Uncertainty Analysis of a Coupled Hydrological-plant Growth Model for Grassland under Elevated CO2
The continuous increase in atmospheric carbon dioxide (CO 2 ) contributes to changes in plant evapotranspiration and terrestrial water budgets in two ways. Firstly, elevated CO 2 can result in a water saving effect, since increasing CO 2 reduces stomatal opening and therefore decreases transpiration. Secondly, CO 2 fertilization increases biomass accumulation and leaf area at plant canopy level, likely increasing transpiration 1 . Vegetation and hydrological models can be used to investigate the CO 2 response and the bidirectional effects outlined above, including their relative contribution to the changes in the water cycle. However, the intrinsic plant-soil interaction and the uncertainty related to model parameterization have rarely been considered. Hence, we coupled a detailed plant growth and soil hydrological model by using the generic model frameworks Plant growth Modelling Framework (PMF) 2 and Catchment Modelling Framework (CMF) 3 . Up to date response mechanisms have been implemented in PMF to simulate the various ways of how plant physiology is influenced by elevated CO 2 . Both models interact by using the Python computer language. Applying the coupled PMF-CMF model we investigate the effects of elevated CO 2 in a number of plant physiological and environmental variables such as biomass, leaf area index and soil moisture using field data of a long-term Free Air Carbon Enrichment (FACE) experiment in Giessen, Germany. In this experiment, various grassland varieties (herbs, legumes, grass) grow under elevated (+20%) and ambient CO 2 since 1997. A Monte Carlo based uncertainty analysis (GLUE) is conducted to investigate the coupled PMF-CMF parameter space. The focus will be on the identification of parameters for plant and soil, which are the drivers for the CO 2 response of the terrestrial water balance. We will present first results of the simulation of biomass accumulation and transpiration under ambient and elevated CO 2 concentrations.