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AgMIP is an international program bringing together research projects on climate, crop modelling and regional agriculture adaptation to climate change. One objective is to better assess the projections of global food availability depending on different staple crops (wheat, rice and maize), taking into account the projections of climate change for the end of century and the uncertainty attached to them. The need for robust estimates, i.e. good crop models for yields and use of natural resources is a prerequisite to benchmark the various cropping systems and local solutions that will ultimately be explored in order to cope with climate change, without bringing about any negative side effects on the environment. Modelers hence work together internationally in order to compare and improve process-based crop simulation models. Maize is a strategic crop, exhibiting high potential radiation and water use efficiencies and is cultivated worldwide. In a first phase, the impacts of CO2 and temperature on the maize yields and water use were studied using 23 crop models on 4 sites with contrasted cool or hot climate conditions, under no water limitation (Lusignan in France, Ames in the United States, Morogoro in Tanzania and Rio Verde in Brasil). Models were run using local soil conditions and climate variables for 30 years (1980-2010) after adjusting the cultivar parameters to the ones used in one experiment in each site. At the four sites studied, the average values across models of simulated yields were closer to the observed local experimental results than the simulation of any individual model. This indicated that ensemble modelling could be a relevant way to approach the impact of climate change on maize yields. There was also a broad agreement between models to simulate a reduction in maize yield in response to temperature, roughly - 0.5 Mg ha-1 per °C increase, with no significant impact on water use, although the latter variable was estimated with a large variability between models. Plant phenology was the mostly altered process with increasing temperature. Shortening of the duration from flowering to maturity in particular reduced the gain in grain weight during that phase. This suggests that genetics could hence play a key role in adapting maize production to climate change, at least under high water availability. Doubling [CO2] from 360 to 720 μmole mole-1 increased grain yield by 7.5% on average across models and sites, with a slight decrease of water use, bringing about an increase in water use efficiency. However, the variability of the response to [CO2] was very high, bringing about the need to better simulate the role of CO2, especially on plant transpiration. In a second phase, models are therefore now being tested against Free Air CO2 Enrichment experimental data, so that variability can be reduced and the actual impact of global change on water use can be assessed with a relevant precision to adaptating agricultural practices. (Texte intégral)

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[Departement_IRSTEA]RE [TR1_IRSTEA]RIE / TRANSFEAU; Analysis of the heavy stable isotopic ratios of water (18O and 2H) in the soil can allow determining its history since the last rainfall event: whether it has just infiltrated, reevaporated from the soil or been extracted by the plants. By using the natural abundance in 18O and 2H of the atmospheric water vapour, and when making specific hypothesis, it is possible to identify and quantify its different sources (vegetation and soil at different scales) and realise a partition of evapotranspiration fluxes into plant transpiration and soil evaporation. The study of all the interactions at the field scale is still very difficult due to the large number of controlling variables describing climate, vegetation and soil characteristics. A monolith experiment (including soil and growing plant) was carried out in a reactor called RUBIC (Reactor Used for Continental Isotopic Biogeochemistry, Bariac et al., 1991). Controlled conditions allowed a monitoring and regulation of climatic parameters (net radiation, air temperature, vapour pressure deficit, CO2 partial pressure, and wind speed). It was also necessary to fix soil (structure, texture, and water content) and vegetation (specie and seeding density) parameters. The collected data allow us to improve our understanding of the partition of evapotranspiration into soil evaporation and plant transpiration. Furthermore, the isotopic ratios of evapotranspiration, under these controlled conditions, give us information regarding the depth of water extracted by the roots when comparing the values to the soil water isotopic profile.

[Departement_IRSTEA]RE [TR1_IRSTEA]RIE / TRANSFEAU; Due to the large variability of rainfall in time and space, as well as the heterogeneity of the continental surfaces, rainfall recycling by evapotranspiration from land covers is certainly the most unknown component of the global water cycle. Traditional measuring methods such as sap flow, micro lysimeter, water and energy balance estimation (Bowen ratio, eddy correlation) have been used since the 70s for a monitoring of real evapotranspiration fluxes over crops and others plant covers. A complementary method consists in using isotopic biogeochemistry: when making specific hypothesis, it is possible to identify and quantify the different sources of the atmospheric water vapour (vegetation and soil at different scales). Analysis of the heavy stable isotopic ratios of water in both liquid and vapour phases: 18O and 2H can allow determining the history of the water in the soil since the last rainfall event (infiltration, re-evaporation) or more specifically the root extraction depths. The study of all the interactions at the field scale is still very difficult due to the large number of controlling variables describing climate, vegetation and soil characteristics. A monolith experiment (including soil and growing plant) was carried out in a reactor called RUBIC (Reactor Used for Continental Isotopic Biogeochemistry, Bariac et al., 1991). Controlled conditions allowed a monitoring and regulation of climatic parameters(net radiation, air temperature, vapour pressure deficit, CO2 partial pressure, and wind speed). It was also necessary to fix soil (structure, texture, and water content) and vegetation (specie and seeding density) parameters. The collected data allow us to improve our understanding of the partition of evapotranspiration into soil evaporation and plant transpiration and to assess the hypothesis (often made in isotopic biochemistry) of a stationary state reached in the two reservoirs (soil and plant) for water. These data also allow the evaluation of the hypothesis included in a transfer module of heavy stable isotopes of water within the bare soil and the plant (Braud et al., 2005). The latter is coupled to a SVAT model (Soil-Plant-Atmosphere Transfer) called SiSPAT (Simple Soil Plant Atmosphere Transfer model. Braud et al., 1995; Braud 2000; 2002) and was extended to take into account isotopes transfer within the vegetation (root extraction and transpiration). The poster will present the experimental device RUBIC I. Then, we will emphasize the functioning mechanisms identified from data analysis and derived from the intrinsic properties of stable isotopes as natural tracers of water movement and history. We will also present our first modelling results of the partition between evaporation and transpiration simulated using SiSPAT_Isotope.

