Last fall, UC Riverside’s Dr. Hoori Ajami co-authored a study looking at how long-term droughts are impacting river flows across the US. We asked Dr. Ajami and The Nature Conservancy’s lead river scientist, Dr. Bronwen Stanford, to tell us about the study and its implications.
First, what is a “baseflow drought” and how is it distinct from a precipitation drought?
Hoori Ajami: Water in a stream has two sources: precipitation and groundwater. “Baseflow” is groundwater’s contribution to a stream’s flow. We were specifically interested to see how a river’s baseflow changes after a precipitation drought. When we talk about “drought,” we typically mean a precipitation drought, but there are different types of droughts, including baseflow and groundwater droughts. Any precipitation changes can impact our water balance, which includes groundwater and baseflows.
Often in a hydrologic system, there’s a “memory effect.” A precipitation drought’s effects can take a long time to show up in baseflow—up to 5 months later. In a 2021 paper, we found that the lag-time for groundwater is longer—up to 185 months—and it could take 30 years or more for groundwater to recover from a drought. This is an invisible resource; we don’t have much monitoring data across many regions.
Why should we care about baseflow droughts? Are these a problem in California, and are they getting worse?
HA: Baseflow sustains streamflow, particularly when there’s no rain. It provides essential water for the ecosystems and communities that depend on that water. I came to California towards the end of the 2012–16 drought, and people thought the drought had ended when the precipitation drought recovered in 2017. But our study shows that the impact of drought could last much longer—and that’s important for water management.
Bronwen Stanford: In California, we don’t get rain for half of the year. During those months, we have water in our rivers because of groundwater. Groundwater sustains wet-season and dry-season baseflows, which are extremely important for the vegetation, including trees, that grows along rivers, as well as the fish and other animals that live in or rely on freshwater. Groundwater is usually a lot cooler than surface water too, which is really important for creating thermal refugia—keeping water cool enough so endangered fish can survive under climate change. And these groundwater-supplied baseflows sustain the system in dry times. When flows become too low, it can devastate the system. In some cases, low flows impact water quality. In other cases, rivers dry up completely.
Humans tend to deplete groundwater in a drought because other sources of water disappear, making the stress on aquatic wildlife and river ecosystems even worse. This study is a great contribution to help us think about how groundwater affects rivers.
HA: We intentionally looked at catchment areas—that is, areas from which rainfall flows into a river or lake—that are not influenced by human activities, and there are not many datapoints in California because of that. We wanted to separate drought from the influence of water management practices. Overall, we show that for regions where we have data, baseflow drought impacts seem to be more severe in arid or semi-arid climates. The data also shows that the severity of baseflow droughts has increased across the US, particularly in the western and southeastern parts of the US.
BS: The Department of Water Resources has just started releasing more guidance about how they plan to think about the interconnection between surface and groundwater under the Sustainable Groundwater Management Act. The Nature Conservancy also mapped groundwater-dependent vegetation to help people understand these ecosystems better; we need to think about them as part of groundwater management. These systems—including wetlands and riparian areas—are deeply affected by groundwater drought.
How might one incorporate the concept of baseflow droughts into water management in California? Is there something we should do differently to prepare for and respond to them?
HA: It’s a challenging problem. After 2016, we had a big water year and people thought the drought was over. But in terms of groundwater, we still haven’t recovered from the 2012–16 drought, especially considering all the pumping. The groundwater system responds slowly: it takes time for it to recover from drought, and overpumping makes this recovery even longer. Management needs to be long term and also adaptive, based on monitoring.
BS: I second what Hoori said. I think we need to recognize that baseflow droughts should be a big part of how we think about and plan for climate change, especially in smaller watersheds where we maybe aren’t monitoring and modeling as closely. Climate change is likely to increase drought frequency, and it will be important to remember that river baseflows might not recover as quickly as we expect them to, or ecosystems might be more stressed than we expect, even after droughts appear to be over. Learning from studies like this can help us be more nuanced and precise. We need to be more realistic about how much water is available.
