poster

Several studies show that uncertainties in the representation of land surface processes contribute significantly to the spread in projections for the hydrological cycle. Improvements in the evaluation of land surface models would therefore translate into more reliable predictions of future changes. The isotopic composition of water is affected by phase transitions and, for this reason, is a good tracer for the hydrological cycle. Particularly relevant for the assessment of land surface processes is the fact that bare soil evaporation and transpiration bear different isotopic signatures. Water isotopic measurement could thus be employed in the evaluation of the land surface hydrological budget. With this objective, isotopes have been implemented in the land surface model ORCHIDEE on 1.5 vertical layers. The model is currently undergoing further development. In particular, a newly discretized (11 layers) hydrology aims at a more realistic representation of the soil water budget and, consequently, at a simulation of isotopic vertical profiles with higher resolution. In addition, biogeophysical processes specific of the high latitudes, as, for instance, the dynamics of permafrost and of its interaction with snow and vegetation, are being included, towards an ORCHIDEE version able to more realistically simulate the land surface hydrological budget in those regions. Model results have been evaluated against isotopes measurements in soil and stem water at different instrumented sites. The seasonality for present day simulations is reasonably well reproduced. Furthermore, a sensitivity analysis investigates to what extent water isotopic measurements in soil water can help constrain the representation of land surface processes, with a focus on the partitioning between evaporation and transpiration. In turn, improvements in the description of this partitioning may help reduce the uncertainties in the land surface hydrological response to climate change.