### 2.11.2.2. Plant Water Demand[¶](#plant-water-demand "Permalink to this headline")

Plant water demand depends on stomatal conductance, which is described in section [2.9.3](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Photosynthesis/CLM50_Tech_Note_Photosynthesis.html#stomatal-resistance). Here we describe the influence of PHS and the coupling of vegetation water demand and supply. PHS models vegetation water demand as transpiration attenuated by a transpiration loss function based on leaf water potential. Sunlit leaf transpiration is modeled as the maximum sunlit leaf transpiration multiplied by the percent of maximum transpiration as modeled by the sigmoidal loss function. The same follows for shaded leaf transpiration. Maximum stomatal conductance is calculated from the Medlyn model [(Medlyn et al. 2011)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#medlynetal2011) absent water stress and used to calculate the maximum transpiration (see 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)). Water stress is calculated as the ratio of attenuated stomatal conductance to maximum stomatal conductance. Water stress is calculated with distinct values for sunlit and shaded leaves. Vegetation water stress is calculated based on leaf water potential and is used to attenuate photosynthesis (see section [2.9.4](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Photosynthesis/CLM50_Tech_Note_Photosynthesis.html#photosynthesis))

(2.11.19)[¶](#equation-11-201 "Permalink to this equation")\\\[E\_{sun} = E\_{sun,max} \\cdot 2^{-\\left(\\dfrac{\\psi\_{sunleaf}}{p50\_e}\\right)^{c\_k}}\\\]

(2.11.20)[¶](#equation-11-202 "Permalink to this equation")\\\[E\_{shade} = E\_{shade,max} \\cdot 2^{-\\left(\\dfrac{\\psi\_{shadeleaf}}{p50\_e}\\right)^{c\_k}}\\\]

(2.11.21)[¶](#equation-11-203 "Permalink to this equation")\\\[\\beta\_{t,sun} = \\dfrac{g\_{s,sun}}{g\_{s,sun,\\beta\_t=1}}\\\]

(2.11.22)[¶](#equation-11-204 "Permalink to this equation")\\\[\\beta\_{t,shade} = \\dfrac{g\_{s,shade}}{g\_{s,shade,\\beta\_t=1}}\\\]

\\(E\_{sun}\\) = sunlit leaf transpiration (mm/s)

\\(E\_{shade}\\) = shaded leaf transpiration (mm/s)

\\(E\_{sun,max}\\) = sunlit leaf transpiration absent water stress (mm/s)

\\(E\_{shade,max}\\) = shaded leaf transpiration absent water stress (mm/s)

\\(\\psi\_{sunleaf}\\) = sunlit leaf water potential (mmH2O)

\\(\\psi\_{shadeleaf}\\) = shaded leaf water potential (mmH2O)

\\(\\beta\_{t,sun}\\) = sunlit transpiration water stress (-)

\\(\\beta\_{t,shade}\\) = shaded transpiration water stress (-)

\\(g\_{s,sun}\\) = stomatal conductance of water corresponding to \\(E\_{sun}\\)

\\(g\_{s,shade}\\) = stomatal conductance of water corresponding to \\(E\_{shade}\\)

\\(g\_{s,sun,max}\\) = stomatal conductance of water corresponding to \\(E\_{sun,max}\\)

\\(g\_{s,shade,max}\\) = stomatal conductance of water corresponding to \\(E\_{shade,max}\\)

