## 2.9.3. Stomatal resistance[¶](#stomatal-resistance "Permalink to this headline")
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CLM5 calculates stomatal conductance using the Medlyn stomatal conductance model ([Medlyn et al. 2011](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#medlynetal2011)). Previous versions of CLM calculated leaf stomatal resistance using the Ball-Berry conductance model as described by [Collatz et al. (1991)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#collatzetal1991) and implemented in global climate models ([Sellers et al. 1996](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#sellersetal1996)). The Medlyn model calculates stomatal conductance (i.e., the inverse of resistance) based on net leaf photosynthesis, the leaf-to-air vapor pressure difference, and the CO2 concentration at the leaf surface. Leaf stomatal resistance is:

(2.9.1)[¶](#equation-9-1 "Permalink to this equation")\\\[\\frac{1}{r\_{s} } =g\_{s} = g\_{o} + 1.6(1 + \\frac{g\_{1} }{\\sqrt{D\_{s}}}) \\frac{A\_{n} }{{c\_{s} \\mathord{\\left/ {\\vphantom {c\_{s} P\_{atm} }} \\right.} P\_{atm} } }\\\]

where \\(r\_{s}\\) is leaf stomatal resistance (s m2 \\(\\mu\\)mol\-1), \\(g\_{o}\\) is the minimum stomatal conductance (\\(\\mu\\) mol m \-2 s\-1), \\(A\_{n}\\) is leaf net photosynthesis (\\(\\mu\\)mol CO2 m\-2 s\-1), \\(c\_{s}\\) is the CO2 partial pressure at the leaf surface (Pa), \\(P\_{atm}\\) is the atmospheric pressure (Pa), and \\(D\_{s}=(e\_{i}-e{\_s})/1000\\) is the leaf-to-air vapor pressure difference at the leaf surface (kPa) where \\(e\_{i}\\) is the saturation vapor pressure (Pa) evaluated at the leaf temperature \\(T\_{v}\\), and \\(e\_{s}\\) is the vapor pressure at the leaf surface (Pa). \\(g\_{1}\\) is a plant functional type dependent parameter ([Table 2.9.1](#table-plant-functional-type-pft-stomatal-conductance-parameters)) and are the same as those used in the CABLE model ([de Kauwe et al. 2015](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#dekauwe2015)).

The value for \\(g\_{o}=100\\) \\(\\mu\\) mol m \-2 s\-1 for C3 and C4 plants. Photosynthesis is calculated for sunlit (\\(A^{sun}\\)) and shaded (\\(A^{sha}\\)) leaves to give \\(r\_{s}^{sun}\\) and \\(r\_{s}^{sha}\\). Additionally, soil water influences stomatal resistance through plant hydraulic stress, detailed in the [Plant Hydraulics](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Plant_Hydraulics/CLM50_Tech_Note_Plant_Hydraulics.html#rst-plant-hydraulics) chapter.

Resistance is converted from units of s m2 \\(\\mu\\) mol\-1 to s m\-1 as: 1 s m\-1 = \\(1\\times 10^{-9} R\_{gas} \\frac{\\theta \_{atm} }{P\_{atm} }\\) \\(\\mu\\) mol\-1 m2 s, where \\(R\_{gas}\\) is the universal gas constant (J K\-1 kmol\-1) ([Table 2.2.7](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Ecosystem/CLM50_Tech_Note_Ecosystem.html#table-physical-constants)) and \\(\\theta \_{atm}\\) is the atmospheric potential temperature (K).

Table 2.9.1 Plant functional type (PFT) stomatal conductance parameters.[¶](#id4 "Permalink to this table")  
| PFT
 | g1

 |
| --- | --- |
| NET Temperate

 | 2.35

 |
| NET Boreal

 | 2.35

 |
| NDT Boreal

 | 2.35

 |
| BET Tropical

 | 4.12

 |
| BET temperate

 | 4.12

 |
| BDT tropical

 | 4.45

 |
| BDT temperate

 | 4.45

 |
| BDT boreal

 | 4.45

 |
| BES temperate

 | 4.70

 |
| BDS temperate

 | 4.70

 |
| BDS boreal

 | 4.70

 |
| C3 arctic grass

 | 2.22

 |
| C3 grass

 | 5.25

 |
| C4 grass

 | 1.62

 |
| Temperate Corn

 | 1.79

 |
| Spring Wheat

 | 5.79

 |
| Temperate Soybean

 | 5.79

 |
| Cotton

 | 5.79

 |
| Rice

 | 5.79

 |
| Sugarcane

 | 1.79

 |
| Tropical Corn

 | 1.79

 |
| Tropical Soybean

 | 5.79

 |
| Miscanthus

 | 1.79

 |
| Switchgrass

 | 1.79

 |

