What four decades of earth observation tell us about land degradation in the Sahel?

The assessment of land degradation and the quantification of its effects on land productivity have been both a scientific and political challenge. After four decades of Earth Observation (EO) applications, little agreement has been gained on the magnitude and direction of land degradation in the Sahel. The large number of EO datasets and methods associated with the complex interactions among biophysical and social drivers of ecosystem changes make it difficult to apply aggregated EO indices for these non-linear processes. Hence, while many studies stress that the Sahel is greening, others indicate no trend or browning. The different generations of sensors, the granularity of studies, the study period, the applied indices and the assumptions and/or computational methods impact these trends. Consequently, many uncertainties exist in regression models between rainfall, biomass and various indices that limit the ability of EO science to adequately assess and develop a consistent message on the magnitude of land degradation. We suggest several improvements: (1) harmonize time-series data, (2) promote knowledge networks, (3) improve data-access, (4) fill data gaps, (5) agree on scales and assumptions, (6) set up a denser network of long-term field-surveys and (7) consider local perceptions and social dynamics. To allow multiple perspectives and avoid erroneous interpretations, we underline that EO results should not be interpreted without contextual knowledge

Local vegetation trends in the Sahel of Mali and Senegal using long time series FAPAR satellite products and field measurement (1982-2010)

Local vegetation trends in the Sahel of Mali and Senegal from Geoland Version 1 (GEOV1) (5 km) and the third generation Global Inventory Modeling and Mapping Studies (GIMMS3g) (8 km) Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) time series are studied over 29 years. For validation and interpretation of observed greenness trends, two methods are applied: (1) a qualitative approach using in-depth knowledge of the study areas and (2) a quantitative approach by time series of biomass observations and rainfall data. Significant greening trends from 1982 to 2010 are consistently observed in both GEOV1 and GIMMS3g FAPAR datasets. Annual rainfall increased significantly during the observed time period, explaining large parts of FAPAR variations at a regional scale. Locally, GEOV1 data reveals a heterogeneous pattern of vegetation change, which is confirmed by long-term ground data and site visits. The spatial variability in the observed vegetation trends in the Sahel area are mainly caused by varying tree- and land-cover, which are controlled by human impact, soil and drought resilience. A large proportion of the positive trends are caused by the increment in leaf biomass of woody species that has almost doubled since the 1980s due to a tree cover regeneration after a dry-period. This confirms the re-greening of the Remote Sens. 2014, 6 2409 Sahel, however, degradation is also present and sometimes obscured by greening. GEOV1 as compared to GIMMS3g made it possible to better characterize the spatial pattern of trends and identify the degraded areas in the study region

Estimating and evaluating GPP in the Sahel using MSG/SEVIRI and MODIS satellite data

The aim of this study was to use data from Meteosat Second Generation’s Spinning Enhanced Visible and Infrared Imager (MSG/SEVIRI) to calculate the gross primary production (GPP) in the Sahel region of Africa for 2011 and 2012. GPP was calculated using the light use efficiency method, which relates GPP to the absorbed photosynthetically active radiation the light use efficiency. The results were compared with the widely used Moderate Resolution Imaging Spectroradiometer (MODIS) GPP product (MOD17A) and ground measurements using the eddy covariance method, from Dahra, Senegal. The results show that MSG/SEVIRI derived GPP more accurately represent the in situ measurements from the Dahra site compared with MODIS GPP, both for short time changes and the magnitude of GPP. MODIS GPP underestimated the ground measurements during the growing season, findings which were consistent with previous studies of the Sahel. MODIS performed well during the dry season and in replicating the change of seasons.Fotosyntes från rymden Data från satelliter är en viktig källa för information om Jorden för forskare. Det finns många olika satelliter och sensorer som används för detta ändamål. Denna studie har analyserat data från två satelliter (MSG/SEVIRI och MODIS) med olika upplösning i tid och rum samt olika omloppstider för att undersöka skillnader mellan dem. Sahelregionen i Afrika ligger mellan Sahara i norr och savannen i söder. Det är ett område som är mycket känsligt för förändringar i klimat och väder. Att förstå hur fotosyntes, kol som tas upp av växter med hjälp av solljus, ser ut i området kan hjälpa oss att förutse framtida svältkatastrofer och för att skapa bättre klimatmodeller. Fördelen med att använda satelliter är att man kan studera stora delar av Jorden samtidigt. Data från satelliterna jämfördes med data mätt i Dahra i Senegal. Studiens visar att de två satelliterna som undersöktes ger väldigt olika resultat. MSG/SEVIRI var betydligt bättre än MODIS-sensorn på att uppskatta fotosyntes. Det betyder dock inte att MODIS är oanvändbar. Tvärt om kan denna datan användas för att enkelt studera trender och mönster medan MSG/SEVIRI är mer lämplig för att studera de faktiska nivåerna av fotosyntes

