Implementation of a Pore Scale Resistance Parameterization for Soil Evaporation in a Land Surface Model

The land surface component of weather and climate models is responsible for calculating the energy, water, and carbon balances at the land surface. The model must also partition the available turbulent energy into heat and water fluxes, with the water fluxes further partitioned into evaporation from the canopy, the soil surface, and transpiration. Previous studies in coupled models have demonstrated that climatology can be impacted by how the surface water fluxes are partitioned. The Community Atmosphere Biosphere Land Exchange (CABLE) model has been shown to have a large positive bias in spring time evaporation due to structural model uncertainty and not parameter sensitivity. Despite the importance of correctly partitioning the surface water fluxes based on available knowledge of the underlying physics most land surface models use empirically based methods for limiting soil evaporation when soil moisture is low. These empirical methods have not resulted in adequate simulation of total evaporation even in offline simulations. Several land surface modeling groups have added additional resistance terms in efforts to limit soil evaporation under soil moisture stress. Here we present the results from incorporating more physically based soil evaporation mechanisms into the under-canopy turbulent transfer parameterizations in CABLE. The model is tested at 20 flux tower sites and compared to the observed latent and sensible heat fluxes. The pore scale resistance model greatly reduces the positive bias in spring time evaporation. The new formulation is demonstrably superior to several previously used empirical formulas for limiting soil evaporation under soil moisture stress.