4.0 KiB
2.9.3. Stomatal resistance¶
CLM5 calculates stomatal conductance using the Medlyn stomatal conductance model (Medlyn et al. 2011). Previous versions of CLM calculated leaf stomatal resistance using the Ball-Berry conductance model as described by Collatz et al. (1991) and implemented in global climate models (Sellers et al. 1996). 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)¶\[\frac{1}{r_{s} } =g_{s} = g_{o} + 1.6(1 + \frac{g_{1} }{\sqrt{D_{s}}}) \frac{A_{n} }{{c_{s} \mathord{\left/ {\vphantom {c_{s} P_{atm} }} \right.} P_{atm} } }\]
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) and are the same as those used in the CABLE model (de Kauwe et al. 2015).
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 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) and \(\theta _{atm}\) is the atmospheric potential temperature (K).
Table 2.9.1 Plant functional type (PFT) stomatal conductance parameters.¶
| 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
|