the water that is available.
The critical role of the Sacramento–San Joaquin Delta
A key driver of recharge opportunities and constraints is the management of the Delta, where the Sacramento and San Joaquin rivers come together before flowing out to San Francisco Bay and the Pacific Ocean (see map). In addition to the river water that becomes Delta outflow, some water that enters the Delta is consumed by local farms and communities, and some is drawn through Delta channels to the large export pumps in the southern Delta, where it then goes to cities and farms in the Bay Area, the San Joaquin Valley, and Southern California.
Under current operating rules, the two export projects—the Central Valley Project (CVP) and the State Water Project (SWP)—are responsible for meeting regulatory standards in the Delta. Because meeting these standards can reduce water supplies, exporters are particularly attuned to how increased upstream diversions might affect their ability to meet Delta regulatory requirements.
One key suite of regulations, relevant for all rivers flowing into the Delta, requires minimum outflows of freshwater from the Delta, both to keep Delta water fresh enough for human uses and to protect endangered fishes. If there isn’t already enough water in the rivers (i.e., if the Delta is not “in excess”), the export projects release water from their upstream reservoirs or reduce export pumping to meet the standards. If upstream recharge actions occur when the Delta is not in excess, exporters may need to make additional reservoir releases or reduce pumping to increase outflow, further reducing their own supplies. In 2023, the Delta was in excess conditions just over half the time.
Diversions for recharge upstream of the San Joaquin River can have additional impacts on Delta exports because of other regulations designed to mitigate impacts on fish from export pumping. One rule, known as the Old and Middle River (OMR) Flow Limit, restricts the rate of “reverse flow” that the export pumps can cause on these two offshoots of the San Joaquin River. Like the San Joaquin River in the Delta, these rivers are tidal (flowing back and forth) but on average naturally flow to the north. However, the pumps can cause the rivers to flow backwards, which can harm fish (see the orange arrows in the map). A second rule, known as the Inflow:Export (IE) Ratio, further limits exports during certain months to a small portion of San Joaquin River inflow.* When flows into the Delta are in flood stage, as occurred for just over a quarter of the 2023 water year, these rules do not apply.
Taken together, these two rules can cause significant cutbacks in exports if upstream diversions increase when the San Joaquin River is below flood stage, especially if export pumps are operating at full capacity: for every 1 acre-foot (af) of reduction in San Joaquin River inflows, exports can be cut back by about 0.75 af in April and May (prior to 2025)*, and by 0.5 af in other months. These significant tradeoffs between regular export supplies and upstream diversions for recharge on the San Joaquin River can occur even though this river generally brings a much smaller volume of water into the Delta, and provides a smaller share of outflow and exports, than the Sacramento River.
An illustration
The following figures illustrate how much additional water might have been available for recharge in the 2023 water year considering these regulatory issues. (The accompanying dataset, “PPIC Water Potentially Available for Recharge, 2017 and 2023,” provides sources, methods, and estimates for both 2023 and 2017, another very wet year.) This analysis provides more nuanced results than the preliminary assessment in our June 2023 report on recharge.
First, we show the simpler case of the Sacramento River, where the main regulatory consideration is meeting Delta outflow requirements. Although outflows on any given day can come from any river flowing into the Delta, we assume for simplicity that rivers contribute to required outflow in proportion to their share of total inflow, and we make a similar assumption for their contributions to exports and in-Delta use. This means that 6.7 million acre-feet (maf) of Sacramento River water would have been required to meet outflow standards, and that the river contributed 4.1 maf to Delta exports and in-Delta use, leaving about 11 maf of water potentially available for recharge (“Simple view”). Of this total, at least 1.8 maf were flood flows—water that is now generally available to divert without a permit (“Detailed view”). Diverting the other potentially available water (9.2 maf) might require a permit from the State Water Board (depending on flood conditions upstream in tributaries), but it would not entail tradeoffs between upstream diversions and exports.
Next, we show the more complicated case of the San Joaquin River. We start with a scenario where the export projects are diverting all the water they are authorized to pump (“maximum exports”). This is when the tradeoffs between upstream diversions and exports are most significant.
In 2023, total outflow requirements from the San Joaquin River were about 2.8 maf—including 1.5 maf to maintain minimum flows on the river itself, and an additional 1.3 maf of contributions we assumed would be needed to proportionately contribute to required Delta outflows. An additional 1.5 maf was this river’s assumed contribution to exports and in-Delta use. This left a total of 3.4 maf of water potentially available for recharge (“Simple view”). Of this total, about 1.1 maf were flood flows; the remaining 2.3 maf of water was in the “tradeoff space,” where increases in upstream diversions require export reductions due to the two rules explained above—the OMR flow limit and the IE Ratio (“Detailed view”).
As with the other potentially available (non-flood) water in the Sacramento watershed, upstream diversions of tradeoff water would generally require a permit from the State Water Board. But if upstream diverters took all this tradeoff water for recharge, exporters would need to reduce their pumping by 1.4 maf, or more than 20% of typical wet-year exports. The net increase in total water diverted, system-wide, would be just 0.9 maf—much less than the 2.3 maf of new upstream diversions (“Granular view”). Facing such a large potential loss of their own supplies, it’s easy to see why exporters have been concerned about possible impacts of greater upstream diversions, even if this could increase overall water supplies.
