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The Ball–Berry stomatal conductance model is commonlyused in earth system models to simulate biotic regulation ofevapotranspiration. However, the dependence of stomatal conductance(gs) on vapor pressure deficit (Ds) and soil moisture must beempirically parameterized. We evaluated the Ball–Berry model used in theCommunity Land Model version 4.5 (CLM4.5) and an alternative stomatalconductance model that links leaf gas exchange, plant hydraulic constraints,and the soil–plant–atmosphere continuum (SPA). The SPA model simulatesstomatal conductance numerically by (1) optimizing photosynthetic carbon gainper unit water loss while (2) constraining stomatal opening to prevent leafwater potential from dropping below a critical minimum. We evaluated twooptimization algorithms: intrinsic water-use efficiency (ΔAn/Δgs, the marginal carbon gain of stomatal opening) andwater-use efficiency (ΔAn /ΔEl, themarginal carbon gain of transpiration water loss). We implemented thestomatal models in a multi-layer plant canopy model to resolve profiles ofgas exchange, leaf water potential, and plant hydraulics within the canopy,and evaluated the simulations using leaf analyses, eddy covariance fluxes atsix forest sites, and parameter sensitivity analyses. The primary differencesamong stomatal models relate to soil moisture stress and vapor pressuredeficit responses. Without soil moisture stress, the performance of the SPAstomatal model was comparable to or slightly better than the CLM Ball–Berrymodel in flux tower simulations, but was significantly better than the CLMBall–Berry model when there was soil moisture stress. Functional dependenceof gs on soil moisture emerged from water flow along thesoil-to-leaf pathway rather than being imposed a priori, as in the CLMBall–Berry model. Similar functional dependence of gs onDs emerged from the ΔAn/ΔEloptimization, but not the ΔAn /gsoptimization. Two parameters (stomatal efficiency and root hydraulicconductivity) minimized errors with the SPA stomatal model. The criticalstomatal efficiency for optimization (ι) gave results consistent withrelationships between maximum An and gs seen in leaftrait data sets and is related to the slope (g1) of the Ball–Berrymodel. Root hydraulic conductivity (Rr*) was consistentwith estimates from literature surveys. The two central concepts embodied inthe SPA stomatal model, that plants account for both water-use efficiency andfor hydraulic safety in regulating stomatal conductance, imply a notion ofoptimal plant strategies and provide testable model hypotheses, rather thanempirical descriptions of plant behavior

Modeling stomatal conductance in the earth system: linking leaf water-use efficiency and water transport along the soil–plant–atmosphere continuum