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 March 03, 2020
Think flash floods are bad? Buckle up for flash droughts

 In late spring of 2012, climactic chaos descended upon the Midwest and Great Plains in the midst of the growing season. A drought is supposed to unfold on a timeline of seasons to years, but in the two weeks between June 12 and 26, the High Plains went from what a monitoring group called "abnormally dry" to "severe drought." The affected area ballooned from covering 30 percent of the continental US in May to over 60 percent by August, with the agricultural losses tallying in the tens of billions of dollars.

The region had crashed into a "flash" drought---think of it like a flash flood, only far bigger and therefore far more consequential. It's a phenomenon science is just beginning to understand, let alone predict. But today in the journal Nature Climate Change, two dozen researchers---atmospheric scientists, computer scientists, climate scientists, and more---are publishing a perspective piece trying to get their community to agree on a standard definition for a flash drought, and to set research priorities for the future. Why, for instance, do flash droughts happen in the first place? How can scientists get better at predicting them and giving water managers warning? And if climate change is making the world drier in general, what does that mean for flash droughts?

"I think the challenge with drought, just in general, that makes it so much different than any other hazard---much more challenging and very costly---is the fact that it has a very potentially large spatial footprint and a very potentially long temporal footprint," says Mark Svoboda, director of the National Drought Mitigation Center and coauthor on the new paper. "Compared to a flood, earthquake, hurricane, tornado, there they have a relatively small impact area, and they last a very short amount of time."

Here's the first tricky bit: Calling a drought a "drought" is both an objective and subjective science. The objective side comes from raw data about precipitation and soil moisture. "But there are also these things that are coming from people on the ground, their opinions and their subjective observations," says Angeline Pendergrass, an atmospheric scientist at the National Center for Atmospheric Research and lead author on the new paper. "And so this is a very rich data set, but it's also not entirely objective."

By experts, she doesn't mean old folks sitting on porches saying that their joints aren't acting up, so there must not be any rain coming. She means water managers, who control the distribution of water to residents and industries, and local officials who talk to farmers and other workers likely to be economically affected by drought.

These insights are combined with those objective measurements by the US Drought Monitor---a collaboration of federal agencies, like the USDA and NOAA, and Svoboda's National Drought Mitigation Center---which updates a map every Thursday showing which parts of the country are in drought and how severely they are affected. The group ranks conditions from D0 ("abnormally dry" yet still not a drought) to D4 ("exceptional," or the worst-case scenario).

It was this body that determined the High Plains jumped from D0 to D2 ("severe drought") over those two weeks in 2012. "So one measure of intensification is changing by two categories of drought in two weeks," says Pendergrass.

The problem is that this score deals in subjective judgments, and necessarily so. For scientists to actually quantify how things might get worse in the future thanks to climate change, they need objective measurements. "We don't have a US Drought Monitor for the future," Pendergrass says. "We need to have a different, more objective definition of flash drought in order to be able to even quantify how models project that flash drought could change."

One way of beginning to quantify flash droughts is something called the Evaporative Drought Demand Index. Essentially, it's a measure of how thirsty the atmosphere is. On a hot, dry day, air sucks more water out of vegetation and soil. On a humid, cool day, it hoovers up less. To produce a flash drought, the air needs to be particularly thirsty. Maybe it's extra hot and dry, and a strong wind blows across the soil, carrying away moisture. Indeed, this was the case in the Midwest's 2012 flash drought. It wasn't caused by just a lack of rain; it was also the hot weather.

This is one of the reasons why drought researchers are worried about climate change: Warmer weather means a landscape can dry out quickly, even after a wet winter and spring, as was the case in the Midwest. Climate change is also making rain storms more intense yet less frequent. "I think the game changer recently has been, with a warmer climate and hotter temperatures, it speeds up the hydrological cycle," says Svoboda. "It speeds up how fast water comes into and leaves our systems, and we're more susceptible to days in between rain events." During those gaps between storms, drying can rapidly intensify, leading to flash droughts. These dry periods can be dire when they coincide with growing seasons, as happened in the Midwest in 2012. "If real estate's about location, location, location, droughts to me are all about timing, timing, timing," says Svoboda.

But it's not so simple as just looking at precipitation and weather. For example, climate change is futzing with vegetation---in a way that might actually help the land hold on to water. "That's a big factor," says Pendergrass. "Some recent research has actually shown that because---with increases in greenhouse gases, specifically carbon dioxide---plants can use water more efficiently. And so they don't have to evaporate as much in order to photosynthesize the same amount." Complicating things even further, crops like corn might react differently to more CO2 than a forest, so it really depends on the landscape.

Another problem with predicting flash droughts is that they unfold over an awkward timescale. "Weather forecasting out to a week is reasonably accurate. And seasonal climate forecasts for the next winter, for example, may be reasonably accurate. But that subseasonal timeframe in between can be much trickier to handle," says University of Wisconsin--Madison atmospheric scientist Jason Otkin, who wasn't involved in this new paper. "So flash drought predictability may actually prove to be very challenging."

Even if scientists are able to accurately predict flash droughts, we have to get much smarter about how we collect and store water. Take it from Los Angeles, of all places. The city is facing a future in which rainfall gets more intense yet less frequent. So city leaders are embarking on an ambitious quest to store the rare deluges, even turning road medians into collection devices.

The hard truth is that we've gotten complacent about water, spoiled even. "Going back four or five hundred years ago in India, our ancestors in many villages dug tanks, because they realized that it's very important to conserve rainfall," says Mannava Sivakumar, who advises the World Meteorological Organization and UNESCO on drought but wasn't involved in this new paper. "It is very important that countries take appropriate steps to conserve water. We have to conserve water."

With this new paper, scientists are charting a way toward better understanding flash droughts and helping communities prepare for the worst. "We've got a lot more work to do," says Svoboda. "But even if that day comes where we get the perfect forecast, we still need to be able to monitor and trigger things within better policies and better plans."