Revisiting the coupling between NDVI trends and cropland changes in the Sahel drylands:a case study in western Niger

The impact of human activities via land use/cover changes on NDVI trends is critical for an improved understanding of satellite-observed changes in vegetation productivity in drylands. The dominance of positive NDVI trends in the Sahel, the so-called re-greening, is sometimes interpreted as a combined effect of an increase in rainfall and cropland expansion or agricultural intensification. Yet, the impact of changes in land use has yet to be thoroughly tested and supported by empirical evidence. At present, no studies have considered the importance of the different seasonal NDVI signals of cropped and fallowed fields when interpreting NDVI trends, as both field types are commonly merged into a single ‘cropland’ class. We make use of the distinctly different phenology of cropped and fallowed fields and use seasonal NDVI curves to separate these two field types. A fuzzy classifier is applied to quantify cropped and fallowed areas in a case study region in the southern Sahel (Fakara, Niger) on a yearly basis between 2000 and 2014. We find that fallowed fields have a consistently higher NDVI than unmanured cropped fields and by using two seasonal NDVI metrics (the amplitude and the decreasing rate) derived from the MODIS time series, a clear separation between classes of fields is achieved (r = 0.77). The fuzzy classifier can compute the percentage of a pixel (250 m) under active cultivation, thereby alleviating the problem of small field sizes in the region. We find a predominant decrease in NDVI over the period of analysis associated with an increased area of cropped fields at the expense of fallowed fields. Our findings couple cropping abandonment (more frequent fallow years) with positive NDVI trends and an increase in the percentage of the cropped area (fallow period shortening) with negative trends. These findings profoundly impact our understanding of greening and browning trends in agrarian Sahelian drylands and in other drylands of developing countries characterized by limited use of fertilizers

Woody plant cover estimation in drylands from Earth Observation based seasonal metrics

peer reviewedFrom in situ measured woody cover we develop a phenology driven model to estimate the canopy cover of woody species in the Sahelian drylands at 1 km scale. The model estimates the total canopy cover of all woody phanerophytes and the concept is based on the significant difference in phenophases of dryland trees, shrubs and bushes as compared to that of the herbaceous plants. Whereas annual herbaceous plants are only green during the rainy season and senescence occurs shortly after flowering towards the last rains, most woody plants remain photosynthetically active over large parts of the year. We use Moderate Resolution Imaging Spectroradiometer (MODIS) and Satellite pour l'Observation de la Terre (SPOT) — VEGETATION (VGT) Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) time series and test 10 metrics representing the annual FAPAR dynamics for their ability to reproduce in situ woody cover at 43 sites (163 observations between 1993 and 2013) in the Sahel. Both multi-year field data and satellite metrics are averaged to produce a steady map. Multiple regression models using the integral of FAPAR from the onset of the dry season to the onset of the rainy season, the start date of the growing season and the rate of decrease of the FAPAR curve achieve a cross validated r2/RMSE (in % woody cover) of 0.73/3.0 (MODIS) and 0.70/3.2 (VGT). The extrapolation to Sahel scale shows agreement between VGT and MODIS at an almost nine times higher woody cover than in the global tree cover product MOD44B which only captures trees of a certain minimum size. The derived woody cover map of the Sahel is made publicly available and represents an improvement of existing products and a contribution for future studies of drylands quantifying carbon stocks, climate change assessment, as well as parametrization of vegetation dynamic models

Major trends in the land surface phenology (LSP) of Africa, controlling for land cover change

Monitoring land surface phenology (LSP) trends is important in understanding how both climatic and non-climatic factors influence vegetation growth and dynamics. Controlling for land-cover changes in these analyses has been undertaken only rarely, especially in poorly studied regions like Africa. Using regression models and controlling for land-cover changes, this study estimated LSP trends for Africa from the enhanced vegetation index (EVI) derived from 500 m surface reflectance Moderate-Resolution Imaging Spectroradiometer (MOD09A1), for the period from 2001 to 2015. Overall end of season showed slightly more pixels with significant trends (12.9% of pixels) than start of season (11.56% of pixels) and length of season (LOS) (5.72% of pixels), leading generally to more ‘longer season’ LOS trends. Importantly, LSP trends that were not affected by land-cover changes were distinguished from those that were influenced by land-cover changes such as to map LSP changes that have occurred within stable land-cover classes and which might, therefore, be reasonably associated with climate changes through time. As expected, greater slope magnitudes were observed more frequently for pixels with land-cover changes compared to those without, indicating the importance of controlling for land cover. Consequently, we suggest that future analyses of LSP trends should control for land-cover changes such as to isolate LSP trends that are solely climate-driven and/or those influenced by other anthropogenic activities or a combination of both

