iCHASM, a flexible land-surface model that incorporates stable water isotopes

Isotope-enabled Global Climate Models (GCMs) have been previously used to investigate water isotope fluxes in present and past climate systems. Water isotope modelling is also important in modern ecosystem studies, for investigating the source and transport of moisture and carbon dioxide fluxes. However, in this modelling, there has only been limited investigation of the effect of parameterisation complexity on stable water isotope partitioning. The present study has two aims. The first is to incorporate a stable water isotope parameterisation into a flexible land surface scheme (LSS): the CHAmeleon Surface Model (CHASM). This scheme offers five different modes, each containing up to four water reservoirs (canopy interception, snowpack, root zone and bare ground storage), and each mode offers the opportunity to modify resistances to evaporative fluxes from these reservoirs. These different modes allow the effect of model complexity on different parameterisations to be explored within a common modelling framework. Two modelling experiments are reported which use the iPILPS Phase 1 forcing: the ‘control’ experiment and one using a simpler mode of CHASM comprising a bucket hydrology plus a fixed stomatal resistance. The additional set of experiments using iCHASM's variable modes shows, contrary to an earlier experiments with CHASM, that a land surface model with only a bucket hydrology scheme and constant surface resistance cannot reproduce the behaviour of an LSS that has additional functionality (such as bare ground evaporation, and canopy interception and aerodynamic, surface and stomatal resistances).