Rain-Use-Efficiency: What it Tells us about the Conflicting Sahel Greening and Sahelian Paradox

Rain Use Efficiency (RUE), defined as Aboveground Net Primary Production (ANPP) divided by rainfall, is increasingly used to diagnose land degradation. Yet, the outcome of RUE monitoring has been much debated since opposite results were found about land degradation in the Sahel region. The debate is fueled by methodological issues, especially when using satellite remote sensing data to estimate ANPP, and by differences in the ecological interpretation. An alternative method which solves part of these issues relies on the residuals of ANPP regressed against rainfall (“ANPP residuals”). In this paper, we use long-term field observations of herbaceous vegetation mass collected in the Gourma region in Mali together with remote sensing data (GIMMS-3g Normalized Difference Vegetation Index) to estimate ANPP, RUE, and the ANPP residuals, over the period 1984–2010. The residuals as well as RUE do not reveal any trend over time over the Gourma region, implying that vegetation is resilient over that period, when data are aggregated at the Gourma scale. We find no conflict between field-derived and satellite-derived results in terms of trends. The nature (linearity) of the ANPP/rainfall relationship is investigated and is found to have no impact on the RUE and residuals interpretation. However, at odds with a stable RUE, an increased run-off coefficient has been observed in the area over the same period, pointing towards land degradation. The divergence of these two indicators of ecosystem resilience (stable RUE) and land degradation (increasing run-off coefficient) is referred to as the “second Sahelian paradox”. When shallow soils and deep soils are examined separately, high resilience is diagnosed on the deep soil sites. However, some of the shallow soils show signs of degradation, being characterized by decreasing vegetation cover and increasing run-off coefficient. Such results show that contrasted changes may co-exist within a region where a strong overall re-greening pattern is observed, highlighting that both the scale of observations and the scale of the processes have to be considered when performing assessments of vegetation changes and land degradation

Evolution of Surface Hydrology in the Sahelo-Sudanian Strip: An Updated Review

In the West African Sahel, two paradoxical hydrological behaviors have occurred during the last five decades. The first paradox was observed during the 1968–1990s ‘Great Drought’ period, during which runoff significantly increased. The second paradox appeared during the subsequent period of rainfall recovery (i.e., since the 1990s), during which the runoff coefficient continued to increase despite the general re-greening of the Sahel. This paper reviews and synthesizes the literature on the drivers of these paradoxical behaviors, focusing on recent works in the West African Sahelo/Sudanian strip, and upscaling the hydrological processes through an analysis of recent data from two representative areas of this region. This paper helps better determine the respective roles played by Land Use/Land Cover Changes (LULCC), the evolution of rainfall intensity and the occurrence of extreme rainfall events in these hydrological paradoxes. Both the literature review and recent data converge in indicating that the first Sahelian hydrological paradox was mostly driven by LULCC, while the second paradox has been caused by both LULCC and climate evolution, mainly the recent increase in rainfall intensity

Entre désertification et reverdissement du Sahel : Diagnostic des observations spatiales et in situ

