Water Storage 101: How 3 Types of Reservoirs Support Long-Term Water Supply

In California, the past 20 years have been the driest in over 1,000 years, but a deluge of storms in January and February 2023 brought more than 24.5 trillion gallons of water to the state. “In the past several years, we’ve said, ‘We welcome every drop we can get,’” Dudek hydrogeologist Jill Weinberger said. But can there be too much of a good thing? When a large amount of water falls too quickly, it causes flooding in rivers when the water level rises above the banks and it causes flooding in cities when our stormwater infrastructure can’t carry water away quickly enough. Many people point to the benefit of this large amount of rain in refilling our depleted reservoirs, but the reality is more nuanced. Three types of water storage reservoirs contribute to a diversified water portfolio in California. Understanding how these different reservoirs contribute to our water supply needs helps California and other western states plan for the sustainability of their water supplies.

Lakes: The Prototypical Reservoir

Typically, you think of a reservoir as a lake, which is created by a dam that captures water that has flowed down a river. The lake may be open for public recreation, but its primary purpose is to store surface water, periodically releasing it when water supplies are needed or when the reservoir is nearing capacity. In California, approximately 40-million-acre feet of surface water is stored in reservoirs. Surface water reservoirs contain the most readily accessible water and are usually the most easily replenished when the state gets wet winters.

Water collects throughout the year in lake reservoirs and must periodically be released when the reservoir reaches capacity.

Snowpack: The Winter Reservoir

You may not immediately think of snowpack as a reservoir, but snowpack in the Sierra Nevada mountains on the east part of the state serves a vital function in ensuring California has water year-round. California receives most of its precipitation between October and March. Wintertime temperatures ensure that snow falling in the mountains stays frozen. After March, when temperatures rise (and rainfall becomes scarce), the frozen snow melts, flowing down rivers, collecting in surface water reservoirs. Though snowpack can provide an abundant water source, unlike surface water reservoirs, water managers don’t have control over how much water is released or when. Ideally, the snowpack remains until late spring so that water is available during the driest summer months. However, when less rain falls and temperatures rise earlier in the spring, that snow melts earlier, traveling to reservoirs that may already be near capacity from rain events, meaning valuable reservoir water must be discharged to accommodate snowmelt.

Panoramic view towards Lake Tahoe on a sunny clear day; the snow covered Sierra mountains in the background; evergreen forests in the foreground.
Snow falls in the winter, with the snowpack serving as a water reservoir until temperatures rise and the snow melts in the spring.

Groundwater: The Hidden Reservoir

Perhaps the most vital reservoir, groundwater has the most significant impact on long-term water sustainability. Jill explains, “Our lake reservoirs are like a checking account; water comes in and it gets released” more frequently. “Groundwater, however, is more like a savings account. It gets stored over the long-term” for use in the future, such as pumping water from a well to water crops. A checking account works because of the frequent in/out flow. But, while a savings account can really help you out in a pinch, if you don’t replenish what’s taken out, its utility is limited.

In the past 20 years, when California has seen comparatively little precipitation, the groundwater reservoir has been severely depleted as cities and farmers relied on groundwater to make up the difference when surface water supplies were limited. Over this same period, the groundwater aquifers were not being replenished by storms. Additionally, the ground is parched because the past two decades have been so dry. As a result, more water is absorbed by the soils without either replenishing the groundwater aquifers or running off into rivers to replenish the surface water aquifers.

Water can be directed from water treatment facilities and lake reservoirs to groundwater recharge basins. There, the water collects and can absorb back into the groundwater aquifer for long-term storage and use.

Leveraging Water Storage for Climate Resilience

With climate change, California and other western states are bracing for more prolonged periods of drought that are offset by large storms that deliver lots of water over a short period of time. Despite the destruction caused by the recent large storms in California, they also present a learning opportunity to better plan and prepare for future storms. Throughout California and the west, water managers and planners are reviewing surface and groundwater supply systems to plan for and improve water storage. Many of these plans include increasing the ability of local agencies to recharge groundwater aquifers through managed flooding of farm fields and construction of new recharge basins. California’s Sustainable Groundwater Management Act is one policy that seeks to improve sustainability in the state’s overdrafted groundwater basins. Dudek is on the leading edge of groundwater sustainability plan (GSP) development, having prepared nine GSPs, three of which have already been approved by the California Department of Water Resources (DWR).

As agency plans are approved, implementation is the next hurdle. Our hydrogeologists, planners, and engineers support successful implementation of GSPs, including project design and analysis, stakeholder engagement and facilitation, annual report preparation, numerical groundwater modeling assessments, climate change analysis, and grant writing support.

“We’re trying to get more water into the ground and save it for drier periods,” Jill said. We have 20 years of experience in California with an increasingly dry climate, but this winter has also revealed the other side of climate variability. When the water comes, states must have planned to capture it for long-term storage and release.

Contact us for more information on how our experts can support your water storage, groundwater sustainability, or water supply planning goals.