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Lund University (2012)

Significance of soil moisture on vegetation greenness in the African Sahel from 1982 to 2008

Ahmed, Mohamed

Titre : Significance of soil moisture on vegetation greenness in the African Sahel from 1982 to 2008

Auteur : Ahmed, Mohamed

Université de soutenance : Lund University

Grade : Master Degree in Geomatics 2012

Résumé
Soil moisture (rainwater accumulated over a period of time in soil) is considered one of the most important factors on vegetation growth in Sahel as the agriculture droughts usually associated with soil moisture deficiency. This thesis study investigates the correlation relationship between soil moisture and vegetation greenness in the Sahel region from 1982 to 2008 at different time lags (maximum five lags used in our analysis). Monthly time series data of remotely sensed Normalized Difference Vegetation Index (NDVI) from National Oceanic and Atmospheric Administration’s Advanced Very High Resolution Radiometer (NOAA-AVHRR) was used in this study as a proxy for vegetation growth, whereas the monthly modeled soil moisture data was provided by the NOAA National Centers for Environmental Predictions (NCEP) Climate Prediction Center (CPC) Global Monthly high resolution Soil Moisture (GMSM). Land cover map and soil map data of Sahel region were used to investigate the effect of land cover and soil type on the correlation relationship. The results were based on pixel by pixel analysis for two time frames : all-year data (dry season included) and only growing season (from July to October). The best correlation between NDVI and soil moisture occurred at lag0 (no time lag difference), lag1 (one month time lag) and lag2 (two month time lag) and the strength of relationship is decreasing by increasing time lags (lag0, lag1 and lag2 are the dominant in the study area). The degree of association between NDVI and soil moisture increased in the northern part of Sahel region by using only the growing season data and this relationship was vague in central and southern part of Sahel region. The significant correlation coefficients varied between low and moderate (0.1-0.6) across the study area suggesting that soil moisture is not only the main driver factor on the vegetation dynamics in Sahel region. Trends of vegetation showed a significant increase during the study period in many locations (center of Chad, Senegal and south of Mali), whereas soil moisture showed small significant locations (center of Sudan, center of Mali and east of Mauritania) from 1982-2008. Land cover type (Croplands and Grasslands) and soil type (Entisols and Alfisols) showed a significant influence on the correlation relationship between vegetation greenness and soil moisture

Mots Clés  : geography, NDVI, soil moisture, time series analysis, Sahel, remote sensing, temporal correlation, physical geography

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