The Sahel region is characterized by a strong bioclimatic gradient and is very sensitive to climatic fluctuations and especially to rainfall variability. Two successive and particularly severe droughts occurred in the 1970's and in the 1980's over the whole Sahel region, which had dramatic impact on the populations and their resources. The theory of a Sahel suffering from desertification was then refueled, as well as the view of a Sahara desert marching rapidly through the rest of the African continent. The analysis of the first remote sensing observations suitable for vegetation monitoring (available since the 1980's) proved to be at odds with these desertification assessments. Indeed, the satellite vegetation indices and particularly the NDVI evidenced strong positive trends, meaning that a significant increase in vegetation productivity occurred at the regional scale. This phenomenon was called "re-greening". However, no regional validation of such trends was possible due to the scarcity of field observations of vegetation. The objective of this work is to evaluate whether re-greening is actually happening over the Sahel or if some degradation trends of Sahelian ecosystems can be detected as well, using both field observations of vegetation and long-term NDVI data (GIMMS-3g NDVI). Within the framework of the AMMA-Catch observatory and of previous field programs, we now have almost 30 years of field observations in the Gourma region in Mali (1984-2011) and almost 20 years in the Fakara region in south-western Niger (1994-2011). The comparison of these two data sources (GIMMS-3g NDVI and vegetation productivity measured on the field) evidences strong re-greening patterns over the pastoral Gourma as well as an increase in vegetation productivity measured on the field. In contrast, the Fakara region in south-western Niger reveals decreasing trends for both variables. The consistency of satellite and field data confirms that NDVI can be used as a reliable tool for monitoring vegetation changes in semi-arid ecosystems and over long periods of time. At the Sahel scale, a strong re-greening pattern is found over the major part of the region over the 1981-2011 period. By analyzing the RUE index (the ratio of production to rainfall) we show that the re-greening observed over the Gourma is mainly explained by the rainfall recovery over the same period, thus showing the ecosystems' resilience to extreme climatic events. This resilience nature, however, is mainly found for the deep sandy soils of the area. Contrasted changes are observed over a small part of the landscape composed of shallow soils which experienced an increase in soil erosion and run-off coefficients, sometimes leading to vegetation decay. The re-greening observed at the Gourma scale should thus consider changes in the ecosystem functioning that may occur in a small portion of the landscape (the shallow soils). The decreasing trend observed over the Fakara region is not explained by rainfall. Changes in land use (strong increase in cultivated areas, shortening of the fallow duration) may lead to a decline in soil fertility which could explain the loss in vegetation productivity over the past 20 years. Thus, we do not observe any extensive desertification of the Sahel region over the past 30 years, but we do observe a global recovery of vegetation which mainly follows the precipitation recovery. It does not exclude though that some degradation may occur in a few regions or at the local scale, for instance over the surfaces under strong erosion or over some agricultural lands, which tends to reconcile both theories.Le Sahel est une région semi-aride caractérisée par un fort gradient bioclimatique et qui est particulièrement sensible à la variabilité des précipitations. Les périodes de très forte sécheresse qui ont sévi sur l'ensemble du Sahel entre les années 1970 et 1980 ont eu des effets dévastateurs sur les écosystèmes, les populations et leurs ressources. La théorie d'une désertification du Sahel a été ravivée, ainsi que celle prédisant une avancée rapide du Sahara sur le reste du continent. Dès les années 1990, l'analyse des premiers indices de végétation satellitaires (NDVI) acquises à l'échelle du globe à une fréquence temporelle journalière a mis en évidence une nette augmentation du NDVI depuis les années 1980. On parle alors de reverdissement du Sahel. L'objectif de cette thèse est de faire la part de ce reverdissement et d'une éventuelle dégradation des écosystèmes sahéliens, sur les 30 dernières années. Les indices de végétation satellitaires basés sur les propriétés optiques de la végétation verte sont de "simples estimateurs" de la production annuelle : la validation des tendances de NDVI sur le long-terme nécessite une vérité terrain. De telles données sont très difficiles à acquérir sur des échelles spatiales compatibles avec la résolution des satellites d'observation de la Terre et sur une période de temps permettant l'analyse de tendances temporelles. Dans le cadre du projet AMMA et de projets antérieurs, nous bénéficions de séries longues de masse de la strate herbacée pour deux régions : le Gourma au Mali (1984-2011) et le Fakara au Niger (1994-2011). La confrontation des deux sources de données (NDVI GIMMS-3g et productivité végétale mesurée sur le terrain) montre que le Sahel pastoral du Gourma est marqué par un reverdissement ainsi que par une augmentation de la productivité végétale mesurée in situ. En revanche, le Sud-ouest Nigérien montre une tendance à la baisse de ces deux variables. La cohérence des données de terrain avec les observations satellitaires confirme que le NDVI peut être utilisé comme outil de détection de l'évolution des écosystèmes semi-arides sur de longues périodes de temps. A l'échelle du Sahel, des tendances au reverdissement sont mises en évidence sur la plus grande partie de la région, sur la période 1981-2011. L'analyse du RUE, le rapport de la production par le cumul de pluie, a montré que le reverdissement du Gourma est majoritairement expliqué par le rétablissement des précipitations, et que les écosystèmes sahéliens sont particulièrement résilients aux évènements climatiques extrêmes. Cette résilience, cependant, est surtout mise en évidence pour les sols profonds sableux. Des changements contradictoires sont en effet observés sur la partie du paysage constituée de sols superficiels qui connaissent une augmentation des coefficients de ruissellement et de l'érosion, provoquant parfois une dégradation du couvert végétal. Le reverdissement observé à l'échelle du Gourma doit donc être nuancé par des changements des écosystèmes pouvant toucher une petite portion du paysage (les sols superficiels). Sur le Fakara nigérien, des tendances à la dégradation du couvert herbacé sont mises en évidence par les deux sources de données et ne sont pas expliquées par les précipitations. Les changements d'occupation du sol (augmentation des superficies cultivées, raccourcissement des temps de jachère) peuvent expliquer une diminution de la fertilité du sol et donc de la capacité de production de la région. Nous n'observons donc pas de désertification du Sahel sur les 30 dernières années mais bien une reprise généralisée de la végétation qui suit globalement le rétablissement des précipitations. Cela n'exclut pas que dans certaines régions, ou à l'échelle locale, une dégradation du couvert puisse aussi être observée, comme par exemple sur les sols soumis à une forte érosion ou sur quelques terroirs agricoles, ce qui tend à réconcilier les deux théories

