By Jay Landers

On the Earth’s surface, the end of a drought is usually obvious. Rainfall arrives, streams and rivers increase in volume or begin to flow again, vegetation perks up, and parched soils regain their moisture. But what about below the earth’s surface? In particular, how does groundwater respond to the end of drought? Given the critical role that groundwater plays in many regions as a source of water for drinking and irrigation, understanding the effects of droughts on aquifers could help shape management decisions regarding these key resources.

Seeking to better comprehend the relationship between droughts and groundwater, researchers at the University of California, Riverside examined decades worth of groundwater data from more than 250 wells across the United States. In contrast, previous research on the effects of drought on groundwater has mostly relied on modeling studies.

By observing the timing and extent to which the various unconfined aquifers were affected by multiyear droughts, the researchers found considerable delay between the start and end of a drought and the point at which groundwater levels begin to decline and recover. The recent findings have implications for groundwater management practices, particularly in areas with significant pumping.

Well data

The research was conducted by Adam Schreiner-McGraw, Ph.D., who was a postdoctoral fellow at the UC Riverside, and Hoori Ajami, Ph.D., an associate professor of groundwater hydrology at UC Riverside. (Schreiner-McGraw now is a research hydrologist for the Agricultural Research Service within the U.S. Department of Agriculture.)

The pair published their findings in an article, titled “Delayed response of groundwater to multiyear meteorological droughts in the absence of anthropogenic management,” that became available Sept. 15 on the website of the Journal of Hydrology.

For their study, the researchers relied on data from wells monitored by the U.S. Geological Survey as part of its Climate Response Network. To be included in the study, a well had to have at least 10 years of associated data during which time it experienced at least one drought. What is more, the well had to be located in a single, unconfined aquifer that was not subject to groundwater pumping. This requirement was included to rule out the effects of pumping or other human management practices on groundwater levels, Ajami says.

All told, the researchers included 266 wells from 19 eco-regions across the contiguous United States. They then developed a drought recognition algorithm to assess the well data and identify meteorological and groundwater droughts while accounting for seasonal cycles of precipitation. This approach detected 322 multiyear droughts among the studied wells. Based on these results, the researchers determined what they identified as the lag time and recovery time for a given aquifer.

‘Surprising’ results

The lag time “represents the time that it takes until changes in precipitation propagate through the vadose zone and/or changes in streamflow in a connected surface water-groundwater system impact groundwater levels,” according to the article. By contrast, the recovery time “consists of the time lag between the cessation of negative monthly precipitation and groundwater anomalies … and the time needed for the groundwater levels to rise to the five-year average pre-drought groundwater levels,” the article states.

Of the wells analyzed in the study, the researchers found that the mean lag time was 20.1 months. “On average, across the country it’s about two years until we see the impact of the precipitation drought on groundwater,” Ajami says. “This is kind of a long time.” That said, some aquifers can take much longer to begin to respond to the effects of drought. “In some cases, this time lag could last up to 15 years,” Ajami says.

Meanwhile, groundwater recovery times were found to exceed the lag times. Based on the wells examined for the study, shallow aquifers take three years, on average, to recover the storage lost during a multiyear drought, the article notes. More broadly, “for 85% of droughts, groundwater levels spanning across multiple aquifer systems have recovered from the drought within 10 years,” according to the article.

However, the return of rainfall does not guarantee that a depleted aquifer will return to its previous levels, Ajami notes. “In some wells across the country, we see that during the time period that we have the data, the wells never recovered from drought,” she says. “That was surprising.”

‘Important implications’

Such findings have “important implications for water management,” Ajami says. For example, she points out that water conservation measures often are discontinued once rains have resumed with regularity. “We may see that precipitation has already recovered or we may see water in the stream, but that doesn’t mean that our aquifers are in good shape,” Ajami says. “Maybe water conservation measures should be continued much longer than when the precipitation drought is ended.” Under such circumstances, farmers could consider switching to crops that require less water, if practical, Ajami says.

The study results also raise the prospect that irrigation may have more of an effect on groundwater than previously realized. “In areas where irrigation is sourced from groundwater, our estimates of the time it takes an aquifer to recover from drought would be conservative,” Schreiner-McGraw says. “If pumping is occurring, we would expect recovery to take longer, if the aquifer ever recovers.”

As a result, Schreiner-McGraw calls for a prudent approach to groundwater use during droughts.

“I hope that our results cause water managers to be cautious about increasing groundwater pumping rates during a drought,” he says. “Because the impacts of the drought on the groundwater might not begin to be seen for several years, it can be easy to pump too much water.”

Climate change also can be expected to exacerbate the effects of drought on groundwater supplies. “Our results showed that when droughts are more severe, the time needed for groundwater to recover increased,” Schreiner-McGraw says. “Droughts are projected to become more severe with climate change, so we would expect the recovery time to increase in the future.”

Other problems

Longer recovery times for aquifers could contribute to other problems, including subsidence. This outcome becomes even more likely in the event of excessive pumping, Schreiner-McGraw says. “If rainfall decreases and water managers and farmers pump a lot of groundwater, this can deplete the aquifer before the reduced precipitation has had a chance to impact the aquifer, because of the lag time,” he notes. “In this case, we can get more subsidence, which prevents the ability of groundwater to recover the lost storage.”

Changes in groundwater levels also can cause other complications, such as degraded water quality. As water levels drop in aquifers, arsenic or other contaminants in groundwater could potentially be mobilized, Ajami says. “So you are starting with a problem of water quantity, but the end result might be a water quality problem,” she says.

On the bright side, climate change potentially offers a way to help offset declining groundwater levels in certain locations, Ajami says. Some global climate models “predict that precipitation events in the future are going to be more intense,” she notes. In such a case, extra water not needed for irrigation could be collected and stored underground by means of managed aquifer recharge measures and then withdrawn during periods of drought, Ajami says. “We need to move toward that direction,” she says.