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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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