PLATIN - PLant-ATmosphere INteraction model. Landbauforschung, Special Issue 319, 2008
L Grünhage and H-D Haenel
Appendix J
Estimation of bulk canopy resistance for water vapour from measured data
Eddy covariance measurements of sensible heat H, latent heat λE, and friction velocity u∗ together
with measurements of air temperature ta and relative humidity rH can be used to estimate bulk canopy
resistance for water vapour Rc, H2O according to:
0.622 ⋅ ρ moist air esat ( d + z 0h ) − e( d + z 0h )
⋅
p
(λE ⋅ λ−1 )
Rc, H2O =
with
ρmoist air
p
esat(d+z0h)
e(d+z0h)
λ
(J1)
density of moist air at absolute temperature T [kg⋅m-3]; cf. (F7) and (F8)
air pressure [hPa]
saturation water vapour pressure at z = d + z0h [hPa]; cf. eqs. (F2) and (F3)
actual water vapour pressure at z = d + z0h [hPa]
latent heat of water vaporisation at z = d + z0h [J⋅kg-1]; cf. eq. (28)
Canopy surface temperature Ts needed for e calculations is related to potential canopy surface temperature as described by eq. (25). Actual water vapour pressure e(d+z0h) is given by:
e( d + z 0h ) = e( z ref, T ) +
with
(λE ⋅ λ−1 ) ⋅ p
⋅ (Rah ( d + z 0m , z ref, T ) + Rb, heat )
0.622 ⋅ ρ moist air
e(zref, T)
Rah(d+z0m, zref, T)
Rb, heat
(J2)
actual water vapour pressure at z = zref, T [hPa]; cf. eq. (F4)
atmospheric resistance [s⋅m-1]; cf. eqs. (2) and (3)
quasi-laminar resistance for sensible heat [s⋅m-1]; cf. eq. (7)
Atmospheric resistance Rah(d+z0m, zref, T), quasi-laminar resistance for sensible heat Rb, heat, and
Monin-Obukhov length L (cf. eq. (5)), needed for the resistance estimations, are calculated using
measured friction velocity u∗ and sensible heat flux H.
As described in Chapter 2.3, bulk canopy resistance Rc, H2O is a composite resistance describing
stomatal and cuticular transpiration and evaporation. In PLATIN, Rc, H2O is approximated by a weighted
combination of soil resistance Rsoil, H2O, bulk stomatal resistance Rc, stom, H2O and bulk cuticle resistance
Rc, cut, H2O known for a fully developed canopy (without senescent leaves) under optimum conditions for
maximal transpiration. The weights β* and β depend on the actual canopy development stage taking
into account the transition from a dense canopy to a sparse canopy as given by eq. (8).
Consequently, for a given canopy development stage bulk stomatal resistance Rc, stom, H2O or bulk
stomatal conductance for water vapour gc, stom, H2O can be calculated by:
g c, stom, H2O =
1 − β∗
Rc, stom, H2O
=
1
Rc, H2O
−
1 − β∗
β
−
Rc, cut, H2O
Rsoil, H2O
(J3)
For a dense canopy, which may be assumed when evaporation from soil is below 5 % of total
evapotranspiration, Rc, H2O is often used as a first estimate of Rc, stom, H2O. We found that this assumption
is associated with a mean error of approx. 10 %. Therefore, eq. (J3) represents a useful tool to derive
stomatal resistance directly from measurement-based latent and sensible heat flux, friction velocity,
Monin-Obukhov length and canopy resistance for water vapour. These measurement-based entities
can serve for calibration of PLATIN during daylight hours with global radiation St ≥ 100 W·m−2 if the
subsequent quality criteria are met:
• consistency of measured data set indicated by: esat(d+z0h) − e(d+z0h) >0 hPa
• consistency of measured λE as indicated by positive values during daylight hours
• no rainfall
• interception reservoir empty for current and previous data set (cf. eqs. (17) and (18))
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PLATIN - PLant-ATmosphere INteraction model. Landbauforschung, Special Issue 319, 2008
L Grünhage and H-D Haenel
• relative air humidity rH < 75 %
• integral turbulence characteristic (ITC) test according to Thomas and Foken (2002)
• stationarity tests for friction velocity, latent and sensible heat according to Foken and Wichura
(1996)
Additionally, as PLATIN is based on the canopy energy balance, data sets to be used for model
calibration are required to closely approach the energy balance closure. Therefore, the following criteria
must be satisfied:
• closure of energy balance: ABS(Rnet − G − λE − H) < 25 W⋅m-2
This holds also for nighttime, where only measured sensible heat fluxes can be used for model calibration.
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