|
E. Linacre |
4/'98 |
It is intuitive that a higher net radiation (Rn) onto an irrigated crop implies a higher rate of evaporation (LE) from the crop, as well as a higher energy use by photosynthesis (Fc), i.e. a higher growth rate of the whole plant. However it is not clear how these variables relate, and at what level of Rn a plateau is reached. Fig 1 (1) shows the variation of these variables for a wheat crop. Here Rn has been modified by subtracting the heat absorbed into the ground (G), to yield the amount absorbed by the leaves.
It can be seen that there is remarkable parallelism of the three terms, due to both evaporation and photosynthesis being controlled by the open-ness of the leaves’ stomates. Table 1 indicates that a milligram of plant material results from 250 - 1000 Joules of radiation, and corresponds to the evaporation of 2 orders of magnitude more water. At higher temperatures, more radiation is needed to produce the same amount of plant material, and more evaporation occurs (at least in this case). All the figures would be a few times more for the production of a milligram of wheat grains alone.
It is interesting to consider the dividend E/D, the relationship between evaporation rate and atmospheric moisture content. It is a measure of the resistance limiting the diffusion of water vapour from the crop. Values in Table 1 are about 2 x 10-7 s/m if the radiation intensity exceeds 300 W/m2, when the air’s dryness is the factor limiting evaporation. However, that dryness becomes irrelevant when the radiation intensity is the factor limiting evaporation, in humid and cloudy conditions.
Fig 1. Values of an Australian irrigated wheat crop’s net radiation intake (Rn, W/m2) reduced by the flow of heat into the ground (G, W/m2), as well as photosynthesis (Fc, microgram/m2. s, i.e. units of 3.6 milligram of carbon dioxide uptake per square metre per second) and evaporation rate (LE, W/m2), during a day in October 1984 (1). The two readings for LE (open squares and black circles) result from two different techniques.
Table 1. Data from Fig 1 and similar data obtained from the same crop a month later. The symbols (Rn - G) and LE have units of W/m2, and Fc and E have units of milligrams per square metre per second. The derivation of E (the evaporation rate from the crop) depends on the equivalence of 1 mm/day to 28.1 W/m2, i.e. 1 W/m2 equals 412 microgram/ m2.s. The air’s saturation deficit D is in hectopascals (mb) and the air temperature T in degrees Celsius.
|
October |
Rn - G |
LE |
Fc |
(Rn - G)/Fc * |
E |
E/Fc ** |
D |
E/D # |
T |
|
8.30 am |
350 |
250 |
1.5 |
233 |
103 |
69 |
4.5 |
23 |
10 |
|
12.30 pm |
700 |
520 |
2.35 |
298 |
214 |
91 |
10 |
21 |
15 |
|
4.30 pm |
180 |
200 |
0.7 |
257 |
82 |
117 |
11 |
7.5 |
16 |
|
November |
|
|
|
|
|
|
|
|
|
|
8.30 am |
400 |
300 |
1.25 |
320 |
124 |
99 |
7 |
18 |
17 |
|
12.30 pm |
700 |
600 |
1.65 |
424 |
237 |
150 |
14 |
17 |
21 |
|
4.30 pm |
300 |
220 |
0.3 |
1,000 |
91 |
302 |
22 |
4.1 |
26 |
* units of Joules per milligram of plant growth
** a dimensionless ratio
#
milligrams per square metre per second per hectopascal, i.e. units of 10-8 seconds per metre, since a Pascal has the dimensions of kg/m.s2 (2).
References
(1) Dunin, F.X., W.S. Meyer, S.C. Wong and W. Reyenga 1989. Seasonal change in water use and carbon assimilation of irrigated wheat. Agric. Forest Meteor., 45, 231-50.
(2) Linacre, E.T. 1992. Climate Data & Resources (Routledge) 366pp.