One of the obstacles to an objective and reasoned reaction to drought in Illinois is uncertainty over its definition. The following discussion is taken from Changnon (1987).
“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 is also a temporary feature of the climate of Illinois, and we know it occurs only when less than adequate precipitation exists for an extended period of time. Because of the complex nature of droughts, there are many definitions, often reflecting a specific area of concern of an individual, a city, or a region.
The most commonly used drought definitions are:
Meteorological or Climatological Drought – a period of well-below-average precipitation that spans from a few months to a few years.
Agricultural Drought – a period when soil moisture is inadequate to meet the demands for crops to initiate and sustain plant growth.
Hydrological Drought – a period of below-average streamflow and/or depleted reservoir storage.
Changnon (1987) proposed the following two-part definition for drought in Illinois. The first is a state-wide precipitation departure from the long-term average for selected durations (below). The second part is that the precipitation departure from the long-term average must cover at least 40% of the state to be considered a “statewide drought”.
|DROUGHT DURATION||MODERATE DROUGHT||SEVERE DROUGHT|
|3 months||45 to 60%||less than 45%|
|6 months||56 to 70%||less than 56%|
|12 months||70 to 80%||less than 70%|
|24 months||78 to 90%||less than 78%|
Reference: Detecting Drought Conditions in Illinois by Stan Changnon, Illinois State Water Survey Circular 169, 1987.