Analysis and modelling of the evaporation-transpiration partition under controlled conditions (RUBIC), using stable isotopes of water

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.