The vertical distribution of transpiration and of the eddy transfer coefficient within the plant communities can be estimated through the principle of energy balance. Now, we suppose that the plant communities are formed of some layers, as shown in Fig, 1. Energy balance of the upper layer is represented by the equation
R
12=l·(E
1-E
2)+(F
1-F
2)+M·C·dθ/dt+l·dx/dt≅(l·E
1+F
1)-(l·E
2+F
2)=(l·E
1+F
1)-K
2{l·(dχ/dz)
2+C
p·ρ·(dθ/dz)
2}
where
R12 is radiation absorbed by the layer, which is measurable, and
l is latent heat of evaporation,
E1 watervapor flux at the surface level, 1,
F1 sensible heat flux at the surface level,
K2 eddy transfer coefficient at the level, 2, (
dχ/dz)
2 gradient of absolute humidity at the level, 2, (
dθ/dz)
2 gradient of temperature at the level, 2. Then, from the equation,
K2 is computed by uses of observed (
dχ/dz)
2 and (
dθ/dz)
2 and
l·E1 and
F1, which is estimated by energy balance method. The transpiration from the layer, (
E1-E2), are estimated.
If
R23, radiation absorbed by the middle layer, (
dχ/dz)
3 and (
dθ/dz)
3 are used, then,
K3,
E3 and
F3 are calculated in the same manner as above. So the transpiration from the layer, (
E2-E3), are also estimated.
The calculated values of the transpiration and of the eddy transfer coefficient are shown in Fig. 6 and 7, using the data obtained from the micrometeorological obserbation in and above the wheat field on May 25, 1961. Owing to insufficiency of the measurement of temperature profile and radiation within the plant layers, the plotted points are scattered, as seen from these figures. However, energy balance approach will be available for estimation of the vertical distribution of the transpiration and of the eddy transfer coefficient within the plant communities like crops or woods.
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