When Precipitation Patterns Change

Part C. How is Drought Measured?

1. Go to the Current Conditions webpage (links will open in a new window). Scroll down through the list and explore some of links to get a sense of all the maps and data products that are available for tracking drought.
2. Read the about Drought Classification and Indicators at the About the Data page from the US Drought Monitor.
3. Several different Drought Indices have been developed, and each has its own strengths. We'll look at an index that has been widely used in agricultural regions of the U.S. Go to the National Weather Service's page on the Palmer Drought Severity Index (PDSI) to learn more about this index.

   Checking In

   • What is the main parameter on which the PDSI is calculated?
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     Soil moisture
   • Describe the range of values for the PDSI. Do positive values represent wet or dry conditions?
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     The values range from positive 4 to negative 4, with positive numbers indicating wet conditions and negative numbers indicating dry conditions.
   Palmer Drought Severity Image for the month of May 2016.
4. Look at the PDSI map on the right (click the image for a larger view). It shows conditions for the month of May 2016. Use the color legend to interpret the conditions that existed across the contiguous U.S. at that time. Based on your interpretation, predict what might have been occurring at that time?
   Show me a description of what was going on in May of 2016
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   Heavy rains in May 2016 resulted in significant flooding in the Missouri and Texas-Gulf watersheds. Tropical storm Bonnie made landfall in South Carolina. Parts of the Eastern Rockies, Lower Midwest, and Texas show several areas with an index value of +4 indicating they are extremely moist. Southern California shows value of -4, indicating they were experiencing extreme drought.