Detection of recent changes in Gambia vegetation cover using time series modis ndvi

Boubacar Solly,Ebrima Sonko,Mamma Sawaneh,Aruna M. Jarju,Sidat Yaffa

doi:10.4000/belgeo.47995

Abstract

In The Gambia, the vegetation cover has undergone significant changes over the past two decades. To assist policy decision-making, this study seeks to detect trends in changes over the past two decades using a time series of NDVI MODIS images. The methodological approach is based on the calculation of the correlation of Kendall (τ) associated with the p-significance test at the 10 % threshold. The results showed a trend of increasing vegetation productivity during the decade 2000-2009 (98.37% of the study area) in almost all administrative regions. This trend is more significant along the river. In contrast, during the decade 2010-2019, it was noted overall significant downward trend in productivity (44.01% of The Gambia) in all administrative regions except West Coast Region and Banjul, however not significant. The result showed that, over the whole period (2000-2019), 61.86% of the national territory was characterized by a positive trend and 38.14% by a negative trend; and that the significant positive trend percentage is 10%, the significant negative trend percentage is 4%. Significant positive trends are observed much more along the river and central of the Central River Region; significant negative trends are observed mainly in the western part of West Coast Region and Banjul, at the eastern end of the North Bank Region and in the Upper River Region.

Highlights

Considering that the literature on the issue throughout the Republic of The Gambia is very little, if not scarce
18 During the decade 2000-2009, the average Kendall correlation generally indicates a positive trend of Normalized Difference Vegetation Index (NDVI) on 98.37% of the study area
The trends observed from the MODIS NDVI in The Gambia are consistent, in terms of change in vegetation cover from the Landsat images (LULCC), with the findings from several studies

Summary

Introduction

Considering that the literature on the issue throughout the Republic of The Gambia is very little, if not scarce. Most of the studies conducted there have focused on adaptation to climate change and agricultural production (Akon-Yamga et al, 2011; Yaffa, 2013; Sanneh et al, 2014; Sonko et al, 2019; Jarju et al, 2021). Studies carried out at the West African or Sahel scale have paid little attention to the analysis of results at The Gambia scale (Herrmann et al, 2005; Dardel et al, 2014; Leroux et al, 2014; Brandt et al, 2016). 10 where n is the length of the data series studied, and the observations made on date, i and j respectively. The Kendall trend rate (τ) is obtained through equation 3: with s the correlation coefficient of Kendall and n the length of the series of data studied. The estimation of the significance of the correlation “tau” is given by equation 4: Z follows a standard normal distribution, and a positive value of Z means an upward trend, and a negative value means a downward trend

Objectives

Results

Discussion

Conclusion