ON THE INFLUENCE OF FLOODS ON ECOSYSTEM SERVICES OF FLOODPLAIN GRASSLANDS : An application to fodder production in the Tana River Delta, Kenya

En Afrique Sub-Saharienne, les zones humides forment d'importantes zones de pâturages pour bon nombre de pastoralistes. Le régime d'inondation de ces plaines influence leur fonc tionnement et leur productivité. Pour comprendre et maitriser ces régimes, il est donc essentiel d'évaluer l'impact de l'infrastructure hydro-électrique sur les dynamiques d'inondations en aval des barrages. Pourtant, le manque de données disponibles, l'absence de modèles validés et la forte variabilité environnementale rendent cet exercice difficile. Cette thèse contribue à quantifier l'impact du changement des ressources hydriques sur la productivité fourragère des prairies inondables dans le Delta du fleuve Tana, au Kenya. 1/ Les caractéristiques de croissance et la productivité d'une prairie inondable à Echinochloa stagnina (Retz) P. Beauv. ont été déterminées pour différents régimes de fauche, d'irrigation et conditions d'inondations. Ensuite, un modèle de croissance adapté à des Graminées en C4, tropicales et pérennes, de prairies inondables a été développé. Il constitue, à notre connaissance, le premier modèle éco-physiologique adapté à ce type de prairies. 2/ Des processus hydrologiques importants pour le fonctionnement des écosystèmes (étendue, période, durée et fréquence d'inondation) ont été caractérisés grâce à un modèle de bilan hydrologique et à l'utilisation de techniques de télédétection, et cela en dépit de la faible instrumentalisation du bassin, du peu de données topographiques et d'un fort couvert nuageux. 3/ Une analyse préliminaire de l'impact de différents scénarios d'inondations sur la production fourragère a été effectuée en utilisant des indicateurs de services écosystémiques. Cette thèse participe à l'amélioration de nos connaissances des services écosystémiques des zones humides par la construction de modèles et par l'évaluation de scénarios dans une région du monde où ce type de données est rare.Wetlands are a vital resource for many pastoralists in Sub-Saharan Africa as they provide dry-season grazing zones. As floods are essential for wetland ecosystems, the assessment of water abstraction and hydroelectric infrastructure on downstream flooding dynamics is crucial. Yet, scarce data, environmental variability and the lack of models make this challenging. This research contributes to quantifying the influence of changing water resources on fodder pro- duction of floodplain grasslands in the Tana River Delta, Kenya. 1/ Growth characteristics of floodplain grasslands of Echinochloa stagnina (Retz) P. Beauv. for different flood and manage- ment options were determined and a quantification of their productivity achieved. This kind of data is scarce for floodplain grasslands. A plant growth model adapted to tropical floodplain conditions and perennial C4 grasses was developed, and is the first known physiologically based model for floodplain grasslands. 2/ Hydrological processes of ecological importance (flood extent, timing, duration, frequency) were characterized in a poorly gauged basin using a water-balance model combined with remote-sensing techniques, despite precise knowledge of discharge rates, topography and a high cloud cover. 3/ A preliminary analysis explored different flooding scenarios and their impact on fodder production through the use of simple ecosystem service indicators. This PhD contributes to the repertoire of wetland ecosystem ser- vices by building biophysically based simulation models and exploring possible scenarios in a region of the world and an ecosystem where these type of evaluations are rare

Agroecology in North African irrigated plains? Mapping promising practices and characterizing farmers’ underlying logics

International audienceIn the irrigated plains of North Africa, the sustainability of productive resources is subject to multiple threats linked to the prevailing productivist model of irrigated agriculture. These threats, such as soil degradation and unequal access to resources, markets, and information, prompt farmers to mobilize depleting natural resources, including soil and water, in an often environmentally unsustainable way. In order to sustain their farming systems and consequent incomes, farmers sometimes update their strategies by setting-up alternative farming practices. This study aims at mapping and analyzing such existing local farming practices with agroecological potentials. Our approach is based on direct observations combined with 150 interviews of farmers in three major irrigated plains in North Africa, namely, the Merguellil, Upper Cheliff, and Saiss plains, respectively, in Tunisia, Algeria, and Morocco. This study shows that a wide range of alternative practices with agroecological potential are emerging (or exist) locally, against the flow of the dominant agricultural model which is rather in favor of intensive practices. The most common practices are developed to improve soil fertility management (production of manure tea, integration of legumes in crop successions), increase per surface agricultural production (relay intercropping, intercropping, agroforestry), or provide multiple ecosystem services (diversification, livestock integration). These practices are jointly used, mostly to (i) increase land-use efficiency, and hence face land fragmentation; (ii) diversify their cropping strategy, and spread out market-related risks; and (iii) reduce expensive production costs related to irrigation and chemical fertilization. The gradient of adoption observed according to national contexts suggests a strong influence of contrasting socio-political and historical factors at the regional to national levels. The analysis of logics in implementing such practices by farmers indicates that economic reasons take precedence over environmental concerns. As such, these practices can be seen as (i) an access to low-cost strategies for small farmers or (ii) a pathway to international markets for agribusiness farmers. Taming the extensive local knowledge related to ecological intensification strategies, as identified in this study, can help to pave the way for a more sustainable agriculture, in this intensively cultivated region of the world

Integrating water availability and accessibility constraints in localized food basin models

International audienceNumerous models have been developed to calculate the territory needed to supply cities with the food required to satisfy their consumption habits (Schreiber et al., 2021). However, the effect of water availability remains little studied. Our communication will have a programmatic orientation. It will seek to outline an approach or methodology for developing a relevant food basin model at a local scale (metropolis or region) integrating water-related constraints (access to water and water availability for farms, crop water requirements). This project is original in the sense that it links two themes that are rarely addressed jointly: food systems and water management

Comparing viewpoints on agricultural development

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Reconstitution of a continuous climatic and rainfall series for the central Sahel (1950-2012): methodology and application.