And because increasing upstream recharge also reduces Delta outflow, this could raise ecological concerns in some periods—such as during early season, “first flush” storms that benefit endangered fish in both the San Joaquin and Sacramento rivers. Although our proportional flow assumptions for meeting minimum regulatory requirements limit the reductions of San Joaquin inflow and Delta outflow beyond what they might otherwise be, regulators might want to limit upstream diversions of tradeoff space water to add an additional margin of safety.
In reality, limitations on water storage south of the Delta in wet years prevent the export projects from pumping to their full capacity in some periods. This shrinks the tradeoff volume and raises the potential for broadly beneficial increases in recharge. In the “actual exports” scenario for 2023 shown below, the projects exported about 0.6 maf less than they could have. All of this water could have been available for diversion upstream without entailing export cutbacks to meet the OMR and IE rules (“Detailed view”). The tradeoff space was correspondingly smaller, at just 1.7 maf (“Detailed view”). And with a smaller tax on exports in this space (about 0.5 acre-foot lost for each additional acre-foot of water diverted upstream, down from 0.6 acre-foot lost in the maximum exports scenario), there are more compelling opportunities for recharge partnerships between exporters and upstream diverters (“Granular view”).
Implications for policy and practice
Since the 2023 legislation authorizing the recharge of flood flows without a diversion permit, there have been ongoing discussions about further changes in the law and regulatory practice to facilitate beneficial recharge activities. One issue involves clarifying when flood flows should be available for diversion without a permit, including possible legislative tweaks to SB 122 that failed to pass earlier this year. Also of interest is whether the State Water Board should move beyond its very conservative approach to recharge permitting by authorizing additional upstream diversions when inflows to the Delta are still quite high, but below flood stage.
While upstream diverters have sought as much flexibility as possible, exporters have proposed a variety of restrictions to limit upstream diversions when this would cost them water. Environmental advocates, for their part, have emphasized ensuring that adequate water remains in the rivers for the ecosystem, another important element in this policy space. Our analysis suggests several important takeaways for advancing beneficial recharge policies:
1. The tradeoffs between upstream diversions and exports can be significant, especially for diversions upstream of the San Joaquin River. Given California’s “area of origin” laws, some upstream diverters may believe that they should have the right to divert more water upstream, even if it impacts Delta exports. But it is important to recognize the tradeoffs at stake. Of the 7.7 maf of inflow to the Delta from that river in 2023, only 1.1 maf was coming in at flood stage—and thus available for recharge without impacting minimum Delta outflows or exports. Anything more entails tradeoffs that would need to be worked out in policy discussions.
2. The fact that exporters don’t operate at capacity during wet years creates more opportunities for recharge. In both 2023 and 2017, an additional 0.6 maf could have been diverted if upstream diverters and exporters had worked together to get that water in the ground upstream of the Delta. There are also clear opportunities for mutual benefit by increasing upstream diversions when the San Joaquin River is below flood stage, as long as exporters are compensated for the reductions in water they can take at the pumps. Having agreements in place that can be acted on quickly when opportunities arise could benefit all parties.
3. More flexibility is needed regarding upstream diversions of flood flows. While the charts above show broad measures of flood stage on both rivers, upstream tributaries can be in flood stage at other times. For example, the upstream community of La Planada in Merced County flooded on January 9, 2023, more than a week before the San Joaquin River reached flood stage at Vernalis. Allowing recharge diversions that can reduce flood damage—even if they impact exports—will be important in such situations.
4. Updates to environmental regulations in the Delta likely will not change the basic story. The export projects will soon have an updated long-term operating plan to comply with the Endangered Species Act, and additional changes are likely under the State Water Board’s pending update of the Water Quality Control Plan for the Bay–Delta. While these changes would increase the minimum required Delta outflow in some years, they are not likely to significantly affect the water available for recharge in wet-year conditions. For example, the new operating plan for the projects reduces exports during dry years but allows additional exports in wet years. It also removes the April-May export restriction that cut back exports more than other months.* Finally, recent analysis suggests that substantially more water could be safely diverted for recharge under a functional flows approach than would be required under the board’s proposed minimum instream flow requirements.
5. Other state and local policies can help make the most of available water. Our survey of recharge experiences in the San Joaquin Valley in 2023 found that authorizing more diversions is just part of the solution. At the state level, a simplified approach to permitting streambed alterations will also be essential to making recharge projects operational, for instance. And local policies that incentivize landowners to put more water on their land can help unlock the enormous potential of recharge on farmland—a cost-effective way to get floodwaters out of harm’s way and into the ground for future use.
Our recharge survey also found that partnerships among different parties have been essential to California’s progress in groundwater recharge to date. Continued collaboration and compromise hold the key to moving past roadblocks and making the most of the opportunities that lie ahead.
*This post was updated to reflect the adoption of the new operating plan for the projects on December 20, 2024. The new rules eliminate the I:E Ratio restriction described above.
