Severe droughts in Amazonia in 2005 and 2010 caused widespread loss of carbon from the terrestrial biosphere. This loss, almost twice the annual fossil fuel CO2 emissions in the EU, suggests a large sensitivity of the Amazonian carbon balance to a predicted more intense drought regime in the next decades. This is a dangerous inference though, as there is no scientific consensus on the most basic metrics of Amazonian carbon exchange: the gross primary production (GPP) and its response to moisture deficits in the soil and atmosphere. Measuring them on scales that span the whole Amazon forest was thus far impossible, but in this project I aim to deliver the first observation-based estimate of pan-Amazonian GPP and its drought induced variations. My program builds on two recent breakthroughs in our use of stable isotopes (13C, 17O, 18O) in atmospheric CO2: (1) Our discovery that observed δ¹³C in CO2 in the atmosphere is a quantitative measure for vegetation water-use efficiency over millions of square kilometers, integrating the drought response of individual plants. (2) The possibility to precisely measure the relative ratios of 18O/16O and 17O/16O in CO2, called Δ17O. Anomalous Δ17O values are present in air coming down from the stratosphere, but this anomaly is removed upon contact of CO2 with leaf water inside plant stomata. Hence, observed Δ17O values depend directly on the magnitude of GPP. Both δ¹³C and Δ17O measurements are scarce over the Amazon-basin, and I propose more than 7000 new measurements leveraging an established aircraft monitoring program in Brazil. Quantitative interpretation of these observations will break new ground in our use of stable isotopes to understand climate variations, and is facilitated by our renowned numerical modeling system “CarbonTracker”. My program will answer two burning question in carbon cycle science today: (a) What is the magnitude of GPP in Amazonia? And (b) How does it vary over different intensities of drought?

Due to an increasing demographic pressure, the Metropolitan Region of Recife (RMR, the fifth largest metropolitan area in Brazil), went through remarkable water and land use changes over the last decades. These evolutions gave rise to numerous environmental consequences, such as a dramatic decline of the piezometric levels, groundwater salinization and contamination. This degradation of natural resources is linked to the increase of water demand, punctually amplified by drought periods which induced the construction of thousands of private wells, hindering global political solutions. The RMR thus appears as a typical "hot spot" illustrating the problems of emerging countries such as urbanization, unequal distribution of wealth, limited effects of political decisions, rapid industrial and touristic development. All these factors induce high pressures on water resources both on quantity and quality in the context of global social and environmental changes. Under these conditions, the COQUEIRAL project proposes an interdisciplinary research program aiming to study the human impact on coastal overexploited aquifers. The project is structured in three principal converging axes: (1) the analysis of pressures on the groundwater resources and their societal and structural reasons, (2) the identification of sources and mechanisms of groundwater quality and quantity degradation, focusing on the physical and chemical processes as vectors of the reaction of the system to the external pressures and (3) the assessment of the regional impact of global changes on water ressources. COQUEIRAL will approach the degradation of the groundwater resources by questioning the specific conditions of urbanization and water administration in Recife at multiple levels: the macro-sociological level with the political and institutional stake of water management; the meso-sociological level with the water’s collective stakes and their perceptions; and the micro-sociological level, meaning the representations, practices, individual and collective uses of water. Geomorphological-urban maps will complete the knowledge. In parallel to the acquisition of new geological, hydrological and hydrogeological data, COQUEIRAL will elaborate methods to determine the origin and processes of salinization, including a multi-tracer approach, to identify sources and pathways of inorganic contamination. and to determine the residence time of water within the aquifer system. Based on the gained knowledge, hydrogeological conceptual and 3D numerical models of the functioning of the aquifer system in its social and environmental contexts will be developed. In the aim to improve existing management tools, COQUEIRAL will propose the outlines for best practices, based on scenarios of groundwater resources evolution resulting from the sociological and climatic scenarios developed in the project. Knowing the outstanding importance of water resources management for the regional development, COQUEIRAL will share the results of this interdisciplinary work with all relevant stakeholders, through a variety of communication networks, including a photographic exhibition. The results obtained in the specific framework of the metropolitan region of Recife are in great part transposable to similar contexts of "hot spots" of human and climatic pressure on water resources in emerging countries. At French level, the project also involves a SME (Geo-Hyd) and will serve as a example for future collaborations with regional enterprises at an international scale. The project has applied for labeling by the “pôle de compétitivité” DREAM.