Remote sensing environmental change in southern African savannahs : a case study of Namibia

Savannah biomes cover a fifth of Earth’s surface, harbour many of the world’s most iconic species and most of its livestock and rangeland, while sustaining the livelihoods of an important proportion of its human population. They provide essential ecosystem services and functions, ranging from forest, grazing and water resources, to global climate regulation and carbon sequestration. However, savannahs are highly sensitive to human activities and climate change. Across sub-Saharan Africa, climatic shifts, destructive wars and increasing anthropogenic disturbances in the form of agricultural intensification and urbanization, have resulted in widespread land degradation and loss of ecosystem services. Yet, these threatened ecosystems are some of the least studied or protected, and hence should be given high conservation priority. Importantly, the scale of land degradation has not been fully explored, thereby comprising an important knowledge gap in our understanding of ecosystem services and processes, and effectively impeding conservation and management of these biodiversity hotspots. The primary drivers of land degradation include deforestation, triggered by the increasing need for urban and arable land, and concurrently, shrub encroachment, a process in which the herbaceous layer, a defining characteristic of savannahs, is replaced with hardy shrubs. These processes have significant repercussions on ecosystem service provision, both locally and globally, although the extents, drivers and impacts of either remain poorly quantified and understood. Additionally, regional aridification anticipated under climate change, will lead to important shifts in vegetation composition, amplified warming and reduced carbon sequestration. Together with a growing human population, these processes are expected to compound the risk of land degradation, thus further impacting key ecosystem services. Namibia is undergoing significant environmental and socio-economic changes. The most pervasive change processes affecting its savannahs are deforestation, degradation and shrub encroachment. Yet, the extent and drivers of such change processes are not comprehensively quantified, nor are the implications for rural livelihoods, sustainable land management, the carbon cycle, climate and conservation fully explored. This is partly due to the complexities of mapping vegetation changes with satellite data in savannahs. They are naturally spatially and temporally variable owing to erratic rainfall, divergent plant functional type phenologies and extensive anthropogenic impacts such as fire and grazing. Accordingly, this thesis aims to (i) quantify distinct vegetation change processes across Namibia, and (ii) develop methodologies to overcome limitations inherent in savannah mapping. Multi-sensor satellite data spanning a range of spatial, temporal and spectral resolutions are integrated with field datasets to achieve these aims, which are addressed in four journal articles. Chapters 1 and 2 are introductory. Chapter 3 exploits the Landsat archive to track changes in land cover classes over five decades throughout the Namibian Kalahari woodlands. The approach addresses issues implicit in change detection of savannahs by capturing the distinct phenological phases of woody vegetation and integrating multi-sensor, multi-source data. Vegetation extent was found to have decreased due to urbanization and small-scale arable farming. An assessment of the limitations leads to Chapter 4, which elaborates on the previous chapter by quantifying aboveground biomass changes associated with deforestation and shrub encroachment. The approach centres on fusing multiple satellite datasets, each acting as a proxy for distinct vegetation properties, with calibration/validation data consisting of concurrent field and LiDAR measurements. Biomass losses predominate, demonstrating the contribution of land management to ecosystem carbon changes. To identify whether biomass is declining across the country, Chapter 5 focuses on regional, moderate spatial resolution time-series analyses. Phenological metrics extracted from MODIS data are used to model observed fractional woody vegetation cover, a proxy for biomass. Trends in modelled fractional woody cover are then evaluated in relation to the predominant land-uses and precipitation. Negative trends slightly outweighed positive trends, with decreases arising largely in protected, urban and communal areas. Since precipitation is a fundamental control on vegetation, Chapter 6 investigates its relation to NDVI, by assessing to what extent observed trends in vegetation cover are driven by rainfall. NDVI is modelled as a function of precipitation, with residuals assumed to describe the fraction of NDVI not explained by rainfall. Mean annual rainfall and rainfall amplitude show a positive trend, although extensive “greening” is unrelated to rainfall. NDVI amplitude, used as a proxy for vegetation density, indicates a widespread shift to a denser condition. In Chapter 7, trend analysis is applied to a Landsat time-series to overcome spatial and temporal limitations characteristic of the previous approaches. Results, together with those of the previous chapters, are synthesized and a synopsis of the main findings is presented. Vegetation loss is predominantly caused by demand for urban and arable land. Greening trends are attributed to shrub encroachment and to a lesser extent conservation laws, agroforestry and rangeland management, with precipitation presenting little influence. Despite prevalent greening, degradation processes associated with shrub encroachment, including soil erosion, are likely to be widespread. Deforestation occurs locally while shrub encroachment occurs regionally. This thesis successfully integrates multi-source data to map, measure and monitor distinct change processes across scales

