Is It Possible for California to Store Current Rainfall for Future Droughts?

Recent weeks have brought an overwhelming amount of rain to California, leaving farmlands flooded and transforming dry creeks and drainage channels into rushing streams flowing toward the sea. Despite this, much of the state remains under severe drought conditions.

The sight of so much runoff during a drought raises an important question: Why isn’t more of this rainwater being captured and stored for the dry months of spring and summer when water is scarce?

As a hydrogeologist at the University of California, Santa Cruz, I focus on exploring ways to capture runoff from major storms on a large scale. There are two main methods for large-scale water storage that could alleviate drought conditions: storing water in reservoirs behind dams and recharging groundwater supplies.

Why Isn't California Capturing More Stormwater Runoff?

During intense weather events like the atmospheric rivers that struck California in December 2022 and January 2023, water officials across the state often wonder why more of this water can't be retained. The answer lies in the complexity of the issue.

California's large dams and reservoirs, primarily located in mountainous regions, can hold significant amounts of water. However, when these reservoirs near their limits, water must be released to prepare for incoming storms. Without additional storage downstream, much of this water flows into the Pacific Ocean.

In urban areas, a major obstacle to collecting stormwater runoff for large-scale use is contamination. Initial runoff from roads often contains pollutants, and flooding can lead to septic system overflows. Treating this water would be necessary before it could be utilized.

One might argue that the collected water doesn’t need to be potable; it could be used for purposes like irrigating golf courses. However, this would require dedicated storage facilities and a separate distribution system with its own pipes and pumps, as it cannot be mixed with drinking water infrastructure.

Recharging Groundwater Supplies

Another viable solution is to direct the water underground, where it can help replenish depleted groundwater reserves.

Managed recharge, a long-established practice, has been employed for decades in numerous regions to actively restore groundwater levels. Recently, as wells dry up due to prolonged drought, these methods have garnered increased interest. Over 340 recharge initiatives have been proposed across California, with the state projecting they could replenish an average of 500,000 acre-feet of water annually if fully implemented.

One approach under consideration by the Department of Water Resources and other entities is Flood-MAR, or flood-managed aquifer recharge. This strategy involves diverting excess river flow during peak periods to flood vast areas, allowing water to seep into underground aquifers. The idea is to flood land in winter and use it for agriculture in summer.

Flood-MAR shows great potential, but its success depends on landowners willing to flood their properties and securing the necessary water rights. Additionally, not all landscapes are suitable for this type of water absorption.

Flooding 1,000 acres (404.6 hectares) of ranchland could result in standing water for days or weeks. While some crops can withstand this, others may suffer damage. There are also concerns about creating environments that attract pests or compromise food safety.

Another hurdle is that the majority of significant river flows occur in northern California, while the most severe groundwater shortages are in central and southern regions. Transporting this surplus water to areas in need involves complex and costly distribution systems.

Motivating Landowners to Participate

In the Pajaro Valley, a key agricultural area near Monterey Bay, my colleagues and I are piloting a unique groundwater recharge initiative. This project targets runoff from hill slopes during heavy storms.

The concept involves redirecting a portion of this runoff to infiltration basins spanning several acres. These basins allow water to collect and seep into the ground, ideally on farmland or open spaces with suitable soil. We prioritize coarse soils, which facilitate water percolation through gaps between particles. However, since much of the terrain has finer soils that hinder rapid infiltration, selecting the right sites is crucial.

A Pajaro Valley initiative incentivizes landowners to join recharge projects by offering a rebate on their water usage fees through a "recharge net metering" system.

Our cost-benefit analysis of this method revealed that, even after accounting for construction expenses and soil removal, the costs are comparable to securing alternative water sources. It is also more economical than desalination or water recycling.

Can the Recent Rainfall Alleviate the Drought?

Addressing the region's prolonged drought will require multiple strategies and several years of above-average rainfall. A single storm or even one wet year is insufficient to resolve the issue.

For groundwater-dependent basins, replenishing aquifers is a slow process. If this marks the end of the rainy season, we could face water shortages again within a month.

Andrew Fisher serves as a professor of Earth and planetary sciences at the University of California, Santa Cruz. His research is supported by funding from the U.S. National Science Foundation, U.S. Department of Agriculture, National Aeronautics and Space Administration, Gordon and Betty Moore Foundation, Santa Clara Valley Water District, and the U.S. Geological Survey. He is also affiliated with the Public Policy Institute of California's Research Network.

This article has been republished from The Conversation under a Creative Commons license. The original article can be accessed here.