An Assessment of Vegetal-Cover Transition in Nigeria Using Temporal Analysis of Satellite-Derived Data

Department of Environmental Management and Toxicology, University of Agriculture, Abeokuta, Nigeria
September, 2011

JOHN ADEBAYO OYEDEPO

“An assessment of vegetation dynamics in Nigeria from 1982 to 2009 was conducted using satellite spectral measurements of the earth. The study established current trends in vegetal-cover transition and its response to environmental changes. Monthly satellite-derived data for vegetation, land surface temperature and precipitation were acquired. Land measured data obtained from Nigerian Meteorological Agency were supplemented with field data. In this study, “Normalized Difference Vegetation Index” (NDVI) data sets from two satellite sensors; the National Oceanic and Atmospheric Administration’s Advanced Very High Resolution Radiometer (NOAA-AVHRR) and the National Aeronautics and Space Administration’s Moderate-Resolution Imaging Spectroradiometer (NASA-MODIS) were combined to characterize seasonal and inter-annual performances of vegetation through time. Harmonic Regression Analysis of Time Series (HARANTS), Empirical Orthogonal Tele-connections (EOT), Fourier Transform Analysis (FTA) and other algorithms were adopted for the assessment. Trends in seasonal and inter-annual variations in vegetal cover transition were examined. Transition in growing seasons and alterations in production efficiency of Nigerian vegetal-cover were equally investigated. The observed trends were correlated with variation in climatic and other environmental conditions in order to identify true change from mere environmental variations. From the results, a habitat index of 19.66 revealed that less than 20% of national land cover remains as natural vegetation as more than 56% of the total geographical area in Nigeria is human dominated. A decline in biomass and corresponding carbon fixation was recorded for the three decades examined. The percentage total tonnage of biomass accumulation and sequestered carbon for 1981-1990 was 54.4% while 1991–2009 accounted for the remaining 46%. Although there was a slight increase in the last decade but this trend clearly portends a precarious condition for terrestrial ecosystem. The Production Efficiency Model showed a general decline in monthly average Net Primary Production (NPP) with values ranging from 280 gm-2 in 1982 to 235gm-2 in 2009. The graph of mean NDVI values for the month of July in all the years indicated a rise from 0.33 in 1982 to a peak of 0.52 in 1994 and a decline to 0.39 in 2009. Phenology of the vegetal-cover revealed shifts in the commencement and termination dates of growing season from one decade to the other; a decline in length of growing season of 10 days was recorded in the south while as high as 80 days was recorded in the North. Positive correlation was observed between the satellite-derived metrics and actual land-measured ancillary data. Precipitation, surface temperature, soil moisture retention and solar radiation revealed high correlation coefficients (r > 0.6). The study demonstrated a general decline in the performance of Nigerian vegetal-cover and also supported utility of remotely sensed data as input to vegetation mapping indicating a very reliable source of real-time monitoring of environmental changes. The result of this study could be applied for early warning against natural disasters like drought and desertification.”