ENVIRONMENTAL SECURITY AND SEASONAL VARIABILITY:REMOTE SENSING AND MODELING APPLICATION FOR THE MONITORING OF SAHELIAN NATURAL RESOURCES

Il lavoro sviluppato in questa Tesi si \ue8 incentrato sullo studio dei sistemi pascolivi delle regioni del Sahel, Africa Occidentale, tramite tecniche e strumenti del telerilevamento satellitare. L\u2019area oggetto di studio \ue8 una fascia di savana semi-arida, rappresentate la zona di transizione tra il Sahara a nord e le foreste del golfo di Guinea a sud. La regione \ue8 caratterizzata da ana marcata stagionalit\ue0, con una breve stagione umida (da Giugno ad Ottobre) in cui concentra gran parte delle produzione di biomassa vegetale e di conseguenza la produzione di derrate alimentari, seguita da una lunga stagione secca (Novembre-Maggio). A seconda della distanza dal Sahara le precipitazioni medie annuali vanno dai 150 mm annui ai 500, con elevata variabilit\ue0 tra le annate. In questo sistema cos\uec mutevole la pastorizia transumante \ue8 l\u2019attivit\ue0 antropica che meglio si adattata alle dinamiche stagionali. Difatti le uniche fonti di cibo sono date dalla pastorizia e, ove possibile, da agricoltura di sussistenza di specie molto rustiche come il Miglio (Pennisetum glaucum). Nonostante questi adattamenti la regione ha subito una serie di crisi umanitarie a partire dagli anni 70\u2019 del secolo scorso, causate da un brusco calo delle precipitazioni annuali. Il fenomeno climatico \ue8 risultato essere dovuto ad anomalie di temperature dell\u2019oceano Atlantico, similmente al fenomeno de El Ni\uf1o. Nonostante le piogge siano in lenta ripresa dall\u2019inizio degli anni 90\u2019, ricorrenti crisi umanitarie continuano ad interessare l\u2019area (l\u2019ultima nel 2010), motivo per cui le strategie da adottare per incrementare la sicurezza alimentare dell\u2019area rimangono questioni dibattute. In particolare, essendo il Sahel un\u2019area marginale a ridosso di una zona iper-arida, non vi sono gli estremi per attuare due comuni strategie di food security, l\u2019incremento delle aree coltivate e l\u2019intensificazione delle produzioni. In questo contesto, in cui strategie top-down sono inefficaci o dannose, \ue8 il monitoraggio del territorio che riveste un ruolo cruciale. In particolare in un\u2019area semi-naturale vasta come quella Saheliana, gli strumenti del telerilevamento satellitare sono strategici grazie alla loro capacit\ue0 di fornire dati spazializzati ed ad elevata risoluzione temporale. Scopo del lavoro \ue8 stato quello di contribuire a due aspetti del monitoraggio delle risorse naturali: lo studio di serie storiche di dati satellitari per individuare zone sottoposte a cronico degrado e studiare parametri correlati allo sviluppo della biomassa vegetale ad al suo stato idrico. Mentre il primo lavoro vuole dare informazioni utili alla pianificazione della gestione delle risorse naturali, il secondo vuole fornire informazioni in grado di fotografare in tempo reale l\u2019andamento della stagione corrente. La prima parte del lavoro ha riguardo il confronto sull\u2019intera Africa Occidentale tra il 1998 e il 2009) dei i trend delle cumulate stagionali di NDVI come proxy dello sviluppo vegetazionale, e delle precipitazioni in quanto variabile climatiche guida. I risultati hanno confermato che larga parte del territorio saheliano ha visto queste due variabili come perfettamente concordi durante il decennio passato. Tuttavia sono state evidenziate aree in cui i trend di produzione vegetale non sono spiegati dalle piogge. Aree in cui le produzioni sono aumentate nonostante la sostanziale stabilit\ue0 delle precipitazioni (Anomalous Greening, AG) risultano pi\uf9 frequenti nelle aree pi\uf9 meridionali dell\u2019Africa Occidentale ove \ue8 preponderante l\u2019attivit\ue0 agricola (West Sudanian savannah, 46% degli AG rilevati), mentre zone localizzate di anomalo decremento dell\u2019NDVI (Anomalous Degradation, AD) sono state rilevate nelle zone pi\uf9 aride del Sahel (Sahelian Acacia savannah, 59% degli AD rilevati). La analisi condotte a scala pi\uf9 di dettaglio con immagini ad alta risoluzione (30 m) hanno mostrato come queste anomalie si correlino ad usi e coperture del suolo differenti, con l\u2019AG in aree agricole l\u2019AD in aree marginali ove \ue8 praticabile unicamente la pastorizia. Due casi particolari di AG hanno mostrato eventi