Flash drought? -

I read an article from Wired talking about flash droughts. It piqued my interest because the words flash and drought seem almost mutually exclusive. It implies a rapidly developing event that sneaks up on you with little notice. This concept is clearly outside of my range of experience.

The example used to describe a flash drought occurred in 2012 in the American Great Plains. It turns out that in the two weeks between June 12th and 26th, conditions changed from “abnormally dry” to “severe drought.” Also, by August that summer, the affected area went from 30 percent of the continental USA to 60 percent.

Flash or not, the 2012 event does present an interesting view of mother nature at work. It turns out that the scientific community is still debating how to precisely define a “flash drought.” But never-the-less the conditions required to bring on such dramatic short-term changes in drought conditions are of interest.

A thirsty atmosphere

The Evaporative Drought Demand Index (EDDI) measures how “Thirsty” the atmosphere is at any point in time. NOAA uses the index to help provide early warning of agricultural drought, hydrologic drought, and fire -weather risk. The EDDI is calculated daily using near-real-time atmospheric data. So, the higher the EDDI percentile category, the dryer the weather. Drought conditions are categorized ranging from ED0 (70% – 80%) to ED4 (98% – 100%). These categories mimic the existing U.S Drought Monitor Index of D0 (abnormally dry) to D4 (exceptional drought – as dry as it can get)

The movement of water from the earth’s surface to the atmosphere is through the process of evapotranspiration. So, the ground moisture at a given location reflects the gains from precipitation and the losses from evapotranspiration. EDDI attempts to measure the total evaporative demand as a proxy for evapotranspiration. The process uses temperature, humidity, wind speed, and solar radiation. However, evapotranspiration is a function of the actual moisture available, but evaporative demand is not constrained by available moisture.

What EDDI does well is alert people to anomalous conditions indicating excessive drought potential. It is not an actual prediction as much as an alert or early warning system that the potential for a problem is rising.

The thirstier the atmosphere is, then the higher the rate that moisture gets sucked from the ground, and the greater the risk of a flash drought.

Flash drought versus flash flood.

The flash drought concept requires that we rethink the geospatial context of a single event. A flash flood affects limited areas determined by the drainage basin shape. However, a drought can have an extensive spatial footprint that could cover very large portions of the country. Time duration is also a consideration. Flash floods persist until the available excess surface water disappears (the rain stops or the snow all melts). But the flash drought may come on quickly and continue for an extended period since it is a response to longer-term weather patterns.

The concept of the flash drought is new to me and still a matter of debate in the science community. But the general consensus is that these types of drought events will happen with increasing frequency as the climate changes and global temperatures rise.