Leaf Energy Balance

Subroutine Lfebal (culfbal.f)

This subroutine calculates the canopy leaf energy budget. It returns:

• TEMPLF(I,J) An estimate of the temp of leaves in angle class I and layer J based on energy budget considerations.

Leaf Temperature Calculations

• A mean air temperature, TMEAN, for the canopy layers is calculated as the arithmetic mean between adjacent layers.

• A first approximation for the leaf temperature in a layer and angle class, TEMP,is set at to the arithmetic mean between TMEAN (the air temp in that layer) and the leaf temperature of the previous time step. [240]

• Saturated vapor pressures (VP's) are calculated for air and leaf temperatures.

• The slope of the saturation-VP function is evaluated at TEMP (see Brutsaert 1982 Eq 3.21). [246]
  • This is used to determine the air->leaf temperature gradient.

• Layers that had evapotranspiration in the last time step are set to ISTATE=0 and IFLGRS=1. Layers that had condensation (CONTOT < 0 !) are set to ISTATE=1 and IFLGRS=0.

• Scale leaf stomatal resistance (RSLEAF) according to fraction of live leaves. Dead leaves offer no resistance.

• HH is the total conductance for evaporating water vapor from the leaves and consists of either: [272]
  • a boundary layer component only (for wet leaves: IFLGRS=0) or
  • a boundary layer resistance and a stomatal resistance (dry leaves: IFLGRS=1).

• The temperature gradient between the leaves and the air, DELT, is calculated on the basis of the energy balance and the new estimates for TEMPLF and its saturated VP (ESTLF) are calculated. [280]

• Check for condensation:
  • If there is condensation (ESTLF > EAIR=EMEAN) on the:
    • 1st iteration through the energy balance equation, ICOND is set to 1 (flag for condensation) and IFLGRS is set to 0. The leaf temperature calculations are then reiterated ignoring the stomatal resistance (return to here).
    • 2nd iteration, leave the leaf temperature where it is.
  • If there is no condensation (ESTLF < EMEAN) on the:
    • 1st iteration, continue on.
    • 2nd iteration, calculate leaf temps one more time (go here, this time reintroducing the stomatal resistance (IFLGRS = 1).

• Calculate:
  • RELH(J) the relative air humidity for the canopy layer
  • GEVAP(I,J) the evapotranspiration rate per unit ground area
  • GHEAT(I,J) the sensible heat flux per unit ground area

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