## 2.9.2. Introduction[¶](#introduction "Permalink to this headline")
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Leaf stomatal resistance, which is needed for the water vapor flux (Chapter [2.5](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Fluxes/CLM50_Tech_Note_Fluxes.html#rst-momentum-sensible-heat-and-latent-heat-fluxes)), is coupled to leaf photosynthesis similar to Collatz et al. ([1991](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#collatzetal1991), [1992](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#collatzetal1992)). These equations are solved separately for sunlit and shaded leaves using average absorbed photosynthetically active radiation for sunlit and shaded leaves \[\\(\\phi ^{sun}\\),\\(\\phi ^{sha}\\) W m\-2 (section [2.4.1](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Radiative_Fluxes/CLM50_Tech_Note_Radiative_Fluxes.html#solar-fluxes))\] to give sunlit and shaded stomatal resistance (\\(r\_{s}^{sun}\\),\\(r\_{s}^{sha}\\) s m\-1) and photosynthesis (\\(A^{sun}\\),\\(A^{sha}\\) µmol CO2 m\-2 s\-1). Canopy photosynthesis is \\(A^{sun} L^{sun} +A^{sha} L^{sha}\\), where \\(L^{sun}\\) and \\(L^{sha}\\) are the sunlit and shaded leaf area indices (section [2.4.1](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Radiative_Fluxes/CLM50_Tech_Note_Radiative_Fluxes.html#solar-fluxes)). Canopy conductance is \\(\\frac{1}{r\_{b} +r\_{s}^{sun} } L^{sun} +\\frac{1}{r\_{b} +r\_{s}^{sha} } L^{sha}\\), where \\(r\_{b}\\) is the leaf boundary layer resistance (section [2.5.3](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Fluxes/CLM50_Tech_Note_Fluxes.html#sensible-and-latent-heat-fluxes-and-temperature-for-vegetated-surfaces)).

