Illinois State Water Survey - University of Illinois at Urbana-Champaign

State Climatologist Office for Illinois

Frequently Asked Questions About Drought

Dr. Jim Angel, State Climatologist

How do you define and measure drought?

Answer: Drought is a complex physical and social phenomenon of widespread significance, and despite all the problems droughts have caused, drought has been difficult to define. There is no universally accepted definition because: 1) drought, unlike flood, is not a distinct event, and 2) drought is often the result of many complex factors acting on and interacting within the environment.

Complicating the problem of drought is the fact that drought often has neither a distinct start nor end. It is usually recognizable only after a period of time and, because a drought may be interrupted by short spells of one or more wet months, its termination is difficult to recognize.

Drought can be measured by departures of precipitation from normal for some extended time, say 3 to 12 months; by drought indices such as the Palmer Drought Severity Index; or by impacts such as low soil moisture, streamflow, or groundwater levels. Typically, a particular precipitation threshold or drought index is chosen as it relates to a particular impact. For example, agricultural droughts are most sensitive to precipitation deficits in June, July, and August while groundwater may respond to a 12-month deficit in precipitation.

What is the frequency and coverage of drought?

Answer: Generally, the warmer temperatures and higher variability of precipitation in southern Illinois makes it more vulnerable to drought. As a result, the 2-year, 3-month drought in southern Illinois is about 48% of normal precipitation while northwestern Illinois is 64% of normal. Three-month droughts are more likely to occur in winter months while droughts lasting a year or longer tend to begin in summer. Severe droughts lasting more than 24 months are infrequent in Illinois with the last being in the early 1950’s. 

While droughts may cover only a portion of the state, they are more likely to be part of the regional pattern because the dry conditions are usually the result of changes in the large-scale circulation patterns. For example, many large-scale summer droughts in the Midwest are caused by warm, high pressure system settling over the entire region. As a result, the impacts may be felt over a wide area.

What are the impacts of drought and the timing of the impacts on the various parts of the hydrological cycle?

Answer: The first part of the hydrological cycle to be impacted by drought is the soil moisture. The changes in soil moisture can be quite rapid during the growing season when demand on it is high due to plant growth. Dry periods in Illinois typically have a near-normal number of days with rain, but the rains are more spotty and less intense. As a result, stream flow usually drops as well due to a lack of heavy rainfall events. Any rain that does fall is first absorbed by the depleted soil moisture, reducing runoff.

Northern Illinois is less vulnerable to short-term drought because a higher percentage of streamflow comes from groundwater discharge into streams. In southern Illinois, this percentage of groundwater discharge is much smaller due to differences in the soils and topography, making it more vulnerable to drought. Smaller streams usually respond more quickly to drought than larger streams due to their smaller drainage area and smaller storage.

Most lakes and reservoirs used for public water supplies are fed by streams. As a result, several months of low stream flow, especially during the high flow periods in spring, can lead to supply problems. Smaller reservoirs usually deteriorate more quickly to drought due to smaller storage and reliance on smaller streams.

Groundwater levels are usually the slowest to respond to drought, only after soil moisture and streams are down. They are the slowest to recover from drought only after precipitation exceeds evapotranspiration and soil moisture demands. Deep groundwater is recharged from the shallow aquifers, which act as reservoirs form which groundwater can percolate slowly downward. As a result, they are buffered from short-term droughts and only show an effect during extended periods of drought.

Are timing aspects of drought important?

Answer: The timing of drought is important. Short but intense droughts that occur in the growing season typically have a significant impact on agriculture. On the other hand, droughts that start in the fall and end in the spring may have minimal impact since they occur in the time of year when the demand for water by both man and the environment are low.

What is the predictability of drought in seasonal and decadal time scales?

Answer: The persistence of drought from one season to the next in Illinois is not as high as in other parts of the U.S., especially the West where multi-year droughts are common. Therefore the ability to predict the onset or continuation of a drought is more problematic. Recent advances in our understanding of large-scale atmospheric and oceanic circulation features, such as El Niño and the Pacific Decadal Oscillation, may lead to some small degree of skill in predicting drought one or two seasons ahead. On the longer scale of multi-decades, no skill has been shown in forecasting drought, even with the application of so-called drought/solar cycles. As global and regional climate models improve we may begin to realize the ability to predict changes in frequency, intensity, or location of drought.