Interpreting Vegetation Greenness

Purpose

Persistent drought in the western U.S. and northern Mexico has often necessitated modification of wildlife management practices due to deteriorating vegetation conditions. Given the large and sparsely populated areas being monitored, it is difficult to determine the effects of decreased rainfall in specific regions. However, a proxy for the impact of rainfall is available through the RangeView website in the form of the satellite measured vegetation greenness.

Tools

This exercise uses the AVHRR Dynamic Animation tool (link opens in new window).

Exercise

View Greenness for Sonora

1. Choose Sonora from the “ZoomTo” menu. Wait a few seconds for the image viewers to reload.

2. Select “Greenness (NDVI)” in the “Data type” menu for both image viewers.

3. Select 1998 as the year for the left image viewer and 2002 as the year for the right image viewer.

4. In the reference layer list on the left side of the screen, check both the label and layer boxes next to “Major Cities.” These . Click Refresh.

5. The image viewers show vegetation greenness for the years 1998 and 2002. Notice the legend below the map. The dark to light brown colors represent the least green areas in the scene. The light to dark green colors represent greener areas in the scene.

1. Which map shows more overall vegetation greenness?

2. Does the 2002 image appear as you would expect in drought conditions?

Navigate to Pinacate Reserve

1. Select the “Zoom In” button in the navigation bar (at the top of the screen) and click twice on northwestern Sonora (west of Nogales and south of the Mexico-United States border) to zoom in.

2. In the reference layer list, uncheck both boxes next to "Major Cities."

3. and check both boxes next to “FS Allot AZ & NM” to display the layer and the label.

4. Check the layer box next to “FS Pasture AZ & NM.”

5. Uncheck both boxes next to “FS Land”. Click Refresh.

6. Click twice on the Walker Basin allotment to zoom in.

7. If necessary, select the “Pan” button in the navigation bar, and click in the image viewer to center the Walker Basin pasture in the viewer (Click just to the right of center if the right side of the allotment is cut off and to the left of center if the left side of the allotment is cut off).

8. Uncheck the label box for “FS Allot AZ & NM”. Add or remove layers to the image viewers to display the information you feel is most relevant.

Discussion Questions

3. Which area is greener, the higher elevation area in the east or the lower elevation area in the west? Why do you think this is?

4. How many days are combined to create a compositing period for 2002?

In the movie, Dave Schaeffer talked about how the Walker Basin pasture greened up early in January because of the rainfall at the end of 2001. Watch the animation for the rest of the 2002 year.

5. When does the Walker Basin pasture begin to appear less green?

Dave also said that in March and April the Cedar Flats area appeared very dry, but they would be moving the cattle to the Cedar Flats area in the beginning of May.

6. Does there appear to be an increase in greenness from April to May that would indicate that the pasture is ready to have cattle on it?

7. Does there appear to be much variation in vegetation greenness along the north-south gradient or is it primarily from east to west?

8. Are there other applications for which you think this tool would be useful? If so, what are they?

9. What additional information would be useful in interpreting the images?

10. How easy is it for you to decipher the change in greenness from one week to the next?