A test of an optimal stomatal conductance scheme within the CABLE Land Surface Model
Geoscientific Model Development Discussions
Stomatal conductance (<i>g</i><sub>s</sub>) affects the fluxes of carbon, energy and water between the vegetated land surface and the atmosphere. We test an implementation of an optimal stomatal conductance model within the Community Atmosphere Biosphere Land Exchange (CABLE) land surface model (LSM). In common with many LSMs, CABLE does not differentiate between <i>g</i><sub>s</sub> model parameters in relation to plant functional type (PFT), but instead only in relation to photosynthetic
... photosynthetic pathway. We therefore constrained the key model parameter "<i>g</i><sub>1</sub>" which represents a plants water use strategy by PFT based on a global synthesis of stomatal behaviour. As proof of concept, we also demonstrate that the <i>g</i><sub>1</sub> parameter can be estimated using two long-term average (1960–1990) bioclimatic variables: (i) temperature and (ii) an indirect estimate of annual plant water availability. The new stomatal models in conjunction with PFT parameterisations resulted in a large reduction in annual fluxes of transpiration (~ 30% compared to the standard CABLE simulations) across evergreen needleleaf, tundra and C4 grass regions. Differences in other regions of the globe were typically small. Model performance when compared to upscaled data products was not degraded, though the new stomatal conductance scheme did not noticeably change existing model-data biases. We conclude that optimisation theory can yield a simple and tractable approach to predicting stomatal conductance in LSMs.