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 percent reduction in annual precipitation? What about a 25 percent departure from average over a period of five years or longer?
You could also investigate the monthly precipitation data to see whether 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 percent of the annual forage is produced by July 1, rainfall received after this time will not greatly benefit grass production.