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Study: Heavy Storms May Be Enough to Recharge California Groundwater

A handful of storms each year could provide enough water to offset annual groundwater overdraft in California’s Central Valley. But Tiffany Kocis of U.C. Davis explains that work is needed to capture and distribute that water.

Written by Matt Weiser Published on Read time Approx. 6 minutes
Stormwater floods the 5-acre almond orchard of Modesto farmer Nick Blom in an experiment in 2016 to restore the drought-depleted aquifer in Modesto, Calif. If the water was not diverted to the Blom farm, the stormwater would flow into the Tuolumne River.Paul Kitagaki, The Sacramento Bee

California’s Sustainable Groundwater Management Act, passed in 2014, requires some 250 groundwater basins throughout the state to halt the overdraft in their aquifers. The big question for everyone is: Where will the water come from to do that?

It could come from “high-magnitude flows” – flooding events, essentially, that occur from just a handful of storms every winter. Tiffany Kocis, a PhD student in hydrologic sciences at University of California, Davis, is the lead author of a new study that attempts to quantify these high flows. It’s one of the first efforts to measure how much water might be available for groundwater recharge from these storm events, and the results are surprising.

Kocis and her coauthor, Helen Dahlke, an assistant professor of hydrology at U.C. Davis, estimate that 2.6 million acre-feet of water is available in an average year from these high-magnitude flows. They analyzed historical streamflow data to arrive at that number. So the estimate is based on real flows that have occurred. They also estimate this water is surplus to both existing water rights and to environmental flow requirements in the Sacramento-San Joaquin Delta.

Coincidentally, this amount of water also nearly equals the average annual groundwater overdraft in the state’s Central Valley. This suggests these high-magnitude flows could be an important tool to recharge stressed aquifers. That is, if the water can be captured by groundwater banking projects, the flooding of farm fields and other means.

Within a few weeks, the results will be presented online in an interactive format at

Water Deeply recently interviewed Kocis to learn more about her findings.

Water Deeply: What’s the most important result of your study?

Tiffany Kocis: California has tons of water, but it’s all allocated and appropriated. We were looking for a potential source of unmanaged water that we could utilize for groundwater banking. What we came across were these really high-magnitude flows that often aren’t captured because it’s during the winter. These flows are often really high and so they’re hard to utilize. And they occur early enough that the reservoirs need to be emptied for flood storage. So a lot of this water, we hypothesize, is flowing out to the ocean.

Some people say a lot of this is “wasted” to the ocean. We wanted to look at whether or not we could actually utilize this water. Was it actually available? And how much is there?

We found there was quite a bit of water. We report that in an average year, there are high-magnitude flows of approximately 3.2 cubic kilometers of flow (2.6 million acre-feet). That’s enough to sort of offset our average annual groundwater overdraft.

Part of our findings that are also important is how quickly these flows occur. These are storms that last a few days. We don’t get really high flows every single year. But when we do get them, we get a lot.

I think it becomes a management question from here.

Water Deeply: That’s a lot of water. Did this result surprise you?

Kocis: Yes and no. I suppose I was surprised by the results in that it’s fairly close to what we see in groundwater overdraft. But having lived here for a few years now and kind of experienced the winters and checking out the Yolo Bypass when it floods, it’s not wholly unexpected anymore for me. It’s incredible to me the volume of water that you see with these high flows.

I think it is a potential part of the solution. I wouldn’t say it’s outright going to solve everything about it. But I think it’s something that really needs to be explored and considered in the future of California water. It’s one thing to look at it from a research perspective, and another thing to get people to agree that it’s available and actually want to use it. That’s why I really stress that I feel like it’s a management issue.

Water Deeply: A report this year from the state Department of Water Resources found there was very little water available for recharge. Why are your results so different?

Kocis: The biggest thing is that the estimates from the DWR report are made from a computer model. You can only rely so much on the accuracy of model data: It doesn’t really capture data at a scale that we’re looking at. Their model looks at monthly data. Our underlying data set is daily. And our results are not based on modeling. It’s all based on historical streamflow data. Nothing has been altered.

Also, DWR is required to respect water rights data. But we feel there’s a lot less water actually diverted than the face value of water rights allow. Particularly during the winter, I would say people are underutilizing their water rights.

Water Deeply: What are these high-magnitude flows, exactly? And are they truly surplus to the needs of the environment?

Kocis: Basically these are times when the Delta is considered in true excess, where basically it’s meeting all the environmental requirements and all the needs of the Central Valley Project and State Water Project. So it’s kind of considering both water rights and environmental concerns in the Delta. I was surprised when I compared the data. It wasn’t just, “Oh, it’s a little bit over this amount.” It’s a lot of water passing into the Delta during these times.

But even if you cut it in half – even if you cut out 1.5 million acre-feet of water for the environment, you’re still left with 1 million acre-feet for recharge. And this may be only part of the solution to groundwater overdraft, so you might not need to utilize the whole thing.

It has been shown these really high flows are critically important for ecosystem function. But a couple of studies have shown it’s really only the first one or two storm events that really drive these processes of sediment transport and habitat formation and channel scouring. So a potential management solution for this is to say, let’s just skip the first two storm events, and after that you can start diverting onto your farms for groundwater banking. But you have to let the first two storms go by.

We’re trying to say, hey, this is available. You could potentially use this. But obviously we need to consider a whole host of other factors. But we don’t also have to pretend like this water doesn’t exist.

Water Deeply: How much do atmospheric rivers drive these high-magnitude flows?

Kocis: The vast majority of these flood events we get in winter are from atmospheric river events. California winter storms tend to be that way. They are these really intense, short-duration storms. You get these intense rain storms in relatively localized areas where you end up with tons of flooding.

A good way to set this up as a management question is to ask where we expect these storms to strike. We need to have plans in place to say this is who’s going to be diverting water where. If there could be some collaborative decision making on this front, these flows could be utilized.

California has a relatively predictable, intense storm pattern. You get a little bit of warning that it’s going to happen. We know that it’s about to come. So I think that having a plan in place could really go a long way to making this work as a groundwater banking solution.

Water Deeply: Do we have the ability to capture and distribute these flows? Are there canals and floodways in place to handle it?

Kocis: As the system exists currently, it could not handle moving all of these flows. It currently is lacking in capacity to do these things. We could do a part of them, but it definitely could not take all the flows all at once.

A potential solution is reoperating reservoirs. These things operate under regulations and there’s very little space in the current system for saying “could we maybe work this differently for the next month.” There’s no flexibility in the system right now to work with these high-magnitude flows we get early in the season to push groundwater banking efforts.

I don’t think there is a future in which California doesn’t invest money improving that. I think it’s an inevitability that California infrastructure will be changed to deal with things like this, because we need the water.

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