Monitoring of seasonal vegetation response to rainfall variation and land use in East Africa using ENVISAT MERIS data

The actual status of vegetation is represented by its spectral and generally characterized by a low reflectance in red and a high reflectance in near infrared. This specific feature of phenological photosynthetic active vegetation is used to calculate different vegetation indices. One index is MGVI (MERIS Global Vegetation Index), the other one is the Red Edge Index, which represents the point of maximum slope (Red Edge Position) of reflectance of green vegetation between 670 nm and 780 nm. With a spectral resolution of 15 bands and a narrow band setting in the red and near infrared spectra ENVISAT MERIS seems particularly suitable for the Red Edge Index. In this study the variation of the Red Edge Position (REP) and MGVI is analyzed in comparison with seasonal rainfall patterns. Both indices are derived from ENVISAT MERIS data for the time period of August 2003 to July 2004 for the Mount Kenya region, East Africa. The study area is characterized by a high population density, extreme rainfall variability and a strong ecological gradient from the rain fed slopes of Mt. Kenya towards the and savannas of the Laikipia plateau. As a result the land use varies from agro-forest and small-scale farms to large-scale farming, rangelands and savannas. Grassland, shrubland and woodland are the main land cover types. The results of our investigation show: a) generally a relative high correlation between MGVI and REP, b) a similar seasonal variation of REP and MGVI for most classes, c) a better discrimination for forest and agricultural fields using REP and a better discrimination for degraded woody, savanna using MGVI and d) a high potential for ecosystem monitoring using the combination of both indices.