EarthLabs > Drought > Lab 4: When Precipitation Patterns Change > 4C: How is Drought Measured?

When Precipitation Patterns Change

Part C. How is Drought Measured?

  1. Go to the Current Conditions webpage (link 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 Introduction section of the Drought Indices page from the National Drought Mitigation Center (link will open in a new window).
  3. Stop and Think

    11. Use your own words to describe what a drought index is.
    12. What are some of the parameters that are commonly taken into account in calculating drought index values?
  4. 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: the Palmer Drought Severity Index (PDSI). Click the "Palmer Drought Severity Index" link at the top of the Drought Indices page to learn more about this index.

    Checking In

    • What is the main parameter on which the PDSI is calculated?
      Soil moisture
    • Describe the range of values for the PDSI. Do positive values represent wet or dry conditions?
      The values range from positive 4 to negative 4, with positive numbers indicating wet conditions and negative numbers indicating dry conditions.
    palmer drought map June 28, 2008 Palmer Drought Severity Image for the week ending June 28, 2008.
  5. Look at the PDSI map on the right (click the image for a larger view). It shows conditions for the week ending June 28, 2008. 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?
    Heavy rains in June, 2008 resulted in significant flooding across the Mississippi and Missouri watersheds. While the Upper Midwest shows several areas with an index value of +4, indicating that they are extremely moist, California, Wyoming, and portions of states in the Southeast show values of -4, indicating that they were experiencing extreme drought.

Stop and Think

13. Access and interpret the current PDSI map image. Write a description of current drought status for areas across the U.S.