particolarmente drammatici in Chad e in Sudan. Entrambi i fenomeni sono risultati, da remoto, in un incremento dello sviluppo vegetazionale non legato alle piogge, dovuto al ritiro delle acque del lago Chad ed all\u2019abbandono delle terre di pascolo a seguito del conflitto del Darfur (2005-2006). I risultati sino a qui ottenuti permettono di sviluppare una mappatura tematica di aree localizzate soggette a cronico degrado, evidenziando in un sistema semi-naturale largamente legato alle precipitazioni zone in cui altre variabili vanno ad incidere sullo sviluppo vegetazionale. Queste possono essere approfondite dagli esperi locali, in modo da verificare se una popolazione rurale in continua crescita demografia sia incidendo sulla capacit\ue0 di carico dell\u2019ecosistema. La seconda parte del lavoro si \ue8 concentrata sullo stima dello stress idrico e della biomassa, due variabili fondamentali nel monitoraggio delle risorse naturali e pascolive in aree semi-aride. Serie temporali di frazione evapotraspirativa (EF) a bassa risoluzione sono state ottenute grazie alla relazione tra albedo e temperature superficiale. L\u2019EF \ue8 una componente del bilancio energetico ed \ue8 strettamente correlata con la disponibilit\ue0 idrica per la pianta. Le stime risultano avere dinamiche spaziotemporali coerenti con quelle che sono le dinamiche ecologiche della regione (piogge, fase vegetativa etc.) . Inoltre, l\u2019EF \ue8 risultata altamente correlata con flussi energetici misurati a terra da una stazione eddy covariance in Niger (r2 > 0.7). Il metodo implementato \ue8 di sicura utilit\ue0 per la stima dello stress idrico su vaste aree come quella Saheliana, frequentemente interessata da siccit\ue0 e piogge scarse. Stime di produzioni di biomassa sono state ottenute dal prodotto operativo satellitare Dry Matter Productivity (DMP) basato su di un modello di Light Use Efficiency (LUE). Le stime satellitari sono state valutate grazie al confronto con dati di produzione di biomassa pascoliva in 46 siti di misura in Niger lungo 10 anni (2000-2009). Le stime da remoto riportano valori di biomassa (kg/ha) in linea con le produzioni medie annuali dell\u2019area, tra i 200 kg/ha (aree iper-aride in annate sfavorevoli) e i 2000 kg/ha (pascoli altamente produttivi). Tuttavia le correlazioni coi dati di campo risultano basse (r2<0.3), ed il lavoro propone due approcci per incrementare l\u2019accuratezza del modello satellitare. La prima consiste nell\u2019integrazione dell\u2019EF come fattore di efficienza di disponibilit\ue0 idrica, attualmente non considerata dal DMP. L\u2019EF ha permesso di incrementare la capacit\ue0 del modello di LUE di spiegare la variabilit\ue0 dei dati di campo, specialmente su quei siti ove \ue8 pi\uf9 marcata la carenza idrica. Inoltre \ue8 stato verificato che il modello pu\uf2 incrementare la sua accuratezza nel caso in cui diverse Radiation Use Efficency (RUE) siano considerate, e seconda delle differenti coperture vegetali presenti al suolo. Le biomasse di queste \u201cunit\ue0 ecologiche\u201d presentano correlazioni staticamente differenti con le stime satellitari, e si differenziano tra di loro per la loro produttivit\ue0 media (max NDVI) e la loro fenologia (inizio della stagione, SoS). In conclusione, una stima satellitare di biomassa corretta per la disponibilit\ue0 idrica e l\u2019efficienza d\u2019uso della radiazione da parte delle diverse specie vegetali, una volta prodotta operativamente potr\ue0 fornire indicazioni sulla capacit\ue0 di carico dei pascoli nel corso della stagione, permettendo, se necessario, di produrre tempestive indicazioni sulle aree soggette a criticit\ue0.The research thesis here discussed focused on the Sahelian semi-arid rangeland, a region characterized by strong rainfall seasonality, with few dry months followed by a long dry season. In that area rangeland vegetation and human livelihoods of pastoralism and rainfed crop relies on this peculiar climatic condition. Unfavorable years whit poor or erratic rain results in reducing food supply from agropastoral activities possibly creating food insecurity condition. The work conducted address to main aspects of natural resources monitoring: long term study to identify critical condition that require further analysis to assess potential unbalanced human activities and near real time production of herbaceous biomass