[Departement_IRSTEA]RE [TR1_IRSTEA]RIE / TRANSFEAU; Rainfall recycling by evapotranspiration from continental surfaces is certainly the most unknown component of the global water cycle. This is due to the large variability of rainfall as well as the heterogeneity of these continental surfaces, both in time and space. Traditional measuring methods including the Bowen ratio, the eddy correlation and the water balance estimation (micro lysimeter, sap flow) have been used since the 70s for a monitoring of real evapotranspiration fluxes over crops and others plant covers. A complementary method consists in using isotopic biogeochemistry. When making specific hypothesis, it is possible to identify and quantify the different sources of the atmospheric water vapour (vegetation and soil at different scales). Analysis of the heavy stable isotopic ratios of water in both liquid and vapour phases: 18O and 2H can allow determining the « history » of the water in the soil since the last rainfall event (infiltration, re-evaporation) or the root extraction depths. Field campaigns measurements (plants and soils), interpreted using the Keeling Plot method (Keeling, 1958) allowed some progress in the partition between evaporation and transpiration understanding (e.g. Yepez et al., 2003; Williams et al., 2004). But the experimental design is not sufficient to mechanistically describe the water processes involved. The study of all the interactions is not possible due to the large number of controlling variables describing climate, vegetation and soil characteristics. The BIIBA research team (Isotopic Biogeochemistry for the study of Biosphere-Atmosphere Interactions) from the Research Institute BioEMCo (Biogeochemistry and Ecology of Continental Environments) studies the vapour and liquid water fluxes within the soil and vegetation (at the plant and small cover scale). Experiments are conducted in the field and in controlled conditions inside biogeochemical reactors called RUBIC (Reactor Used for Continental Isotopic Biogeochemistry). Controlled conditions allow a monitoring and regulation of climatic parameters (net radiation, air temperature, vapour pressure deficit, CO2 partial pressure, and wind speed). It is also easier to fix soil (structure, texture, and water content) and vegetation (specie and seeding density) parameters. A monolith experiment (including soil and growing plant) was carried out in one of our reactors. The collected data allow us to improve our understanding of the partition of evapotranspiration into soil evaporation and plant transpiration and to assess the hypothesis often made in isotopic biochemistry of a stationary state reached in the two reservoirs (soil and plant). These data also allow the evaluation of the hypothesis included in a transfer module of heavy stable isotopes of water within the bare soil and the plant (Braud et al., 2005). The latter is coupled to a SVAT model (Soil-Plant-Atmosphere Transfer) called SiSPAT (Simple Soil Plant Atmosphere Transfer model. Braud et al., 1995; Braud 2000; 2002) and was extended to take into account isotopes transfer within the vegetation (root extraction and transpiration). The poster will present the experimental device RUBIC. Then, we will emphasize the functioning mechanisms identified from data analysis and derived from the intrinsic properties of stable isotopes as natural tracers of water movement and history. We will also present our first modelling results of the partition between evaporation and transpiration simulated using SiSPAT_Isotope.

