Groundwater is a critical resource for most living things in California. But while human communities can increase groundwater pumping when surface supplies diminish during droughts, ecological systems often have no backup supply. We talked to Sandi Matsumoto, associate director of the Nature Conservancy’s California Water Program, about determining which ecosystems are particularly dependent on groundwater and what can be done to help them cope with dropping water levels.

PPIC: What are “groundwater-dependent ecosystems”?

Sandi Matsumoto: These are familiar places to everyone—wetlands, rivers, estuaries, springs, and associated plants and animals that rely on groundwater to meet some or all of their needs. They’re found in nearly all of the state’s groundwater basins and are often supported by surface waters, such as rivers and streams fed by rain or melting snow. Even deserts have them—where springs support desert oases, for example.

The Nature Conservancy has been working with California’s Departments of Water Resources and Fish and Wildlife to develop a comprehensive statewide map of vegetative indicators of groundwater-dependent ecosystems. Using plants as the prime indicators, we’ve mapped 2.2 million acres of groundwater-dependent ecosystems.

PPIC: How well do we understand the connections between groundwater and the ecosystems that depend on it?

SM: We’re definitely early in our understanding of this complex relationship. To fully understand it you’d have to get a hydrogeologist and an ecologist in the same room together—and you don’t see that pairing very often.

I like to think of rivers and streams as expressions of a healthy groundwater system. In California, rivers can flow well into summer months, long after the snowpack has melted. In areas with high groundwater levels, a lot of that river flow can come from groundwater. High levels of pumping can reverse this dynamic—when groundwater levels drop lower than the bottom of the river channel, water starts seeping into the aquifer. It’s called a “losing stream.”

Going forward, I think the Sustainable Groundwater Management Act (SGMA) will bring a much greater understanding of ecosystem conditions. By monitoring certain vegetation types we could improve our understanding even more. For example, we can look for signs of declining health in plants or changes in the number of acres of these “indicator plants.”

The trend has been a decline of groundwater-dependent ecosystems. The goal of SGMA is to stabilize things, but if groundwater sustainability agencies make a concerted effort, we may even be able to reverse the decline. For example, colleagues at the Nature Conservancy in Arizona are doing groundwater recharge projects near rivers, boosting their flow and improving conditions for riparian ecosystems. In California we’re trying to mimic seasonal wetlands by flooding farmland to support migratory birds. These kinds of projects help farmers while also providing habitat and even groundwater recharge benefits.

I also think that SGMA will force us to figure out what we care about and how to sustain those things. To get to groundwater sustainability we have to talk to each other, and experts will have to come out of their silos. If SGMA is done right, we’ll see hydrogeologists working with ecologists and other unlikely pairings. That’s what I’m most hopeful about.