relate parameters to support on-going season early warning. In order to achieve the first goal satellite time-series of vegetation index and estimated rainfall were exploited (1998-2009) to identify areas where the two variables have opposite trends. These areas of anomalous hot spots highlight situations where the trend in the development of vegetation is locally driven by other factors mainly linked to human activity, rather than climatic driving force. In the humid regions of the southern part of the study area an increase of NDVI was observed even in conditions where rainfall remained stable (i.e. no significant trend), or even decreased (anomalous greening). These patches of increased NDVI are associated to crop land and savannah land cover classes. A number of hot spots of anomalous conditions along the boundary between the Sahelian and the Sahelian-Sudanian zones were analyzed in details using multi-temporal Landsat TM/ETM+ images and a more detailed analysis was conducted on a test area in Niger analyzing the anomalies in terms in changes of land cover and land use through years. The analysis of changes occurred between pairs of images acquired over the same area confirmed at local scale the trends of land degradation or recovery identified at the coarser resolution of 1km. It is important to underline that these anomalous situations are driven by local causes. Anomalous greening occurring north of Lake Chad is indicative of a critical environmental situation: the shrinking of Lake Chad has uncovered new lands colonized by new agricultural fields. On the contrary, small pockets anomalous degradation have been identified mainly in the Northern part of the study area, in the belt from West Mali to the Chad-Sudan border, which is well-known as fragile zone, where increasing population and human activity (rainfed agriculture, pastoralism and wood exploitation) are in instable equilibrium. Their strong dependence on climatic conditions determines frequent humanitarian crises due to food shortage. In Niger anomalous greening corresponds to the intensification of cropping in a fertile floodplain, whereas in Western Sudan it is associated to the abandonment of agro-pastoral land as a consequence of Darfur conflict. In areas where anomalous vegetation degradation is observed, the demographic framework and associated increase of the exploitation of environmental resources provide the general framework but are not sufficient to explain the local patterns. This result would be a support for natural resources exploitation planning, highlighting local chronic rangeland condition that need detailed analysis to identify causes and specific strategies to compensate the negative effect. The second part of the thesis focused on the estimation of two crucial variables in rangeland monitoring, the water availability for vegetation and the biomass production. Time series of evaporative fraction (EF), strongly linked to the vegetation water status and able to increase the performances of biomass estimation , were estimated from low resolution satellite data exploiting the albedo vs. land surface temperature relation. EF satellite derived resulted highly correlated to flux tower evapotranspiration (ET) measurements. In order to monitoring regional biomass the reliability of an operational LUE based product called Dry Matter Productivity (DMP) was evaluated in Niger rangeland thanks to ground biomass measurements on 46 sites over 10 years. In order to improve this useful biomass prediction at large scales the contribution of EF as a water stress efficiency in DMP algorithm was tested. Moreover the DMP performances were analyzed in relation to different ecological units, homogeneous in terms of vegetation cover and vegetation seasonal behaviour. Results suggest and discuss feasible LUE modelling improvement over the Sahel, taking into account satellite estimation of water availability and different radiation use efficiency for distinct plant communities. In conclusion, satellite biomass estimation corrected by water availability and including eco-types radiation use efficiency, once operationally produced and validated, could provide the necessary information for i) the creation of near real time bulletin of ongoing season and ii) if the case, the identification potential critical situation occurrence due to food shortage

