[en] In order to identify areas where desertification may have occurred over the last two decades, 109 stations located in three bioclimatic zones of Niger are analyzed. Time series of remote sensing data are well suited to this task. Here, we use time series of NOAA AVHRR normalized difference vegetation index (NDVI) data and rainfall data. In this study, trend analysis is used to identify areas within the biomes from 1981 to 1999 in Niger with apparent land degradation in semi-arid areas. It appears that most of Niger (82,5%) recorded increasing rainfall while only 78,9 % experienced increasing in integrated vegetation index (iNDVI). However, the combination of the two variables through the ratio iNDVI/RR shows a widespread reduction of the rain-use efficiency with highly significant trend for the stations grouped by bioclimatic area (p<10-6) during the studied period (1981-1999). These negative trends suggest a progressive diminution of the vegetation productivity as highlighted by the lowering of the iNDVI/RR ratio. This situation suggested a consistently environmental degradation and that desertification processes continued during the last two decades over most of the Sahelian belt of Niger. [fr] Dans l’optique d’identifier les endroits potentiellement affectés par la désertification au cours des deux dernières décennies, 109 stations réparties dans trois zones bioclimatiques du Niger ont été étudiées. L’analyse conjointe des séries chronologiques de précipitations et d’images satellitales AVHRR de NOAA, transformées en indice de végétation normalisé (NDVI) a permis d’appréhender la dynamique du couvert végétal au sein des biomes considérés entre 1982 et 1999. L’évaluation qualitative de la dynamique à long terme du couvert végétal a été réalisée grâce à l’analyse de tendance des valeurs annuelles du rapport du NDVI intégré (iNDVI: juin – octobre) par rapport aux précipitations (RR) pendant la période de végétation active (mai – octobre), pour chacun des points étudiés. Sur la période analysée, il ressort que 82,5% des résultats obtenus pour les précipitations contre 78,9% des valeurs du iNDVI montrent une tendance positive allant de modérée à très forte pour l’ensemble des pixels. Par contre, la combinaison des deux variables à travers le rapport iNDVI/RR montre un déclin généralisé de l’efficacité de l’utilisation de l’eau par la végétation avec une tendance hautement significative (niveau p<10-6) sur la période analysée (1981-1999), lorsque les stations sont regroupées par zones bioclimatiques. Les tendances négatives traduisent une diminution progressive de la productivité végétale qui se reflète dans la baisse du ratio iNDVI/RR. Cette situation suggère une dégradation environnementale continue et met en lumière que les processus de désertification se sont probablement perpétués au cours des deux dernières décennies sur plus de la moitié de la frange sahélienne du territoire nigérien.
Hountondji, Yvon-Carmen ; Université de Liège - ULiège > Sciences agronomiques > Laboratoire de Toxicologie environnementale
Nicolas, Jacques ; Université de Liège - ULiège > Département des sciences et gestion de l'environnement > Surveillance de l'environnement
Sokpon, Nestor
Ozer, Pierre ; Université de Liège - ULiège > Département des sciences et gestion de l'environnement > Département des sciences et gestion de l'environnement
Language :
French
Title :
Mise en évidence de la résilience de la végétation sahélienne par télédétection basse résolution au Niger à la suite d’épisodes de sécheresse.
Alternative titles :
[en] EVIDENCE OF SAHELIAN VEGETATION RESILIENCE IN NIGER AFTER DROUGHT PERIODS BY USING LOW RESOLUTION SATELLITE DATA
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