THE INFLUENCE OF SOIL TYPE ON THE RELATIONSHIPS BETWEEN NDVI, RAINFALL, AND SOIL-MOISTURE IN SEMIARID BOTSWANA .1. NDVI RESPONSE TO RAINFALL

This study examines the variability of the normalized difference vegetation index (NDVI) over Bostwana during the period 1982-1987. Its relationship to rainfall is evaluated from 26 stations representing six vegetation formations and sir soil types. The study demonstrates a linear relationship between rainfall and NDVI as long as rainfall does not exceed approximately 500 mm/yr or 50-100 mm/month. Above these limits, a ''saturation'' response occurs and NDVI increases with rainfall only very slowly. This occurs at much lower thresholds of rainfall over Bostwana than over East Africa or the Sahel. The ratio of NDVI to rainfall in Botswana is several times higher than in savannas in these other two regions. Both this ratio, which represents the rain-use efficiency, and the total productivity of vegetation, as assessed from NDVI, vary with vegetation and soil type. The highest productivity and efficiency occur on the clay-rich vertisols, the lowest on the sandy arenosols and solonchaks. In general, the drain-use efficiency appears to be more a function of the underlying soil than of the vegetation formation.