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Illinois Water Science Center

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DROUGHT: Perceptions, Effects, and Assessment


DROUGHT, Perceptions, What is it?: "Prolonged Dry Weather; Want of Rain; Lack" from Webster’s New Expanded Dictionary.

Droughts can be considered as the opposite of floods. Floods result from an overabundance of rainfall, droughts from a lack of rainfall. Floods can result from one large rainstorm lasting anywhere from minutes to hours to days (depending on the drainage area considered--The 1993 Flood on the Upper Mississippi River lasting months), whereas droughts can last months or even years. One large rainstorm in the period of a drought may not end a drought. It may takes months of precipitation to end a drought.

For a more detailed discussion of drought and its many definitions click on:


DROUGHT, Impacts:

There is often a perception in the Midwest that water is an ever-abundant and inexhaustible resource. Whereas it is true that we in Illinois do enjoy an abundant source of water, both surface and ground water, it is not an inexhaustible resource. This fact is driven home during periods when the precipitation is below what is normally expected for a period of time.

Drought, although hard to believe, can have a greater impact, financially and on human health, than floods. Floods usually affect only a small portion of the State, whereas droughts can have widespread effects. Some of the effects are as follows:

  • Damage/Dimished Crop Harvests: This can result (although historically not a problem here in the U.S.) in famine. Damaged or diminished harvest usually results in economic hardship for farmers and increases in prices of products to consumers.

  • Loss of Electricity Generation: Diminished streamflow (from lack of precipitation) can result in the shutdown of electrical generation plants. Nuclear and coal- and gas-fired electrical generation plants require large amounts of water. These plants withdraw water either from surface water supplies (streams or lakes) or ground-water supplies (aquifers). When precipitation is lower than expected, streamflow and ground-water levels are dimished (from the lack of precipitation). When insufficient water from these sources is available, the electrical generation operation has to shut down.

  • Poor Surface Water Quality: Dimished natural streamflow can result in poor water quality, as the effluent from wastewater treatment plants contributes a bigger percentage of the streamflow during droughts. This can result in poor quality drinking-water supplies from surface water intakes and aquatic/ecosystem damage.

  • Scarce Drinking Water Supplies/Water Rationing: Lack of precipitation results in lowered ground-water levels and dimished streamflow that replenishes water-supply reservoirs. Lowered ground-water levels can result in some private and public water wells going dry (or water levels fall below the level of the pump). Lower levels in water-supply reservoirs limits the amount of water available to users.

  • Recreation Opportunities Lost: Many reservoirs and lakes are used for recreation purposes. Lowered lake and reservoir levels can result in inadequate water depths for boating, skiing, and fishing. Lower dissolved oxygen levels (which is the result of inadequate inflow of streamflow) can negatively affect fisheries; thus, hampering recreational fishing and other activities.

  • Transportation Problems: Lower water levels on the larger rivers can affect transportation of commodities by barge. Many barges are stranded and are unable to move at low river levels. The Army Corps of Engineers spends considerable time, effort, and resources in dredging activities during drought periods.


DROUGHT, Assessment -- Tools to help you analyze the drought :

There are many indices that scientists and engineers use to measure a drought and its severity. These indices can be based on one or a combination of the following: precipitation, forecasted precipitation, snowpack, streamflow, and ground-water levels.


Streamflow Conditions Index (USGS)

The U.S. Geological Survey (USGS) maintains over 7,000 streamflow gages throughout the United States. One index of drought condition would be to determine how the current streamflow compares with the historical record of streamflow at that location for that date. This type of analysis is called flow duration. In Illinois, the USGS operates over 170 streamflow gages providing data on a real-time basis. These gages are used to produce a current map of the index of streamflow conditions in the State. This index of streamflow conditions is expressed in percentiles of streamflow. A percentile is a value on a scale of 100 that indicates the percent of a distribution that is equal to or below it. For example, on the map of daily streamflow conditions a river flow at the 90th percentile is equal to or greater than 90 percent of the flow values recorded on this day of the year during all years that measurements have been made. In general:

a percentile greater than 75 is considered above normal

a percentile between 25 and 75 is considered normal

a percentile less than 25 is considered below normal

In some hydrological studies, particularly those related to floods, a variation of the percentile known as the "percent exceedance" is used. It is simply obtained by subtracting the percentile scale value from 100 percent. For example, a discharge at the 75th percentile is the same as a discharge at the 25th percent exceedance (100-75=25).

When clicking on the Illinois streamflow conditions map , the areas with brown to red dots are areas where the streamflow is dimished and below normal. The areas with blue are where streamflow is above normal.


Other Indices

Among the many indices available are the following: Standardized Precipitation Index, Palmer Drought Severity Index, Crop Moisture Index, Surface Water Supply Index, and Reclamation Drought Index. For a discussion of these indices click here:

The Drought Monitor is a comprehensive drought monitoring effort between the USDA (JAWF/NRCS), NOAA/CPC (NCEP/NWS) and the National Drought Mitigation Center. The Drought Monitor looks at conditions of drought throughout the United States using the multi-index drought classification scheme. This map may be located at and the scheme used for the map is located at:


Another map of Drought Impacted Areas can be found at:


Statistics Used in Streamflow Characterization

To characterize streamflow, many times it is useful to know the frequency of occurrence of certain flows. In floods, we want to know the frequency of occurrence of a certain flood discharge so that we can design for it. For example, you may want to build a bridge, but you want to build it high enough over the river to prevent the river from overtopping it very often. For design purposes, you make the decision that in any one year, you want the chance of the river overtopping the bridge at 5%, this would correspond to the 20 year flood (1/20 = .05 or 5%).

For low-flow conditions, such as during a drought, we also try to characterize the streamflow in terms of frequency. The purpose for frequency analysis at low flow may be for design of water-supply reservoir or permitting of wastewater-treatment plant effluent permits. One important characterization that is used in frequency analysis is the annual minimum flow for 7 consecutive days. This is determined by looking for the lowest flow for 7 consecutive days for the climatic year (April 1 to the following March 31). Statistical analysis on many years of data allows the estimation of the frequency of occurrence of certain low flows. The most commonly used frequency is the 7 day, 10 year frequency, or termed 7dayQ10. The 7dayQ10 is the flow for 7 consecutive days that has a chance of occurrence of 1 in 10 in any one year. If stated analogous to flood frequency, we would term this as the 10 year 7 day low flow.

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Page Last Modified: Tuesday, 25-May-2010 08:23:13 CDT