Vegetation and Rainfall Dynamics in Niger Using NDVI and Ground-Based Data

An analysis of NOAA'S Advanced Very High Resolution Radiometer (AV HRR) Normalized Difference Vegetation Index (NOVI) was made with gauged rainfall data and potential evapotranspiration estimates ( derived from the Penman equation) for the Republic of Niger's 1985 through 1992 rainy season. The purpose of this study is to (1) analyze the spatial and temporal variations in ground-derived NOVI in the Republic of Niger during the 1992 rainy season, (2) examine the variability and strength of the long-term and interannual relationship between satellite derived NOVI, grouped rainfall and potential evapotranspiration estimates and (3) compare the 1992 ground-based measurements of NOVI with long-term and short term satellite-derived NOVI. The West Central site of the National Atmospheric and Space Administration's (NASA) Hydrologic Atmospheric Pilot Experiment (HAPEX), located in the Republic of Niger, West Africa, 13° 32'N and 02° 31 'E was chosen to evaluate the accuracy and applicability of satellite data for use in areas where ground-based measurements are not possible. A transect of three climatological stations in Southern Niger was constructed. A general trend of decreasing rainfall and increasing potential evapotranspiration estimates with increasing latitude was observed. This transect demonstrated how variable rainfall is over a small region in Niger. The Republic of Niger's rainy season as a whole was examined over five different vegetation zones. The rainy season for Zones I and II (located furthest sou~) began in April, in May for Zone ill, and in August for Zone IV (located furthest north). Zone V which is located east of Zones II and ill, showed the rainy season starting in May. To demonstrate how vegetation responded to this spatial climate variation, I looked at the view taken by satellite NOVI. Zone I showed a sine curve distribution of greenness where maximum greenness occurred during the month of September, representing a one month lag after the peak rainy season in August. Zone I had the greatest NOVI values with Zone II showing lower NOVI values bQt with a similar sine curve distribution. Zone ill and IV showed little to no satellite • J NOVI response with the rainy season. Zone V showed a great increase in the satellite NOVI at the peak of the rainy season. NOVI data gathered at the ground of the HAPEX SAHEL West Central site taken at three homogeneous field sites (Fallow, Degraded and Grassland), were compared to NOAA' S satellite NOVI. Overall, satellite NOVI recorded lower values in comparison to the ground-based NOVI. This was due to the influence of moisture and water vapor in the atmosphere during the onset of the rainy season. Differences of scale in the two types of observations also played a role.Digitized from paper copies provided by the Department of Hydrology & Atmospheric Sciences