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The AVHRR Vegetation Index of Africa

The Landsat and SPOT systems participated in crop control and inventories of various kinds. As stated earlier, Landsat Thematic Mapper ( TM)4 and Multispectral Scanner (MSS) 6 and 7 bands (and SPOT band 3) are the most sensitive for detecting IR reflectances from plant cells (modified by water content). TM band 3 and MSS band 5 (and SPOT band 2), which measure reflectances in the visible red, provide data on the influence of light-absorbing chlorophyll. Ratio images in these bands help to quantify the amount of vegetation, as biomass, involved in signature responses. The ratio of TM band 4 to band 3 or MSS bands 6 or 7 to band 5 is one expression of the Vegetation Index (VI variants depend on other combinations of these variables, such as TM (4 - 3)/(4 + 3) ).

General classification of land-cover types for all of Africa, April 1982 through February 1983.

VI distributions for entire continents can be monitored in one view from geostationary satellite,s such as the meteorological satellite, NOAA-7. Bands 2 and 1 of its Advanced Very High Resolution Radiometer (AVHRR) sensor are roughly equivalent to TM bands 4 and 3. (see Section 14 for a review of metsat systems) . Using AVHRR data (and supporting ground truth) grouped into 21-day periods for eight observing intervals from April 1982, through mid-February 1983, J.C. Tucker and his associates at NASA's Goddard Space Flight Center have produced, using Principal Components Analysis, a general classification of land-cover types for all of Africa. In the class map below medium blue = thickets and bushlands; dark blue = interspersed tropical forests and grasslands; purple = Sudan-type grasslands and woodlands; medium green = semi-arid wooded grasslands and bushlands; dark green = woodlands; yellow = deciduous bushlands and wooded grasslands; orange = semi-deserts to deserts; and red = tropical rainforests and montane forests.

This group and others have continued to apply metsat AVHRR to observe seasonal changes in biomass ("green wave") over all of Africa, as illustrated below for the following dates: A. April 12-May 2, 1982; B. July 5-25, 1982; C. Sept. 27-Oct. 17, 1982; D. Dec. 20, 1982-Jan. 9, 1983. This has proved invaluable in determining crop shortfalls and drought conditions in Ethiopia and in areas of the Sahel (northern desert regions) during periods in the last decade where starvation was a mass threat. This observational technique has now been applied worldwide.

General classification of land-cover types for all of Africa for all four seasons, 1982-1983.
Key to previous image.

3-9: Knowing that the desert in northern Africa is called "The Sahel", in which of the four above panels has the "drought" moved farthest south; which panel marks the most significant northern encroachment of vegetation, as disclosed by a higher VI? ANSWER

The Volga Wheat Drought; the Salton Sea, California/Mexico

In the mid-70s, another example of crop failure on a grand scale, but not in Africa, made media headlines. In the Volga and other key wheat growing areas of the former Soviet Union, a severe drought in parts of Russia led to a threatening production shortfall that forced the leaders to seek help from outside wheat markets. They approached the U.S. and Australia governments, in particular, to furnish enough wheat and other grains to forestall possible starvation in several regions. Some critics claimed that the leaders were faking the shortage to take advantage of good prices elsewhere. But, the camera doesn't lie. Landsat images proved the veracity of the Russian plea for help, as is clearly depicted in this before and after image:

Color Landsat subscene of the Volga River region during the 1974 drought.

In the 1974 subscene that embraces a large bend in the Volga River, the fields are already in normal crop stages. A year later, and three weeks beyond the 1974 time, when mature crops should have increased the scene redness, instead much of the farmland is fallow (darker grays and tans), confirming the drought claims.

3-10: Does the drought appear localized or regional; what is the nature of the red colored area within the great curved bend of the Volga? ANSWER

Before leaving this agricultural theme, we want to explain one more example using a Landsat TM subscene, instead of SPOT, to show one of the most fertile and prolific growing areas in North America. This image of the Salton Sea and Imperial Valley covers as the Imperial Valley of California and a part of Mexico at the northern end of Baja California. The town of El Centro (bluish-black patch) lies about 16 km (10 miles) north of the Mexican border (very evident because of the sharp contrast with the agricultural activity). South of El Centro is Mexacali.

Color Landsat TM subscene of the Salton Sea/Imperial Valley region, California and Mexico.

This agricultural center extends south of the Salton Sea, a saline body of water more than 49 km (30 mi) long, that fills a basin about 82 m. (269 ft) below sea level. This "Sea" was created by an overflow of water from the distant Colorado River (a small segment is visible in the upper right corner) shortly after the beginning of the 20th Century. Floodwaters poured through low dry washes, traveling westward more than 64 km (40 miles) to empty into the lowest part of the Coachella Valley. In this desert climate, that water is slowly evaporating and turning brackish (moderately salty) and is thus not suited for direct irrigation.

Today, canals from the Colorado River transport water to the sea. The biggest canal in this scene is the obvious, All-American canal. Mild winters promote year-round farming (up to three harvests) in the Valley, with cotton, sugar beets, lettuce, and citrus being the main crops. Most fields are in full growth in this April scene, as indicated by the bright, uniform reds. Differences in land use practice and availability of water (no major canals) account for the pronounced decrease in agriculture on the Mexican side. The lake-like water at the image bottom is Laguna Salada, which undergoes seasonal drops in level that at times reach a dry state, exposing playa lake beds.

East of the Valley are the Chocolate Mountains, part of the Basin and Range system, against whose flanks are conspicuous alluvial fans. The bright strip in the right half of the subscene is the Algodones Dunes field, derived from beach sands left at the surface after an ancient predecessor to the Salton Sea had occupied the Salton Trough, a structural basin between the Coast Ranges (lower left) and the eastern mountains.

3-11: How might a sequence of Landsat or SPOT scenes taken over the months or even years be beneficial to the economic and environmental management of this region? ANSWER

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Primary Author: Nicholas M. Short, Sr. email:

Collaborators: Code 935 NASA GSFC, GST, USAF Academy
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