[Departement_IRSTEA]RE [TR1_IRSTEA]RIE / TRANSFEAU; Rainfall recycling by evapotranspiration from continental surfaces is certainly the most unknown component of the global water cycle. This is due to the large variability of rainfall as well as the heterogeneity of these continental surfaces, both in time and space. Traditional measuring methods such as sap flow, micro lysimeter, water and energy balance estimation (Bowen ratio, eddy correlation) have been used since the 70s for a monitoring of real evapotranspiration fluxes over crops and others plant covers. A complementary method consists in using isotopic biogeochemistry. When making specific hypothesis, it is possible to identify and quantify the different sources of the atmospheric water vapour (vegetation and soil at different scales). Analysis of the heavy stable isotopic ratios of water in both liquid and vapour phases: 18O and 2H can allow determining the history of the water in the soil since the last rainfall event (infiltration, re-evaporation) or the root extraction depths. Field campaigns measurements (plants and soils), interpreted using the Keeling Plot method allowed some progress in the partition between evaporation and transpiration understanding. But the experimental design is not sufficient to mechanistically describe the water processes involved. The study of all the interactions is difficult due to the large number of controlling variables describing climate, vegetation and soil characteristics. A monolith experiment (including soil and growing plant) was carried out in a reactor called RUBIC (Reactor Used for Continental Isotopic Biogeochemistry, Bariac et al., Geochim. Cosmochim. Acta., 1991). Controlled conditions allowed a monitoring and regulation of climatic parameters (net radiation, air temperature, vapour pressure deficit, CO2 partial pressure, and wind speed). It was also necessary to fix soil (structure, texture, and water content) and vegetation (specie and seeding density) parameters. The collected data allow us to improve our understanding of the partition of evapotranspiration into soil evaporation and plant transpiration and to assess the hypothesis (often made in isotopic biochemistry) of a stationary state reached in the two reservoirs (soil and plant). These data also allow the evaluation of the hypothesis included in a transfer module of heavy stable isotopes of water within the bare soil and the plant (Braud et al., Journ. of Hydrol., 2005). The latter is coupled to a SVAT model (Soil-Plant-Atmosphere Transfer) called SiSPAT (Simple Soil Plant Atmosphere Transfer model, Braud et al., Journ. of Hydrol., 1995) and was extended to take into account isotopes transfer within the vegetation (root extraction and transpiration). The experimental design of RUBIC as well as the first modelling results will be presented.

[Departement_IRSTEA]RE [TR1_IRSTEA]RIE / TRANSFEAU; Dans les systèmes agricoles comme dans tout écosystème, les économies de l'eau et de l'azote sont étroitement mêlées. Différentes variables synthétiques caractérisent l'efficacité du système de culture face au déficit hydrique. Une valeur élevée du rapport entre l'azote exporté et l'eau consommée par la culture (efficience de l'eau pour l'azote : NWUE , mg L-1) laisse espérer que le système est moins sujet aux pertes d'azote minéral (ou gazeux). Maximiser la quantité d'azote exportée par rapport à la consommation d'eau est l'une des voies pour une agriculture respectueuse de l'environnement. Nous cherchons à comprendre l'influence du fonctionnement hydrique de la culture sur différentes composantes du fonctionnement azoté : extraction de l'azote minéral du sol, influence sur le fonctionnement trophique et morphogénétique du peuplement cultivé. Ces différents processus interviennent à des échelles diverses, et sont encore mal hiérarchisés. Le principal objectif de ce travail est d'établir le poids respectif de différents processus impliqués dans la contrainte hydrique et son influence sur l'état de nutrition azotée (en termes quantitatifs et de temps de réponse). Un effort particulier a porté sur les situations de compétition entre espèces végétales se partageant la surface cultivée, dans la mesure où ce type d'occupation est souvent perçu comme un avantage pour l'environnement et la régularité de la production agricole. Nous avons étudié ces processus sous différents niveaux de contrainte hydrique et azotée à différents stades de croissance en milieu contrôlé et dans le cas de la prairie temporaire et de la vigne enherbée. Dans les deux cas, l'état de nutrition azoté est souvent limitant, notamment durant l'été.

Actual evapotranspiration (ETR) is one of the major component of the hydrological cycle, but also one of the most difficult to measure. Its quantification requires the knowledge of its components: evaporation from soils, transpiration from plants and evaporation of intercepted water. The study focuses on the partition between evaporation and transpiration and is based on the use of stable water isotopes (oxygen 18 and deuterium). Indeed, the isotopic signatures of water vapour produced by evaporation and transpiration are in general different. In combination with the sampling of the isotopic signature of liquid water in the soil and in the plant, its measurement allows the quantification of the importance of each reservoir. The associated processes are studied within the PIETE (Isotopic Partition Evaporation Transpiration Evapotranspiration) using both modelling and laboratory and field experiments. The methodology used is described in this paper and some preliminary results are presented.; L`évapotranspiration réelle (ETR) constitue l`un des termes essentiels du cycle hydrologique et l'un des plus difficiles à mesurer. Sa quantification requiert la connaissance de ses composantes: évaporation du sol, évaporation directe de la pluie ou de l'irrigation interceptée par le couvert végétal, transpiration des couverts végétaux. L'étude se concentre sur la séparation entre évaporation directe du sol et transpiration des couverts végétaux et s'appuie sur les isotopes stables de l'eau (oxygène 18 et deutérium). En effet, les signatures isotopiques de la vapeur issue de l'évaporation du sol nu et de la transpiration sont en général distinctes, ce qui permet de quantifier l'importance de chaque réservoir, en combinant les mesures dans la vapeur atmosphérique, dans l'eau du sol et des végétaux. C'est ce que se propose d'étudier le projet PIETE (Partition Isotopique Evaporation Transpiration Evapotranspiration) en combinant modélisation mécaniste et expérimentations de laboratoire et de terrain. La méthodologie utilisée est présentée dans cet article ainsi que quelques résultats préliminaires.