More rain is falling on New Hampshire. Why do we still hurt for water?

The amount of rain falling on New Hampshire every year is going up, and projections say it will continue rising. But in what might seem like cruel irony, the storm patterns contributing to that trend are also a reason we’re seeing our landscape get drier, according to a study published last week by researchers from Dartmouth College.

It’s an example of how climate change is disrupting the balance we rely on to sustain groundwater supplies in the Northeast and around the world, experts said. Study coauthor Justin Mankin said facing up to these conditions will require us to be prepared for diverging extremes.

“It’s not as simple as the binary of wetter or drier, and kind of managing for that. What climate change asks society to do is manage both at the same time, and I think these results really exemplify that,” he said.

Mankin and study coauthor Corey Lesk used global rainfall data spanning decades to map the frequency and intensity of rainstorms and to understand how that is affecting water availability as the climate warms.

Across much of the planet, they found, rain events are becoming consolidated. In other words, storms are getting more intense, and they are arriving in clusters. 

There are some regional outliers, including across far northern latitudes, where rain events appear to be becoming more spread out. But in the Northeast, rainfall seems to be consolidating in line with the pattern Lesk and Mankin identified, said New Hampshire State Climatologist Mary Stampone, who was not involved in the study.

Will more water translate into more groundwater? It’s not that simple, said the researchers. In fact, their study concluded that increased concentration of rainstorms throughout the year has a drying effect on the landscape in the long term.

Furthermore, as the climate continues to warm, this pattern will be reinforced, say Lesk and Mankin. Projecting their findings into the future, assuming the planet warms an additional 1 degree Celsius — which we remain on track to see — the researchers show that this effect could lead to a widespread shift to abnormally dry conditions for many regions of the world.

“Here we uncover a new way in which a warmer world is a drier world,” said Lesk.

One reason for this, he said, is that intense storms may pelt the earth with water faster than it can be soaked up into underground aquifers. 

Excess water ends up pooling on the ground in water bodies or puddles. From there, it is more susceptible to evaporation when the sky clears, while water that filtered down into groundwater reserves stays put.

Joseph Ayotte, hydrologic interpretive branch chief with the United States Geological Survey’s New England Water Science Center and a groundwater scientist who was not involved in the study, summed up that phenomenon with a metaphor.

“If you have a pint glass, and you get a pint of water every day and you can use it, that’s one thing,” he said. “But if you get two pints, and you have a pint glass, and it spills over and you have to wait two days to get your next refill, then you can see quickly how all that water at once is not useful to you.”

The impact of evaporation on groundwater is compounded by another factor: with rainfall becoming more concentrated, stretches of clear, sunny weather — prime time for evaporation — are more drawn out and plentiful.

Lesk and Mankin’s study describes these phenomena at a global and long-term scale. But the processes involved can also help explain what we’re seeing here in New Hampshire and the Northeast, scientists said.

“What we’re seeing on the ground is what they’re demonstrating,” said Stampone. “… We’ve been saying it, but this really shows it with the data.”

Scientists in the Northeast have been pointing to this idea in their efforts to communicate why flash droughts have emerged in the region in recent years despite the ongoing increase in precipitation, she said. (That includes the drought that emerged last summer after a particularly rainy spring and continues to grip some regions of New Hampshire and neighboring states.)

Consistent with Lesk and Mankin’s findings, New Hampshire’s climate is indeed getting warmer and wetter, Stampone said, with rainfall consolidating, storms intensifying, and — on the whole — more rain falling on an annual basis.

There is some regional variation to this pattern, said Eric Kelsey, a meteorologist and researcher at Plymouth State University who was not involved in the study. For example, long-term precipitation data from Hubbard Brook Experimental Forest, in the White Mountains, shows that rain there may be getting more spread out, he said.

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But overall, Stampone said, precipitation is becoming increasingly clustered, and the drying effect described by the study can help explain the “conundrum” of facing droughts nonetheless.

Climate change drives this phenomenon by adding energy, which we feel as heat, into the Earth’s atmosphere. Higher-energy air holds more water for longer, which extends the time between storms and creates more intense storms as that water is finally released. Meanwhile, during the intervening periods, higher-energy air evaporates more water than cooler air would.

“This whole process, it’s a positive feedback loop that just enhances itself,” Stampone said.

Kelsey said the paper was an important link that advances scientists’ understanding of the many factors that influence groundwater availability.

“Ultimately we want to know, as a society, and people around the world want to know, how is water availability going to change in a warming climate and with changing land use on a big scale,” Kelsey said. From shifting precipitation patterns to increased paving and deforestation, many things will impact the future of the water supply, he said. 

“We want to understand how all these things are going to affect how much fresh water is available, because we need fresh water to live,” said Kelsey.

Ayotte, who studies groundwater availability in New England, said it was crucial not to oversimplify our understanding of drought. Lesk echoed that idea.

“I think what we’re trying to draw attention to is an effect where it’s not just the amount of water coming in versus going out, but the character of that, how that water comes — the nature of the precipitation,” he said. 

By illuminating another factor that contributes to groundwater depletion, the study helps advance conversations around drought, Ayotte said, and can help us plan for the future. That could mean adjusting our approach to drawing groundwater to accommodate changes in the way aquifers recharge, he said.

Kelsey, who has done extensive research on New Hampshire’s snowpack, said shifting precipitation patterns could affect that, too. When temperatures are warmer, rainfall can deplete snow cover in winter and the shoulder seasons, adding to the pressures on New Hampshire’s shrinking snowpack. 

To better prepare the built environment for these pattern shifts, severe rainstorms have already inspired state officials to rethink some New Hampshire stormwater and road infrastructure, Stampone said. And to ensure we have access to enough drinking water, Stampone also suggested we take a more proactive and conservative approach to water use and storage to better prepare for increasingly volatile water availability. 

New Hampshire communities often respond reactively to drought conditions as they arise, Stampone said. But it could be more productive to view dry conditions as something expected, to be handled proactively during seasons when rainfall is concentrated. 

“We need to start planning, instead of just reacting,” Stampone said. 

Adaptation also requires overcoming the “cognitive dissonance” of planning both for challengingly wet and dry periods, Mankin said. 

Mankin and Lesk will be looking more at how the long-term changes identified in their work translate into phenomena like flash drought on the ground. The two have been working together to better understand precipitation intensity and water availability for years; “I’ve been kind of obsessed with what determines water availability for people for a long time,” Mankin said. 

Lesk said he was motivated to keep going in his work by the need to understand how climate change is complicating these important questions about resource availability. 

“We need to understand the impacts. This is happening, and we’re not making very quick progress on preventing it,” he said. 

Mankin said that while communities work to adapt to increasingly extreme conditions, they must also weigh investments in climate mitigation. Like the cognitive dissonance of preparing for both droughts and floods, this task, too, requires some balance, he said. 

“That’s, kind of, the other duality of warming,” Mankin said. “… It’s also managing the equitable distribution of resources towards adaptation, towards impacts (of climate change) that are already manifesting, and also reducing our reliance on fossil fuels in order to decarbonize our economy and prevent preventable future impacts from climate change.”