Monitoring Drought

 How to Create a Local Drought History

Although most of us can recall a few severe droughts in the past, it seems as if we are still surprised when drought recurs. It is important to remember that droughts, including multiple-year droughts, are a normal part of climate in nearly every part of the world. Understanding the historical occurrence of drought will provide a better basis on which to make informed management decisions for the future. This section provides guidelines on researching the history of drought in a region.

Where to Start?
You’ll need to start with a good understanding of historical precipitation in your area. Some people acquire at least part of this data by measuring daily precipitation at their home or business.  Others will have to rely on state and federal monitoring efforts such as:

CLIMVIS

This program provides historical drought occurrence by climate division since 1895, based on the Palmer Drought Severity Index.

State Climate Offices
State climatologists are in charge of keeping long-term precipitation records in each state.

Regional Climate Centers
The United States has five regional climate monitoring districts.

National Climatic Data Center (NCDC)
The NCDC is the national depository for historical weather station data.

Determining the right source to research depends on the scale of information you need. If you want general regional drought information, sources such as CLIMVIS would be suitable. Other state and federal sources will be able to provide specific historical data for weather stations near your location. This will allow you to conduct your own drought history investigation.

How to Analyze Precipitation Data

Once you’ve located the historical precipitation data for a nearby weather station (either annual or monthly), you can begin to calculate drought frequency and severity for your location. As an example, precipitation data for Lincoln, Nebraska, has been collected since 1887. From 1887 to 2002, the city’s average annual precipitation has been 27.6 inches. However, the climate records show that the city received less than 20 inches of annual precipitation in thirteen of those years, and as little as 14.09 inches in 1936. In fact, the years 1934-1939 averaged 20.8 inches of rainfall per year. This information is valuable for designing your operation or activity to withstand such departures from the average. What changes would you have to make in your activities to survive a 50% reduction in annual precipitation? What about a 25% departure from average over a period of five years or longer?

You could also investigate the monthly precipitation data to see if the seasonality of precipitation is more important than annual departures from the average. For example, according to a study by Smart et al. (2005), the amount of spring precipitation during April, May, and June is a good indicator of the current year’s forage production on ranches in the northern mixed-grass prairie of the Great Plains. Since more than 90% of the annual forage is produced by July 1, rainfall received after this time will not greatly benefit grass production. Therefore, understanding the historical occurrence of drought during April-June can be very important for ranch planning in the northern Great Plains.

Drought is a normal part of climate. However, people often forget about drought until the next one comes along and surprises them. This should not be the case--drought is inevitable. Analyzing the historical occurrence of drought provides an understanding of the range of climate possibilities for your region, resulting in more informed management decision making.