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Garden with Insight v1.0 Help: Plant params transpiration group


Vapor pressure deficit (VPD) threshold: The vapor pressure deficit (amount of water the air can hold minus the amount it is holding) at which water will start to slow down its evaporation from the leaves of this plant. Normal values range from 0.5 to 1.0 kiloPascals.

VPTH - Threshold VPD (SPA) (F=1.) 

In EPIC, leaf conductance is insensitive to VPD until VPD (calculated hourly) exceeds the threshold value, VPTH (usually 0.5 to 1.0 kPa).

Leaf stomatal conductance maximum: The fastest rate at which water vapor can evaporate from the leaves of this plant during high solar radiation and low water vapor pressure. Sets a physical upper limit to transpiration due to the structure of its leaves and stomata. Ranges from 0 to 5.
GSI - Maximum Stomatal Conductance 

The crop parameter GSI is the maximum stomatal conductance (ms-1) at high solar radiation and low vapor pressure deficit. Korner et. al (1979) reported maximum stomatal conductance values for 246 species and cultivars. (Korner, C. H., J. A. Scheel, and H. Bauer. 1979. Maximum leaf diffusive conductance in vascular plants. Photosynthetica 13(1):45-82.)

VPD at which leaf conductance is 75% of maximum: When the vapor pressure deficit falls below a parameter (threshold VPD), water will start to slow down its evaporation from the leaves of this plant. This is the VPD at which leaf conductance slows to 75% of the maximum rate. A value around 4 is normal.
VPD2 - VPD value (KPA) / F2 1 

In EPIC, leaf conductance declines linearly as VPD increases above VPTH. VPD2 is a double parameter in which the number on the left of the decimal is some value of VPD above VPTH (e.g. 4.0), and the number of the right of the decimal is the corresponding fraction of the maximum leaf conductance at the value of VPD (e.g., 0.7). [Note: we fixed the X value at 0.75.]

Canopy resistance for transpiration: Parameter used to modify the resistance of the plant canopy to water transfer during plant transpiration (used in the Penman-Monteith equation). Ranges from 1.0 to 2.0; higher means more resistance to transpiration.

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