Canopy Transpiration

14.5 Canopy Transpiration

The equation for canopy transpiration is, again, similar to the one for leaf transpiration. We treat the canopy as a big leaf, so all that is needed is to add soil heat flux to Eq. (14.11) and compute the

and conductances using the appropriate equations. The canopy transpiration equation is:

This is the well known and widely used equation (Monteith, 1965) for estimating evapotranspiration from plant commu- nities. As we have presented it here, it appears just to provide canopy transpiration estimates at a particular instant, but it is now commonly

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) any plant community, whatever its canopy conductance, if the canopy conductance is known. The canopy conductance, however, gen- erally 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

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 re- quirements are substantial (radiation, temperature, wind, vapor pressure, canopy conductance, and canopy height), but one usually obtains bet- ter results using the

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 compu- tation 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 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 esti- mates with and without the stability parameters have shown no advantage to using them in the calculation). Allen et al. (1994) recommend using

= = when wind is measured at a height of 2 m. The vapor conductance is therefore

0.6 x

and the apparent psychrometer constant is

Only the convective conductance (rather than 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

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.

Simple Assimilation Models

Photosynthesis

The assimilation of carbon by leaves follows the general reaction