A comparison of global agricultural monitoring systems and current gaps

Global and regional scale agricultural monitoring systems aim to provide up-to-date information regarding food production to different actors and decision makers in support of global and national food security. To help reduce price volatility of the kind experienced between 2007 and 2011, a global system of agricultural monitoring systems is needed to ensure the coordinated flow of information in a timely manner for early warning purposes. A number of systems now exist that fill this role. This paper provides an overview of the eight main global and regional scale agricultural monitoring systems currently in operation and compares them based on the input data and models used, the outputs produced and other characteristics such as the role of the analyst, their interaction with other systems and the geographical scale at which they operate. Despite improvements in access to high resolution satellite imagery over the last decade and the use of numerous remote-sensing based products by the different systems, there are still fundamental gaps. Based on a questionnaire, discussions with the system experts and the literature, we present the main gaps in the data and in the methods. Finally, we propose some recommendations for addressing these gaps through ongoing improvements in remote sensing, harnessing new and innovative data streams and the continued sharing of more and more data

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

Information for Meeting Africa’s Agricultural Transformation and Food Security Goals (IMAAFS)

The organizers of this international Conference on Information for Meeting Africa’s Agricultural Transformation and Food Security Goals (IMAAFS) included the African Union, the UN Economic Commission for Africa, and the European Commission (through the Joint Research Center). The Conference took place at the UN Conference Centre in Addis Ababa from 1 to 3 October 2014, to widen the availability and use of evidence-based information for agricultural growth and improved food and nutrition security. With over 180 international participants, the event brought together scientists and policy makers from a wide range of institutions and research organizations from Africa, Europe and the United States, as well as major UN agencies. The Conference took place over the course of three days and included nine presentation and discussion sessions (each with a chairperson and a rapporteur), executive morning briefs, break-out working groups, and a final decision-grid exercise to summarize the expert opinion of participants regarding the most promising strategies.JRC.H.4-Monitoring Agricultural Resource

Mapping crop phenology using NDVI time-series derived from HJ-1 A/B data

With the availability of high frequent satellite data, crop phenology could be accurately mapped using time-series remote sensing data. Vegetation index time-series data derived from AVHRR, MODIS, and SPOT-VEGETATION images usually have coarse spatial resolution. Mapping crop phenology parameters using higher spatial resolution images (e.g., Landsat TM-like) is unprecedented. Recently launched HJ-1 A/B CCD sensors boarded on China Environment Satellite provided a feasible and ideal data source for the construction of high spatio-temporal resolution vegetation index time-series. This paper presented a comprehensive method to construct NDVI time-series dataset derived from HJ-1 A/B CCD and demonstrated its application in cropland areas. The procedures of time-series data construction included image preprocessing, signal filtering, and interpolation for daily NDVI images then the NDVI time-series could present a smooth and complete phenological cycle. To demonstrate its application, TIMESAT program was employed to extract phenology parameters of crop lands located in Guanzhong Plain, China. The small-scale test showed that the crop season start/end derived from HJ-1 A/B NDVI time-series was comparable with local agro-metrological observation. The methodology for reconstructing time-series remote sensing data had been proved feasible, though forgoing researches will improve this a lot in mapping crop phenology. Last but not least, further studies should be focused on field-data collection, smoothing method and phenology definitions using time-series remote sensing data