Rain-Use-Efficiency: What it Tells us about the Conflicting Sahel Greening and Sahelian Paradox

Rain Use Efficiency (RUE), defined as Aboveground Net Primary Production (ANPP) divided by rainfall, is increasingly used to diagnose land degradation. Yet, the outcome of RUE monitoring has been much debated since opposite results were found about land degradation in the Sahel region. The debate is fueled by methodological issues, especially when using satellite remote sensing data to estimate ANPP, and by differences in the ecological interpretation. An alternative method which solves part of these issues relies on the residuals of ANPP regressed against rainfall (“ANPP residuals”). In this paper, we use long-term field observations of herbaceous vegetation mass collected in the Gourma region in Mali together with remote sensing data (GIMMS-3g Normalized Difference Vegetation Index) to estimate ANPP, RUE, and the ANPP residuals, over the period 1984–2010. The residuals as well as RUE do not reveal any trend over time over the Gourma region, implying that vegetation is resilient over that period, when data are aggregated at the Gourma scale. We find no conflict between field-derived and satellite-derived results in terms of trends. The nature (linearity) of the ANPP/rainfall relationship is investigated and is found to have no impact on the RUE and residuals interpretation. However, at odds with a stable RUE, an increased run-off coefficient has been observed in the area over the same period, pointing towards land degradation. The divergence of these two indicators of ecosystem resilience (stable RUE) and land degradation (increasing run-off coefficient) is referred to as the “second Sahelian paradox”. When shallow soils and deep soils are examined separately, high resilience is diagnosed on the deep soil sites. However, some of the shallow soils show signs of degradation, being characterized by decreasing vegetation cover and increasing run-off coefficient. Such results show that contrasted changes may co-exist within a region where a strong overall re-greening pattern is observed, highlighting that both the scale of observations and the scale of the processes have to be considered when performing assessments of vegetation changes and land degradation

Chapter 6. Between desertification and regreening of the Sahel

Introduction This chapter is aimed at reviewing the ‘desertification’ of the Sahel, a source of polemic for several decades, and its ‘regreening’, a term that appeared with the first satellite observations of plant cover in the 1970s. The debate between the backers of these two diametrically opposed theories is real, and particularly important because this part of the world is known for its high sensitivity to climatic events. The specific contribution of this work lies in the combined use of..

Chapitre 6. Entre désertification et reverdissement du Sahel

Introduction Ce chapitre vise à faire le point sur la « désertification » du Sahel, source de polémiques depuis quelques décennies, et son « reverdissement », un terme apparu avec les premières observations satellitaires de la couverture végétale datant des années 1970-1980. Le débat entre les partisans de ces deux théories, diamétralement opposées, est en effet bien réel, et d’autant plus important que cette région du monde est connue pour sa forte sensibilité aux aléas climatiques. L’apport..

Pastoralisme peul : de la rigueur climatique à la marginalité politique

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Zones semi-arides : le Sahel sensible aux variations de pluie

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Chapter 9. Paradoxical pond changes in the non-cultivated Sahel

Introduction An unexpected effect of the multi-decadal drought that affected the Sahel from the 1970s onwards has been an increase in surface water flows that have caused various phenomena described collectively as the ‘Sahelian paradox’. This paradox, that can be summarised succinctly as ‘less rainfall but more water in rivers’, is described by Descroix et al. (see Chapter 7). Most of the observations of the paradox were made in a cultivated Sahelian environment and the phenomenon coincided ..

Chapitre 9. Évolutions paradoxales des mares en Sahel non cultivé

Introduction Une conséquence inattendue de la sécheresse multi-décennale qui affecte le Sahel depuis les années 1970 a été l’augmentation des écoulements d’eau en surface, conduisant à différents phénomènes rassemblés sous le terme de « paradoxe sahélien ». Ce paradoxe, qui peut se résumer par la formule lapidaire « moins de pluies, mais plus d’eau dans les rivières », est décrit par Descroix et al. (chap. 7, ce volume). La plupart des observations de ce paradoxe ont été effectuées en milieu ..