Riyadh Al-Naseri: Parametrizing stomatal conductance for Australian vegetation in JULES land surface model
Riyadh Al-Naseri1, Clare Stephens1,2, Siyuan Tian3, Laura Williams1, Valentina Marchionni3 and Belinda Medlyn1
1Hawkesburry Institute for the Environment, Western Sydney University, Sydney, Australia; 2Univeristy of New South Wales, Sydney, Australia; 3Bureau of Meteorology, Canberra, Australia.
Stomatal conductance plays a key role in the exchange of carbon and water between vegetation and the atmosphere, by controlling photosynthesis and transpiration in plants. However, its representation in land surface models (LSMs) is still considered a major source of uncertainty. The Medlyn model is widely used in LSMs to describe how strongly stomata and the model parameter g1 respond to carbon uptake and atmospheric dryness. This parameter is often introduced as a fixed value in global LSMs for broad vegetation types despite evidence that stomatal behaviour varies with plant water availability, particularly in water limited ecosystems such as those found in large parts of Australia.
We estimated g1 using observational data obtained from three intrinsic water use efficiency techniques: leaf gas exchange, stable carbon isotope discrimination, and eddy covariance. These approaches together can provide a comprehensive knowledge on the estimation of g1 across a range of spatial and temporal scales, from leaf to ecosystem and from short to long term responses. To account for water stress, we relate g1 resulted from these three methods to leaf predawn water potential or soil water potential. We found that g1 varied systematically across Australian plant functional types (PFTs) and these differences among PFTs were consistent across methods.
Simultaneously, we modified how drought stress was represented in JULES by replacing soil moisture by predawn water potential. We will be running the model for a group of Australian flux sites with the new parametrization and evaluate model outputs against flux site observations.
Audience: Land surface modellers, vegetation and carbon cycle researchers and anyone who is interested in improving the representation of Australian vegetation in ACCESS-ESM and other coupled models.
Keywords: Stomatal conductance, g1, PFT, Land surface models, water stress, eddy covariance, stable isotopes, JULES.