In 2012, a fast-moving drought struck the central U.S. during the midst of its growing season. The drought was widespread and devastating, with the National Centers for Environmental Information (NCEI) reporting an estimated $31.2 billion loss primarily from widespread damage to corn, soybeans, forage crops, and pasture.
While there is no universally accepted definition of drought, contemporary definitions look at a percentage of precipitation over a protracted period of time, in most cases over the course of a year. Some say this approach leaves people and communities vulnerable to a different type of drought – flash droughts.
Illinois State Climatologist Trent Ford warns that defining droughts as solely long protracted events with minimal precipitation can underestimate short-term drought risk. Ford is at the forefront of research to better understand flash droughts and the unique impacts they create, which can help those vulnerable to its effects better prepare.
“Tangible characteristics are getting lost in translation because of how we often categorize drought,” Ford said.
With flash droughts the same impacts can be felt in a couple of weeks, instead of lasting for three to four months. Measuring drought as a percentage of normal precipitation over a 12-month period masks some particularly critical details.
That year, Illinois received 75 percent of its normal precipitation (that is, the average amount of precipitation expected per year using data from 1981 to 2010). So measuring the 2012 drought using a 12-month interval dramatically underestimates the drought’s intensity. Illinois as a whole, experienced six dry months, three very dry months, and three near-normal months of precipitation in 2012.
“Droughts themselves are hard because they don't have a clear onset. Usually, when people know they are in a drought they have been in one for a long time. We should focus on traditional droughts but we can't disregard the rapid onset and intensity of flash droughts,” Ford said.
Case study: southern Illinois
Ford points to southern Illinois (south of Interstate 64) as a prime example.
“What the research is telling us is that drought is happening faster in southern Illinois. It’s coming on more quickly, but not persisting as long,” said Ford.
Southern Illinois began 2019 with one of the wettest first halves of the year on record before the onset of a flash drought, which lasted between four to five weeks in late August through September. In fact, it was so dry, the southernmost parts of the state didn't record any measurable precipitation during this time.
“In this example, if we view that drought in anything more than a three- to four-month timeframe of precipitation, then it doesn't look like anything serious, but in reality this flash drought had a significant impact on those communities,” Ford said.
Ford believes there are clues as to what makes southern Illinois more prone to flash droughts based on how the region’s soil responds differently to drought.
A lot of the research starts with precipitation deficits and strong evaporation demand. Evaporative demand is a combination of increased temperature and solar radiation, decreased humidity, and increased wind speed.
A region that experiences 30-60 days of strong evaporative demand then rapidly depletes the soil moisture. This restricts the crops’ ability to pull water from the soil resulting in stress. Soil moisture is a key indicator that can predict a flash drought is about to occur.
Ford is working with data collected by the Water and Atmospheric Resources Monitoring Program (WARM)’s 19 soil monitoring stations in Illinois, to understand these extremes, how quickly they come on, and how long they last. WARM has been monitoring weather and soil conditions across the state for nearly 30 years providing current and long term data for soil moisture. Early research suggests that monitoring soil moisture could provide an early warning up to one to two weeks before a flash drought occurs.
Spring flooding, summer drought
“Projections of future climate conditions suggest spring should continue to get wet, but later into summer could get drier, and southern Illinois may actually be seeing this rhythm of spring flooding, summer droughts,” said Ford.
“What we saw in southern Illinois last year was a rapid trend of wet to dry, and we are asking, ‘Does it have a precedent? What causes it?’ We don’t know if there is predictability in extreme change, but our preliminary findings of wet to dry extremes over the last 70 years are some of the first work being done on this type of precipitation whiplash in the Midwest,” Ford said.
Projections do indicate a decrease in soil moisture during the evaporative demand, but for the Midwest, there's not a long projected trend because the overall long-term variability outweighs the projected trend.
What we know is that a sufficient drop in precipitation can cause flash drought. If the Illinois summer climate gets slightly drier and warmer, these conditions will increase the risk of flash droughts. To date, there is no clear signal of change in droughts from projections because drought is so complicated in how it occurs and how it is measured, this can affect the long-term trends.
This work is funded by the NOAA National Integrated Drought Information System (NIDIS).