A test of an optimal stomatal conductance scheme within the CABLE Land Surface Model

Abstract

Abstract. Stomatal conductance (gs) 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 gs model parameters in relation to plant functional type (PFT), but instead only in relation to photosynthetic pathway. We therefore constrained the key model parameter "g1" 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 g1 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.

A new version of the CABLE land surface model (Subversion revision r4546), incorporating land use and land cover change, woody vegetation demography and a novel optimisation-based approach to plant coordination of electron transport and carboxylation capacity-limited photosynthesis

“…CABLE agrees very well 20 with the LandFlux latitudinal profile, except for an underestimate in the tropics of up to 0.4 mm d -1 (although note LandFlux 1σ uncertainty of ~1 mm d -1 in this region), an underestimate that has been noted in previous evaluations of CABLE global ET (De Kauwe et al, 2015;Decker, 2015) and is particularly noticeable in the Amazon.…”

Optimal stomatal behavior with competition for water and risk of hydraulic impairment

“…1, when applied to biophysical models of photosynthesis and transpiration, closely matches the functional forms of empirical stomatal models, which themselves have extraordinary empirical support (2,5,7,27,34). For this reason, the WUEH is widely used to guard against out-of-sample behavior in the novel climates created in ecosystem models (35)(36)(37)(38)(39). Finally, several notable efforts have been made to extend WUEH to situations including soil moisture stress (11,12,28,33,(40)(41)(42) and competition (43) and to develop a theory of the value of λ over the short time scales during which it should be approximately constant and how λ should change over weeks, months, or seasons (9,11,12,28,32).…”

“…In particular, knowledge of the large costs associated with low xylem potential, which was obtained after 1977, might have obviated the need for the assumption of a fixed water supply in CF77 (6). On the other hand, WUEH is widely applied where such costs are thought to be high, as in ecosystem models under climate change (35)(36)(37)(38)(39). We include all combinations of i-iii simply to explore the impact of each component on the predicted stomatal behavior and in recognition of the fact that Eq.…”

Improved representations of coupled soil–canopy processes in the CABLE land surface model (Subversion revision 3432)

“…Improvements were demonstrated at five European FLUXNET sites, with model performance dependent on a site-specific drought tolerance parameter. Modification to the vapourpressure deficit response of stomatal conductance in CABLE (De Kauwe et al, 2015b;Kala et al, 2015Kala et al, , 2016 has also been featured in recent studies, but it is evident that deficiencies in the predictions of seasonal cycles of evaporation are not resolved by this modification (De Kauwe et al, 2015b;Fig. 3).…”