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Adoption factors and structural characteristics of irrigated olive grove agroforestry systems in Central Tunisia

International audienceIrrigated olive grove agroforestry systems in the Southern Mediterranean have rarely been studied. In the context of increased interest for agroecological approaches, this study questions why and how farmers undertake such associations, in the case of smallholder irrigated agriculture in Tunisia. The objectives were to characterize the physical structure of existing olive-summer vegetable associations and describe the rationales of farmers implementing them. Quantitative and qualitative approaches were used to collect data from 132 olive groves and 31 semi-directive interviews in the Merguellil plain, Central Tunisia. Dual crop input-intensive agroforestry systems were predominant, although agroforestry structures with a high species' diversity also existed. Adoption reasons and implementation of agroforestry systems varied. The latter were often perceived as an economically viable solution in a context of difficult access to productive resources. Particularly, limited and fragmented access to land or water was a strong driver of adoption, inducing contractual arrangements between farmers to share resources. Farmers implemented agroforestry systems mainly to maximize income, reduce production costs or reduce risks through a crop diversification strategy. Most characterized agroforestry olive-summer vegetable associations may fail to meet the principles of agroecology. Being already adopted by farmers, they may however serve as a base to conceive improved cropping systems

L’approche par les services écosystémiques pour la gestion opérationnelle des bassins versants : état de l’art

After an introduction on the definition of ecosystem services (ES) and existing ES classifications, this report presents some examples of using ES approach for watershed management in the world. ES provided by soils are then detailed. Finally, proposals are made for adapting the ES approach to the assessment of water and soil conservation policies in Tunisia.Après avoir rappelé la définition des services écosystémiques (SE) et les typologies existantes, ce rapport présente des exemples d'utilisation de l'approche SE pour la gestion des bassin-versants dans le monde. Les SE des sols font ensuite l'objet d'un développement particulier. Finalement des propositions sont faites pour adapter l'approche à l'évaluation des politiques de conservation des eaux et des sols en Tunisie

Chapter 1. Climate warming observed in the Sahel since 1950

Introduction Research on climate warming has considerably grown over the past three decades or so. However, existing studies in West Africa are focused mainly on precipitation trends, with very little attention paid to the evolution of temperatures. The climate warming observed during the last 60 years is contrasted from one region to another. It is more marked in continental zones and stronger at night than during the day (IPCC, 2013). Until now, most regional studies have concentrated on cl..

Modelling the growth of floodplain grasslands to explore the impact of changing hydrological conditions on vegetation productivity

International audienceThis study (i) presents a coupled vegetation-soil model adapted to perennial C4 seasonally flooded semi-arid grasslands; and (ii) applies the model to evaluate changes in the annual productivity of the grasslands of the Tana River Delta, Kenya, under changing flooding conditions. Main plant growth processes are modelled within coupled plant carbon balance and soil water budget modules: photosynthesis, allocation of photosynthates, respiration, translocation of root phytomass to aerial phytomass, senescence and litter production. Aerial phytomass can also be subtracted from the system, to simulate grazing or cutting. New features concern the inclusion of effects of floods on energy conversion efficiency, photosynthate allocation, senescence and litter production. The vegetation model, composed of four phytomass compartments (leaves, stems, roots, aerial dead matter), simulates three main growth phases related to flooding: floods, a post-flood phase and a non-flooded phase. It was designed to be used with limited climatic data. Data collected during a 14-month experiment (2010–2012) in the Tana River Delta, Kenya, in which different irrigation and cutting treatments and flood events were recorded, were used for calibration and validation purposes. Fourteen parameters, selected through a sensitivity analysis, were calibrated on half of these treatments. Uncertainty in parameter estimation was expressed through a stochastic ensemble of simulations. The remaining independent data were used for model validation. Overall, the model predictions are in good agreement with the experimental data. This model can be used to assess the impact of rain variability, grazing or flooding patterns on the annual primary productivity of Sub-Saharan floodplain grasslands composed mainly of Echinochloa stagnina (Retz) P. Beauv. In particular, simulations for the Tana River Delta suggest that past changes in the hydrological regime of the river, as well as future changes due to the construction of hydroelectric infrastructure, have led and will certainly lead to an important decrease of the floodplain grassland productivity. As local and regional livestock keeping activities rely heavily on the dry seasons’ grazing resources available within the wetland, future development plans should seriously consider the negative effects of these changes on local activities and livelihoods

Chapitre 1. Le réchauffement climatique observé depuis 1950 au Sahel

Introduction Depuis une trentaine d’années, le réchauffement climatique fait l’objet de travaux de plus en plus nombreux. Cependant, en Afrique de l’Ouest, les études existantes se sont principalement intéressées aux évolutions des précipitations, très peu à celles des températures. Le réchauffement climatique observé au cours des soixante dernières années se décline de manière contrastée suivant les régions du globe. Il est plus marqué sur les zones continentales, et plus important la nuit q..