What are flash droughts? | Discover the magazine

You’ve heard of flash floods, but have you ever heard of flash droughts? These events are relatively new to natural disasters and happen quickly, with conditions changing from normal to extremely dry in less than a month. This means people don’t have time to prepare for the consequences, which can include withered crops, dried up streams or exhausted wells.

According to Ben Cook, a climatologist at NASA’s Goddard Institute for Space Studies, efforts have been made over the past six years to better understand flash droughts. One of the goals is to identify early indicators that could help predict these events and give more warning before they happen.

“The classic view of droughts is that they move slowly and take a long time to develop, Cook explains. “But like heavy rains, floods or heat waves, flash droughts are very quick – all of a sudden, you’re there.”

A North American clothes dryer

A well-documented flash drought swept through the central United States in 2012. Normal winter and spring precipitation led farmers, ranchers and meteorologists to assume it would be a normal summer. But it suddenly stopped raining in May, causing the driest summer on the Great Plains since 1895even eclipsing the Dust Bowl summers of 1934 and 1936. Drought decimated crops in six states, resulting in agricultural losses of $35.7 billion.

“Many farmers had already seeded all their fields,” says Cook. “In places without irrigation, they could do nothing but watch their fields dry out completely.”

A drought, such as that of 2012, is “a period of abnormally dry weather long enough to cause a serious hydrological imbalance”, according to the American Meteorological Society. It is characterized according to the season in which it occurs, the type of water it limits (snow, rain, groundwater) or the impact it has on different sectors (agricultural, cultural, ecological).

For example, a snow drought could reduce a community’s drinking water supply during the summer. And an agricultural drought could affect specific crops, while a multidecadal mega-drought could cause ecosystem-wide changes in vegetation.

Flash droughts have an unusually rapid onset and deplete water availability – especially soil moisture – within weeks or even days. Above average temperatures combined with a lack of precipitation are usually the cause.

“A warmer, drier atmosphere sucks more moisture out of the soil and plants, all things being equal,” Cook says. “When you add a rainfall deficit, it speeds up surface drying.”

The 2012 disaster prompted researchers to focus on flash droughts. In a study published in 2021, Cook and his colleagues used tree-ring data, combined with data on soil moisture and the main atmospheric circulation pattern. They then reconstructed the frequency and magnitude of flash droughts in the central plains over the past 500 years. Although they found significant variations from century to century, the model showed that more than a third of all flash droughts since 1500 AD occurred in the 20th century.

Another study published in 2021 found that global warming is shift the rhythm of the hydrological cycle: Peak soil moisture or snowmelt runoff occurs earlier in the year. That means places like the Great Plains can expect drier conditions in early summer that last longer into the fall. Additionally, even in places where the total amount of precipitation is not expected to change, Cook says some areas may experience more consecutive dry days between rain events — priming the pump for more frequent flash droughts.

“All the evidence strongly suggests that a warmer world translates to a drier western North America,” Cook says.

Hard to predict

An article published this year in Nature looked at spatial patterns of where flash droughts developed around the world from 2000 to 2020 and came to the same conclusions. The researchers determined that atmospheric aridity combined with soil moisture depletion “will create an environment prone to sudden drought.” Although rapid drying does not occur more frequently in most parts of the world, they found evidence that flash droughts do occur more quickly: 33-46% of global flash droughts in the past 20 years have developed in only five days.

However, while science provides insight into where and how these rapid drying outs occur, we still don’t know how to predict them.

“We know the physical ingredients that cause flash droughts, but we don’t really understand what triggers them in the soil column,” says Justin Mankin, professor of geography at Dartmouth College and co-lead of the National Oceanic Drought Task. and Atmospheric Administration. Strength.

Part of the reason flash droughts are difficult to predict is that there is a lack of on-the-ground monitoring, including real-time measurements of soil moisture. And that’s not just in the United States, but around the world, Mankin explains. Tools like the US Drought Monitor rely on satellite imagery of vegetation as well as other geophysical data. But they only provide an estimate of soil moisture, rather than showing what’s happening below the surface.

“Plants do the majority of the water exchange between the earth and the atmosphere,” Mankin explains. “This exchange occurs in what we call the root zone about a meter underground, which is difficult to monitor from a satellite.”

According to Mankin, the installation of integrated networks of on the spot soil moisture observations would provide better benchmarks for drought prediction models, including their representation of plant-soil water interactions. This could help people prepare for a flash drought. For example, given enough warning, a rancher could look for other feeding options for livestock before a pasture dries up, or a farmer could harvest crops early before they wilt.

“The forecasting tools we have for longer-term droughts are simply not good enough to anticipate these flash events,” Cook agrees. “A warmer world makes the atmosphere drier, increases moisture losses from the surface and leads to a much drier baseline. This is obviously going to have ramifications for water resources.

Whether droughts come slowly or in the blink of an eye, careful monitoring of water stored in soils, snow, streams and aquifers will help us mitigate their impacts on our communities.

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