Activity #4
Comparing Vegetation to Precipitation (Jul 1987–Aug 1994)

In this activity, you can view the Earth from the unique perspective of outer space. You will examine monthly snapshots (or global datasets) of our planet’s surface and atmosphere. During this activity, you will investigate complex interaction of the Earth’s vegetation productivity and precipitation by looking for patterns and changes over time. You will make connections to global environmental issues.

Background:
To measure and map the density of green vegetation across the Earth's landscapes, scientists use satellite sensors that observe the distinct wavelengths of visible and near-infrared sunlight that is absorbed and reflected by the plants. Calculating the ratio of the visible and near-infrared light reflected back up to the sensor yields a number from minus one (-1) to plus one (+1). The result of this calculation is called the Normalized Difference Vegetation Index, or NDVI. An NDVI value of zero means no green vegetation and close to +1 (0.8 - 0.9) indicates the highest possible density of green leaves.

Scientists now have a modified green vegetation data set called FASIR NDVI. In short, FASIR NDVI is a highly corrected global data set composited into one-month time periods. Scientists removed a variety of erroneous artifacts from the data, ranging from sensor degradation, to volcanic aerosol effects, cloud contamination, short-term atmospheric effects (e.g., water vapor and aerosol effects), sun angle variations, and missing data. These data were collected from January 1982 through December 1990 by the Advanced Very High Resolution Radiometer (AVHRR) flying aboard NOAA-7, NOAA-9, and NOAA-11 satellites. (“FASIR” stands for Fourier-Adjusted, Solar-zenith-angle corrected, Interpolated, Reconstructed data.)

Researchers estimate precipitation across the entire globe by considering many different kinds of input data. Rain gauges are the most accurate, but limited to populated land areas. A variety of satellites provide complete global coverage, but they present two major challenges: First, the various satellite sensors only observe quantities related to precipitation, and algorithms must be developed to get the best estimate from each particular sensor. Second, the mix of available data is constantly changing in space and time, forcing researchers to cope with the particular reliabilities and biases at each time/space grid box. The precipitation data set displayed in this loop is computed in the NASA/GSFC Laboratory for Atmospheres as a contribution to the GPCP, an international research project of the World Meteorological Organization’s Global Energy and Water Exchange program.

The animation will play through once automatically. To replay, simply click the play button. To step through the frames individually, use the left and right arrows on the keyboard to decrease or increase the month. Black frames represent months where there is no data.

Questions:

1. What patterns of vegetation and precipitation do you see?
    
    
2. What is the relationship between vegetation and precipitation over time?
    
    
3. Notice on the globes where there is no vegetation, where the amount of green vegetation remains constant, and where the amount of green vegetation changes seasonally. Can you identify the biomes associated with these regions?
    
    

1. The Water Cycle (http://Earthobservatory.nasa.gov/Library/Water/)
    
2. Measuring Vegetation (http://Earthobservatory.nasa.gov/Library/MeasuringVegetation/)
    
3. Precipitation Data (http://rsd.gsfc.nasa.gov/912/gpcp/)
    
4. Watching Planets Dance to the Rhythms of the Ocean (http://Earthobservatory.nasa.gov/Study/SSTNDVI/)
    
5. From the Dust Bowl to the Sahel (http://Earthobservatory.nasa.gov/Study/DustBowl/)
    
6. Grasslands Initiative (http://Earthobservatory.nasa.gov/Study/Grassland/)
    
7. Growing Data (http://Earthobservatory.nasa.gov/Study/GrowingData/)
    
8. Dry Times in North America (http://Earthobservatory.nasa.gov/Study/NAmerDrought/)
    
9. Mission: Biomes (http://earthobservatory.nasa.gov/Laboratory/Biome/)
    

next activity (#5): Comparing sea surface temperature to vegetation index anomalies
previous activity (#3): Comparing 1km² fires to precipitation during the period from April 1992 to December 1993