Droughts of the Past
Part C: How Common is Drought?
You've seen the spatial variability of drought; now we'll take a quick look at its temporal distribution. In this part of the activity, you'll see what percentage of time specific locations have been in drought.
- On the map above, read the title information and examine the color legend to interpret the information.
- As past conditions are quite often a key to predicting the future, which areas of the continental U.S. would you expect to experience drought in the future?
- If a place experiences droughta condition that is a deviation from "normal"20% of the time or more, what can you conclude about the normal range of conditions in that place? The total range of yearly precipitation amounts for places that experience drought greater than 20% of the time must be wider than for other places. "Normal" is really only a statistic: for a location to be below the mean annual precipitation more than 20% of the time, precipitation would have to be substantially above the mean value at other times.
- At the Historical Maps of the Palmer Drought Index page, you can view maps, similar to the one above, which show the percent of time areas experienced severe or extreme drought during different time periods.
Find and compare the maps for each decade (1900-1909, 1910-1919, 1920-1929, and so on) from the 1900's through the 1980's. Note that the color scale on these decadal maps is different from the map above: the darkest color on the decadal maps indicates that an area was in severe or worse drought for 50% or more of the time.
Stop and Think4. By visual inspection, which decade of the 1900's appears to have had the most drought days? Which decade appears to have had the least?
5. As each year has 365 days and each decade has 10 years, what is the minimum number of days in the 1930's that the black areas such as Nebraska and Kansas experienced severe or extreme drought during that decade?
How common is drought?
- Go to the National Drought Mitigation Center's historical graphs of the Palmer Drought Index.
- Click any river basin to access a graph that shows the percentage of area in the basin that experienced severe to extreme drought each year since 1895.
- For three or more of the river basins, estimate the total number of years out of 100 that over 50% of the basin was experiencing severe or extreme drought. Your teacher may ask you to make this estimate for specific river basins and report your results so they can be shared with the whole class. Draw a line across the graph at the level of 50% area, and count the number of individual red bars that extend above that line.
Stop and Think6. If you were to select a state or river basin to start or invest in a farm, which river basin would you choose to reduce your chances of dealing with a drought? Why?
Procedures for reconstructing drought history into the recent past are well established. Many of these studies involve measuring tree rings or studying pollen or diatoms in the sedimentary layers of lakes.
To go back further in time, beyond about 500 years, scientists also use other methods. NOAA's Paleoclimatology Program describes a range of studies and research methods that help scientists discover and document past changes in climate.
- Learn about some of these methods at the Introduction to Paleoclimatology site. As these methods are refined, scientists are applying them to answer questions about drought over the past several centuries.
- Use an Internet search engine to search for the terms "megadrought" and/or "medieval drought." Skim through several articles to see the range of research that scientists are currently conducting on drought.
Stop and Think7. Describe some of the research that groups are conducting to identify droughts in the past. Include brief descriptions of the methods that they are using to gather evidence.