Plants and Plant Communities
used to estimate transpiration over days, weeks, or months. It is better to
supply at least daily radiation, temperature, wind, and vapor pressure data
to compute daily transpiration, and then sum these for weekly or longer
transpiration estimates, rather than to average radiation, wind, etc. over a
longer period to compute weekly or monthly transpiration.
Equation (14.12) can be used to compute the evapotranspiration (ET)
from any plant community, whatever its canopy conductance, if the
canopy conductance is known. The canopy conductance, however, generally is not known except when it is at a maximum. This condition of
maximum conductance is important, though, since it sets an upper limit
to the rate of water use, and is the rate of water use by a plant community
which is not water stressed. The water requirements of agricultural crops
are often near the maximum ET rate because the crops are managed to
avoid water stress. In recent years the Penman-Monteith equation has
found increasing use for estimating crop evapotranspiration. The data requirements are substantial (radiation, temperature, wind, vapor pressure,
canopy conductance, and canopy height), but one usually obtains better results using the PenmawMonteith equation and estimating missing
data, than by using a simpler equation that does not use such a mechanistic
approach.
A particular use of the Penman-Monteith equation is for the computation of reference ET. Reference ET is the ET from a 12 cm high grass
crop that completely covers the ground and is not short of water (Allen
et al., 1994). The canopy conductance of the grass crop is assumed to
be 0.6 mol m-2 s-'. Equation (14.9) is used to compute boundary layer
conductance. For a fixed measurement height, the conductance is just a
constant multiplied by the wind speed (a number of comparisons of estimates with and without the stability parameters have shown no advantage
to using them in the calculation). Allen et al. (1994) recommend using
g~~ = gva = 0 . 2 ~ when wind is measured at a height of 2 m. The vapor
conductance is therefore
0.6 x 0.224
gv = 0.6 + 0 . 2 ~
mol m-2 s-' ,
and the apparent psychrometer constant is
Only the convective conductance (rather than gHr) is used in computing
y*. It is not clear why, but the estimated reference ET comes closer
to the measured ET without including the radiative conductance (Allen
et al., 1994). With these substitutions, computation of reference ET is
straightforward. The ET of tall crops, or crops that do not completely
cover the ground, is determined by multiplying the reference ET by an
empirically determined crop coefficient.
used to estimate transpiration over days, weeks, or months. It is better to
supply at least daily radiation, temperature, wind, and vapor pressure data
to compute daily transpiration, and then sum these for weekly or longer
transpiration estimates, rather than to average radiation, wind, etc. over a
longer period to compute weekly or monthly transpiration.
Equation (14.12) can be used to compute the evapotranspiration (ET)
from any plant community, whatever its canopy conductance, if the
canopy conductance is known. The canopy conductance, however, generally is not known except when it is at a maximum. This condition of
maximum conductance is important, though, since it sets an upper limit
to the rate of water use, and is the rate of water use by a plant community
which is not water stressed. The water requirements of agricultural crops
are often near the maximum ET rate because the crops are managed to
avoid water stress. In recent years the Penman-Monteith equation has
found increasing use for estimating crop evapotranspiration. The data requirements are substantial (radiation, temperature, wind, vapor pressure,
canopy conductance, and canopy height), but one usually obtains better results using the PenmawMonteith equation and estimating missing
data, than by using a simpler equation that does not use such a mechanistic
approach.
A particular use of the Penman-Monteith equation is for the computation of reference ET. Reference ET is the ET from a 12 cm high grass
crop that completely covers the ground and is not short of water (Allen
et al., 1994). The canopy conductance of the grass crop is assumed to
be 0.6 mol m-2 s-'. Equation (14.9) is used to compute boundary layer
conductance. For a fixed measurement height, the conductance is just a
constant multiplied by the wind speed (a number of comparisons of estimates with and without the stability parameters have shown no advantage
to using them in the calculation). Allen et al. (1994) recommend using
g~~ = gva = 0 . 2 ~ when wind is measured at a height of 2 m. The vapor
conductance is therefore
0.6 x 0.224
gv = 0.6 + 0 . 2 ~
mol m-2 s-' ,
and the apparent psychrometer constant is
Only the convective conductance (rather than gHr) is used in computing
y*. It is not clear why, but the estimated reference ET comes closer
to the measured ET without including the radiative conductance (Allen
et al., 1994). With these substitutions, computation of reference ET is
straightforward. The ET of tall crops, or crops that do not completely
cover the ground, is determined by multiplying the reference ET by an
empirically determined crop coefficient.