A. Anandacoomaraswamy et al. Agricultural and Forest Meteorology 103 2000 375–386 379
Table 1 Meteorological conditions during the experimental period from 1 January to 13 February 1999
a
Day of Mean solar irradiance
Mean maximum Mean minimum
Mean vapour pressure the year
MJ m
− 2
per day temperature
◦
C temperature
◦
C deficit kPa
0–7 22.26
23.8 9.9
0.52 8–14
20.51 24.7
12.0 0.62
15–21 23.63
24.4 9.7
0.81 22–28
24.11 24.5
7.7 0.88
29–35 22.91
22.9 10.3
0.82 36–42
21.37 21.4
8.6 0.85
a
Means of temperatures and vapour pressure deficits were measured at 1.2 m above soil surface and are 24 h mean values. No rainfall occurred during the experimental period.
3. Results
3.1. Meteorological conditions during the experimental period
The experiment was done during a rain-free pe- riod. There were clear skies and high irradiance levels
Table 1. Maximum and minimum temperatures were typical for this agroclimatic zone. The 24 h mean sat-
uration deficit did not exceed 1 kPa throughout the experimental period.
3.2. Variation of transpiration with soil water content The simultaneous variation of transpiration rate
and top soil 0–15 cm depth water content with day of the year during the period between 1 January and
14 February 1997 is shown in Fig. 1. The soil water content S decreased from the field capacity water
content of 44 at Day 1 to 15 at Day 23. The transpiration rate also showed a similar pattern with
1.59 l per plant per day near field capacity to 0.7 l per plant per day at 15 which was below the permanent
wilting point. After Day 23, S increased up to 24 due to irrigation. Transpiration rate also increased up
to 1.1 l per plant per day Fig. 1.
Fig. 2 shows the variation of transpiration rate with S. The transpiration rate declined only slightly when S
decreased from field capacity 44 to 33. However, the transpiration rate showed a rapid decline between
S of 33 and 15.
The diurnal variation of transpiration for two plants on Day 1 S near field capacity and on Day 28 S
near permanent wilting point is shown in Fig. 3. The transpiration rates were much less on Day 28 as
compared to Day 1. When S was near field capac- ity, maximum transpiration rates of 0.53–0.93 l per
plant per day occurred between 1000 and 1500 h. The corresponding maxima when the S was near perma-
nent wilting point were 0.27–0.53 l per plant per day. Although irradiance and saturation vapour pressure
deficit D were higher on Day 28, the transpiration rates were lower than on Day 1. This indicates the
important role of S in controlling transpiration rate in rainfed conditions. Fig. 3 also shows that transpira-
tion rates vary appreciably from plant to plant even within the same plot. This could be due to differences
in both canopy area and rooting patterns. Although plants with approximately similar stem bases were
Fig. 1. Variation of transpiration rate triangles of mature clonal tea and soil water content circles during the experimental period.
380 A. Anandacoomaraswamy et al. Agricultural and Forest Meteorology 103 2000 375–386
Fig. 2. Relationship between soil water content and transpiration rate of mature clonal tea. The curve is drawn by hand.
selected for measurement, the number of branches, the canopy foliage area and the depth and spread of
the root system could be different for different plants. A plant with a greater canopy area could intercept a
greater amount of solar irradiance and consequently could have a greater transpiration rate than a plant
with a similar stem base, but a smaller canopy size. Likewise, a plant with a more extensive root system
may have a greater transpiration rate because of its greater rate of water extraction from the soil.
3.3. Variation of transpiration with shading Fig. 4 shows the relationship between transpiration
rate and irradiance as a percentage of the unshaded control. Transpiration decreased linearly with decreas-
ing irradiance throughout the range of radiation lev- els tested at a rate of 0.031 l per plant per day per
reduction in solar irradiance.
The diurnal variation of transpiration for the un- shaded control and 85 shade treatment i.e. 15 irra-
diance is shown in Fig. 5. It shows that the reduction of transpiration due to shading occurred during the
period around mid-day between 1000 and 1500 h. The sudden drop in transpiration rate and its subsequent
increase around 1500 h in the unshaded plant could probably be due to the shadow of a nearby Grevillea
tree. Because of the diurnal variation of solar angle, even tea plants in the open i.e. not under the canopy of
Grevillea trees could experience transient shading. Fig. 5 shows similar transpiration rates in both the
shaded and unshaded treatments from 1500 h on- wards. This could probably be due to sunlight coming
under the shade at lower sun angles and increasing the transpiration rate of the shaded treatment. It is
also possible that D was lower under the shade than in the open. Although, this may have increased stomatal
conductance in the afternoon, the lower canopy-air vapour pressure gradient could negate any significant
increase in transpiration rate due to greater stomatal conductance.
The effect of natural shade provided by a 12-year-old G. robusta on transpiration of tea is shown
in Fig. 6 as a comparison with the effects of artificial shade. The daily transpiration rates of the two tea
plants 1.07 and 0.42 l per plant per day under the Grevillea tree were considerably lower than the value
of tea plants in the open, 3.511 l per plant per day Fig. 4. The variation between the two plants in their
transpiration rates was probably due to the varying levels of shade provided by the Grevillea canopy as
determined by the respective location of each plant in relation to the canopy overhead. The transpiration
rates of the two plants corresponded to shading levels of around 75 and 85 in Fig. 4 i.e. 25–15 irradi-
ance. Grevillea had a daily transpiration of 2.2 l per plant per day. Except during early morning and late
evening, the diurnal variation pattern of transpiration rates were similar for tea and Grevillea.
3.4. Effects of change in canopy reflectance on transpiration
Fig. 7 shows the effects of spraying Kaolin on the diurnal variation of canopy temperature and transpi-
ration rate. Kaolin decreased the canopy temperature by 2–4
◦
C especially around mid-day Fig. 7a. This is probably because of the increased reflectance of
Kaolin-sprayed canopies. Kaolin also decreased tran- spiration slightly during the period between 1000 and
1500 h Fig. 7b.
3.5. Transpiration efficiency T
E
There was a linear relationship between leaf yield in terms of made tea and transpiration Fig. 8.
A. Anandacoomaraswamy et al. Agricultural and Forest Meteorology 103 2000 375–386 381
Fig. 3. Diurnal variation of transpiration rate of two tea plants solid and broken lines: a at field capacity Day 1 and b near permanent wilting point Day 28. The insets show the diurnal variations of solar irradiance solid line and vapour pressure deficit broken line on
the two respective days. Total irradiance: 17.4 and 22.8 MJ m
− 2
per day and mean D: 0.45 and 0.83 kPa on Days 1 and 28, respectively.
382 A. Anandacoomaraswamy et al. Agricultural and Forest Meteorology 103 2000 375–386
Fig. 5. Diurnal variation of transpiration rate of mature clonal tea under open conditions unbroken line and at 85 artificial shade broken line.
Fig. 4. Response of transpiration of mature clonal tea to shad- ing over a period of 1 day. Estimated linear regression line
is: transpiration=0.238+0.031× irradiance. Standard error of regression slope=0.004 and adjusted R
2
= 0.95.
T
E
as given by the slope of this relationship was 9.637 kg made tea ha
− 1
mm
− 1
of water transpired. The re- lationship between total dry matter yield and the ratio
between transpiration and mean D also was linear. The proportionality constant was 6.9 g kg
− 1
kPa. Standard- ization of transpiration values with D did not increase
the precision of the linear relationship significantly.
4. Discussion
