A Drought by Any Other Name
What’s a drought?
That seems like it should be such an easy question, one especially that those of us working in water should know the answer to. But I’ve found that my definition has really fallen apart over the past few years. I know less than I did.
Traditionally, a drought is simply water scarcity, normally from a lack of rain. This is essentially the definition that we’ve inherited from 10,000 years of rain-fed agriculture. In a more nuanced form, it has also come to refer to water competition — too many straws in the drink.
A few years ago, a close friend in northern Europe approached me and said, “As a country, we’re really just beginning to experience and learn about drought, and our primary operating hypothesis is that droughts and drought responses should be the opposite of how we assess and respond to floods.”
Through a systematic study of his question, what I've come to understand is that my definitions of both flooding and droughts are actually all wrong (but I will leave flooding for a separate essay). I think we tested his hypothesis and found it wanting. And he came to agree with us too.
Indeed, how we diagnose and define droughts actually has a huge influence on how we respond to and anticipate water scarcity more generally. Our definitions can shape the narrative in profound and subtle ways, which will determine if we are making useful decisions.
For instance, regions such as southern Africa, North America’s Colorado River basin, and perhaps the Murray-Darling in Australia as well have all potentially entered “megadroughts,” which are very long-term water scarcity events that can last decades. The Colorado for instance was declared to have entered a megadrought that began at least twenty years ago, stressing century-old governance and infrastructure systems. You can still get a lot of rain in a megadrought, but the overall pattern is a new more water-scarce “normal.”
In contrast, I got a text message from my regional (county) government last week saying that we were having a flash drought — high air temperatures in combination with low humidity that create intense evapotranspiration. Fields and forests can literally wilt before your eyes as moisture leaves plants and soil.
The big risk in our area with flash droughts is triggering runaway forest fires, which is important when 50 percent of my state (about 12 M ha / 30 M acres) is covered in forests, many of them old-growth stands. My house and AGWA’s home office sit on the edge of one of these large forests, so I take a personal interest in the matter! A flash drought a few years ago triggered fires in our valley that had not experienced burning for millennia, turning our sky orange, our radios turned on for evacuation alerts. Several towns disappeared and dozens were killed. My wife and I cautiously looked at maps of fires, trying to decide which way we might evacuate.
There are other options besides the extremes of a megadrought and a flash drought. More traditional types are meteorological, hydrological, and socioeconomic droughts, while ecological droughts are relatively newer. Ecological droughts occur when water availability drops below a level necessary for species, ecological communities, and/or ecosystem services to function or survive. For communities reliant on ecosystem services such as fisheries or reeds, an ecological drought is also a socioeconomic drought.
The thresholds for drought status will differ between impact classes, so that a city may suffer drought conditions that are quite distinct than those experienced by a factory, and both different again than for a farm or an ecosystem.
I’ve never run across the term, but I’d like to suggest a new category as well: phenological droughts, which seem widespread with rapid climate change. Phenology refers to seasonal behavior, such as when you should plant seeds, harvest grain, move livestock from low to high elevations (transhumance), or (for other species) bloom, migrate, or breed. Many of our cultural and economic activities have a strongly seasonal component. When the South Asian monsoon becomes unreliable, or when East Africa rains show up two weeks late, we experience a deep phenological mismatch — even if we still call the shift in timing a drought. Often, phenological droughts are only changes in phase or timing rather than an actual change in annual precipitation; we get the same amount of rain but at the “wrong” time. A farmer or a migratory bird might have a violent disagreement with a meteorologist about the use of the term drought in such cases. Both sides could be correct.
Understanding who is affected by scarcity and the type of drought also tells you a great deal about your preparations and responses; misreading either quality can exacerbate or extend those impacts, perhaps irreparably. A megadrought could potentially renormalize a system, and investments that are not flexible to switch back may build in additional crises. Flash droughts might require some emergency response from a DRR system. A response to a more traditional agricultural drought would be both inappropriate for the beginning of a megadrought, and probably ineffective for an ecological drought or the kinds of shortages we’ve seen in manufacturing in recent years such as with semiconductor manufacturing. Indeed, for megadroughts, we can experience many years, even decades of political and social denial of the new conditions.
“Drought” by itself doesn’t provide much information, much less clear direction or response. You need more information and the humility to understand what might really be happening.
Stayed tuned for floods.
John Matthews
Corvallis, Oregon, USA