Simulating hydrology with an isotopic land surface model in western Siberia: what do we learn from water isotopes?

Abstract. Improvements in the evaluation of land surface models would translate into more reliable predictions of future climate changes, as significant uncertainties persist in the quantification and representation of the relative contributions of soil and vegetation to the water and energy cycles. In this paper, we investigate the usefulness of water stable isotopes in land surface models studying land surface processes. To achieve this, we implemented ¹⁸O and ²H and the computation of the oxygen (δ¹⁸O) and deuterium (δD) stable isotope composition of soil and leaf water pools in a~recent version of the land surface model ORCHIDEE. We performed point-wise simulations with this new model and evaluated its performance on vertical profiles of soil water isotope ratios measured in summer 2012 at four experimental sites located in a boreal region of the Artic zone of western Siberia. The model performed relatively well in simulating some features of the δ¹⁸O soil profiles, but poorly reproduced the d-excess profiles, at all four stations.

The response of the simulated δ¹⁸O profiles to variations in key hydrological parameters revealed the importance of the choice of a correct infiltration pathway in ORCHIDEE. Our results show also that the strength of the evaporative enrichment signal plays a role in shaping the profiles, too and, therefore, the relevance of the vegetation and bare soil characterization. We investigated furthermore to which extent we are able to determine the relative contribution of the evaporation to the evapotranspiration.

This study's results confirm that the use of water stable isotopes measurements helps constrain the representation of key land surface processes in land surface models.