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

Ahmed, Mohamed LU (2012) In Student thesis series INES NGEM01 20122
Dept of Physical Geography and Ecosystem Science
Abstract
Popular science
The Sahel is a semi-arid eco-climatic transition zone in northern Africa separating the Sahara desert from the Africa’s tropical forest. The Sahel word in Arabic language means “shore” which is linguistically describes the appearances of vegetation as a shoreline defining the boundary of the Sahara desert. 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 occurs due to soil moisture deficiency. The projected number of Africans in semi-arid locations will suffer from increasing water stress by 2020s is between 75-250 million and this number is projected to increase to be between 350-600 million by... (More)
Popular science
The Sahel is a semi-arid eco-climatic transition zone in northern Africa separating the Sahara desert from the Africa’s tropical forest. The Sahel word in Arabic language means “shore” which is linguistically describes the appearances of vegetation as a shoreline defining the boundary of the Sahara desert. 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 occurs due to soil moisture deficiency. The projected number of Africans in semi-arid locations will suffer from increasing water stress by 2020s is between 75-250 million and this number is projected to increase to be between 350-600 million by 2050s. This study aims to evaluate the relationship between soil moisture and vegetation growth in Sahel region during 1982-2008 at different time lags. Land cover and soil texture data were used to investigate whether the relationship between soil moisture and vegetation growth are related to land cover and soil type or not. Satellite remote sensing data (vegetation index), modelled soil moisture data land cover map and soil type map were mainly used to achieve the purpose of this study.
The most important findings of this study is the best correlations between vegetation greenness and soil moisture occurred at lag0 (no time lag differences), lag1 (one month time lag) and lag2 (two months’ time lags). The correlation relationship varied between low and moderate values in Sahel region indicating that soil moisture variable is not only the main driver for vegetation dynamics in the study area and maybe other factors such as human impacts could have a great contribution on vegetation changes in Sahel. Croplands and Grasslands are the main land cover types that increasing the correlation relationship between soil moisture and vegetation growth, whereas Entisols (occur in flood plains and steep slopes) and Alfisols (occur under forest and mixed vegetation cover) are the main soil types showing a positive effect on the correlation relationship between soil moisture and vegetation dynamics. Finally, good understanding the temporal relationship between water availability and vegetation dynamics can help us to know water affects plant growth and to predict the future relationship within a season between vegetation growth and soil moisture which can be used for detecting famine possibilities. (Less)
Abstract
This study investigates the temporal correlation relationship between vegetation greenness and soil moisture in the African Sahel from 1982 to 2008 at different time lags (maximum five lags used in this study) and determines the extent which soil moisture explains vegetation dynamics in the Sahel. Monthly composites of remotely sensed Normalized Difference Vegetation Index (NDVI) from National Oceanic and Atmospheric Administration’s Advanced Very High Resolution Radiometer (NOAA-AVHRR) were used in this study as a proxy for vegetation growth, whereas modeled soil moisture data (1.6m column depth) provided by the NOAA National Centers for Environmental Predictions (NCEP) Climate Prediction Center (CPC) Global Monthly high resolution Soil... (More)
This study investigates the temporal correlation relationship between vegetation greenness and soil moisture in the African Sahel from 1982 to 2008 at different time lags (maximum five lags used in this study) and determines the extent which soil moisture explains vegetation dynamics in the Sahel. Monthly composites of remotely sensed Normalized Difference Vegetation Index (NDVI) from National Oceanic and Atmospheric Administration’s Advanced Very High Resolution Radiometer (NOAA-AVHRR) were used in this study as a proxy for vegetation growth, whereas modeled soil moisture data (1.6m column depth) provided by the NOAA National Centers for Environmental Predictions (NCEP) Climate Prediction Center (CPC) Global Monthly high resolution Soil Moisture (GMSM) was used as an indicator of moisture availability for plants. The analyses were applied for all-year months data (dry season included) and only for growing months season (from July to October) to estimate the effect of long dry season on the association between vegetation growth and soil moisture. Trends in vegetation greenness, soil moisture and NDVI residuals were calculated separately in Sahel to investigate the changes occurred in vegetation growth and soil moisture during the study period. The correlations relationship were evaluated against land cover and soil texture data to estimate the influences of land cover and soil type on the strength of correlation relationship between vegetation growth and soil moisture.
The results showed a significant correlation relationship between vegetation greenness and soil moisture at lag0 (no time lag differences), lag1 (one month time lag) and lag2 (two months’ time lag) with a better association in northern parts of Sahel region by using only the growing season data. However, the significant correlations covered a larger area by using all the year data (long dry season included). The results indicated that using AVHRR NDVI data for studying the vegetation growth in response to soil moisture availability is limited in the southern parts of the study area. The significant correlation coefficients (r) are varied between low and moderate values (0.1-0.6) in the study area, suggesting that soil moisture is not only the main driver of vegetation dynamics in Sahel. Vegetation greenness showed a significant increase during the study period in many locations in Sahel region (center of Chad, Senegal and south of Mali), whereas soil moisture showed a small significant locations in the study area (center of Sudan, center of Mali and east of Mauritania) during the study period from 1982-2008. Land cover type (Croplands and Grasslands) and soil texture (Entisols and Alfisols) showed a significant association and high influences on the correlation relationship between vegetation greenness and soil moisture at lag0, lag1 and lag2. (Less)
Abstract
Scientific abstract
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... (More)
Scientific abstract
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. (Less)
Please use this url to cite or link to this publication:
author
Ahmed, Mohamed LU
supervisor
organization
alternative title
Importance of water availability on vegetation growth in the African Sahel
course
NGEM01 20122
year
type
H2 - Master's Degree (Two Years)
subject
keywords
geography, NDVI, soil moisture, time series analysis, Sahel, remote sensing, temporal correlation, physical geography
publication/series
Student thesis series INES
report number
265
funder
European Commission, Erasmus/Socrates Program
language
English
id
3287989
date added to LUP
2012-12-18 14:26:52
date last changed
2013-02-19 16:02:22
@misc{3287989,
  abstract     = {Scientific abstract
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.},
  author       = {Ahmed, Mohamed},
  keyword      = {geography,NDVI,soil moisture,time series analysis,Sahel,remote sensing,temporal correlation,physical geography},
  language     = {eng},
  note         = {Student Paper},
  series       = {Student thesis series INES},
  title        = {Significance of soil moisture on vegetation greenness in the African Sahel from 1982 to 2008},
  year         = {2012},
}