Thirty years of land cover and fraction cover changes over the Sudano-Sahel using landsat time series

Historical land cover maps are of high importance for scientists and policy makers studying the dynamic character of land cover change in the Sudano-Sahel, including anthropogenic and climatological drivers. Despite its relevance, an accurate high resolution record of historical land cover maps is currently lacking over the Sudano-Sahel. In this study, 30 m resolution historically consistent land cover and cover fraction maps are provided over the Sudano-Sahel for the period 1986&ndash;2015. These land cover/cover fraction maps are achieved based on the Landsat archive preprocessed on Google Earth Engine and a random forest classification/regression model, while historical consistency is achieved using the hidden Markov model. Using these historical maps, a multitude of variability in the dynamic Sudano-Sahel region over the past 30 years is revealed. On the one hand, Sahel-wide cropland expansion and the re-greening of the Sahel is observed in the discrete land cover classification. On the other hand, subtle changes such as forest degradation are detected based on the cover fraction maps. Additionally, exploiting the 30 m spatial resolution, fine-scale changes, such as smallholder or subsistence farming, can be detected. The historical land cover/cover fraction maps presented in this study are made available via an open-access platform

Spatio-temporal Patterns of Normalised Difference Vegetation Index Trends in Parts of Northern Nigeria

Vegetation cover change has been, and still is a problem in Nigeria. The main objective of the study is to assess Spatio-temporal variability of vegetation cover in parts of Northern Nigeria using remotely sensed SPOT VEGT data sets. The data covers between 1999 and 2018 and consists of 10-day synthesis NDVI (S10) data. The maximum value composite technique was used to remove atmospheric moisture, cloud and simple haze in the imageries. The 10 days, monthly and later annual composites were generated in image calculator tool in IDRISI selva 17.0 software. Time series trend analysis was performed to determine the Spatio-temporal trends of the annual NDVI. The annual composites were imported into GIS environment to analyse spatial variations of NDVI in the study area. The composite of 2011 was noisy hence was not used in the analysis. The result indicated that NDVI is characterised by moderate Spatio-temporal heterogeneity. The relatively high value of NDVI (above 0.4) is mainly distributed in Jos East, Riyom, Bokkos which are located south of the area while the low value (slightly below 0.3) is obtained in Gamawa Gurri and Ringim. Furthermore, significant positive trends in overall greenness were observed in the southern region while negative trends are also present in the north. This study recommended studies on vegetation dynamics (with climate and anthropogenic influences) with high-resolution satellite data to remove the uncertainties associated with coarser resolution remote sensing-based vegetation data. Keywords: vegetation cover, Spatio-temporal, time-series imageries, NDV

Biomass production and management practices in mixed crop-livestock systems in the west African Sahel: Opportunities and constraints

The Sahel is characterized by a marked inter-annual climate variability and has experienced a number of food security crises following the severe droughts during the 1970s and 1980s. Due to recent challenges such as rapid population growth, climate change, environmental concerns and market changes which cause major impacts to their production systems, the sahelian people have been shifting and adapting their production systems and the way they live to cope with uncertainties. The objective of the present report is to review the various biomass production and management issues in the mixed crop-livestock systems in West African Sahel. An elaborated literature survey of peer reviewed papers mostly, was conducted. The studies were based on the Sahel scale research, more specifically research that had been published on the West African Sahel, including studies published between 1990s and 2016. Results show that many factors have contributed to the changes, among which, rainfall variability, population growth, human induced-activities, land tenure systems and the effects of globalization. Various biomass production and management practices are employed in West African Sahel for both on-farm and off-farm biomass improvements. Some of the best practices are mulching, soil and water conservation techniques, composting, farmer managed natural regeneration, agroforestry, etc. These practices have overall contributed to increase agricultural productivity, ecosystem services provisioning and have sometime deepened the difference between men and women, rich and poor, young and old people. Most of the constraints associated with large adoption of the best practices in the Sahel are land tenure systems, the huge gap between inputs and output investment costs but, the climate conventions are offering new opportunities that will ultimately contribute to positive changes. This will be possible only when land tenure systems in the region are reinforced, institutional linkages are strengthened, and new information systems are used to inform farmers on climate issues and new agricultural practices