Twenty one years of satellite derived vegetation index and rainfall-related data for Africa
by Fred Snijders
In May 2002, the above authors introduced a new tool to graphically analyse the large collection of satellite derived information held by the ARTEMIS system of the FAO Environment and Natural Resources Service (SDRN). Since 1988, this system routinely processes and disseminates data from a number of satellites for operational use in the field of early warning for food security. Through analysis of this data, the status of the growing season over Africa can be closely monitored and an early warning given of possible crop failures.
This new tool, the ARTEMIS Charting Applet (ACA), consists of a very small but powerful charting programme written in Java and a file containing the statistics extracted from all the dekadal (a period of ten days) and monthly ARTEMIS imagery for approximately 400 zones of Africa. The programme and the data file are combined in one single web page. After being loaded by the web browser of the user, all the statistical data resides locally and no more interaction with the web-server is required. This makes the creation of charts or graphs very fast and also allows the information to be included in one-way information dissemination initiatives, such as used by RANET. The original article "Bringing Africa data to the desktop: the ARTEMIS Charting Applet" (link below) gives a complete description of the charting programme, how to use it, the characteristics of the satellite data, and the background of the creation of the 400 homogeneous zones used to extract the statistics.
The set of satellite data incorporated in the ACA tool has now been updated to include the full 21 year data span of Vegetation Index data and is available in the next Section of this article. Furthermore, some examples have been prepared that illustrate how the ACA tool can be used to analyse environmental conditions (desertification) and growing conditions (drought) over Africa. These include the 1982-1989 period over the Sahel with very dry conditions, accompanied by widespread crop failures and famine conditions; and very wet conditions, during which the "green wave" advanced further north than it had done in many years, thereby sending theories about the expanding Sahara back to the drawing boards. The second example describes the 1991/1992 season over parts of Southern Africa, during which many crops failed as a result of the very short growing season. For the first example, only Vegetation Index data derived from the NOAA satellite are used. For the second example use has been made of both Vegetation Index data and the Cold Cloud Duration data derived from METEOSAT.
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