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
Results of the present experiment indicated two key factors which control transpiration of tea, namely
soil water content S and irradiance. With respect to S, it was identified that transpiration of tea did not
decrease significantly until S reached a limiting value of 33 which corresponded to a depletion of 65 of
available water in the top soil layer 0–15 cm. Ritchie and Jordan 1972 also identified a similar two-stage
variation pattern for crop evapotranspiration with S. In comparison to the limiting value of 65 in the present
A. Anandacoomaraswamy et al. Agricultural and Forest Meteorology 103 2000 375–386 383
Fig. 6. Diurnal variation of transpiration rate of Grevillea robusta unbroken line and two tea plants broken and dotted lines under the shade of Gravillea.
experiment, Ritchie 1973 found that in maize, about 80 of available water had to be depleted before
transpiration fell significantly. However, this limiting value of available water also depends on the atmo-
spheric demand for water vapour Denmead and Shaw, 1962 as determined by irradiance, vapour pressure
deficit and boundary layer resistance Penman, 1948.
In the present experiment, S was measured only in the first 15 cm of the soil profile. In the clonal tea
used here, a high proportion of roots were present in this layer. However, most probably, the roots were ab-
sorbing water from deeper layers in the soil profile as well to maintain transpiration rates at maximum lev-
els. Therefore, the limiting depletion value of 65 available water was most probably determined by the
soil water availability in the deeper layers of the soil profile and the depth and extent of the root system as
well. Transpiration rate could be expected to decline at a higher level of soil water availability when the
soil water availability and rooting depths were lower and the atmospheric demand was higher. Neverthe-
less, the limiting value of 65 depletion of available soil water has a practical significance for the specific
conditions prevailing in the tea-growing regions at higher altitudes in Sri Lanka. Based on the results of
Carr 1969, 1974; Willat 1971; Stephens and Carr 1989; Stephens and Carr 1991, it was concluded
that for tea growing in Tanzania and Malawi, the ac- tual evapotranspiration begins to decline significantly
below the potential evapotranspiration when approxi- mately 30–40 of the soil available water is depleted.
This was a much lower limiting depletion level than the value of 65 found in the present experiment.
This was probably because of the lower atmospheric demand as indicated by the lower D 1 kPa, Table 1
of the present experiment as compared to 2 kPa in Stephens and Carr 1991.
The decrease of transpiration rate with the decrease of available soil water can be due to a combination
of several phenomena such as increased canopy re- sistance Monteith et al., 1965, increased hydraulic
resistance within the xylem and increased resistance at the soil–root interphase Passioura, 1988a. Further
experimentation is needed to separate the relative con- tribution of each of the above factors.
The other factor that was identified to have an influ- ence on transpiration of tea was solar irradiance which
is the main source of energy for evaporation of water
384 A. Anandacoomaraswamy et al. Agricultural and Forest Meteorology 103 2000 375–386
Fig. 7. Diurnal variation of canopy temperature a and transpiration rate b of Kaolin-sprayed unbroken line and unsprayed broken line tea canopies.
A. Anandacoomaraswamy et al. Agricultural and Forest Meteorology 103 2000 375–386 385
Fig. 8. Relationship between transpiration and yield of made tea for weekly periods during the experiment. Estimated linear regression
line is: yield=9.637×transpiration. Standard error of regression coefficient=0.953 and adjusted R
2
= 0.64.
in the canopy. The sensitivity of transpiration to irra- diance is dependent on the degree of coupling of the
canopy to the environment McNaughton and Jarvis, 1986 with the sensitivity increasing with decreased
coupling. Tea has a short about 1 m high and smooth canopy which could be expected to have a low degree
of coupling to the surrounding environment Jones, 1992. This agrees with the observed sensitivity of
transpiration rates of tea to irradiance in the present experiment.
On the other hand, the environment in which the present experiment was done experiences strong
winds which increases the coupling between the canopy and the surrounding environment to a certain
extent. Therefore, the decoupling coefficient or the McNaughton and Jarvis factor of tea canopies
of the present experiment could have been around 0.5 on the basis of the values given in Jones, 1992
which makes the transpiration of tea sensitive to both available energy supply i.e. irradiance and stomatal
factors as determined by the soil water availability Passioura, 1988b.
The value of transpiration efficiency T
E
for leaf yield of tea in the present experiment 9.637 kg ha
− 1
mm
− 1
is higher than the range of water use efficien- cies of 1.5–5.2 kg ha
− 1
mm
− 1
observed by Stephens and Carr 1991. This may be partly because Stephens
and Carr 1991 values are based on water use which includes both transpiration and soil evaporation. The
value of 6.9 g kg
− 1
kPa for the product between T
E
and D observed in the present experiment is slightly higher than the maximum of 5.0 g kg
− 1
kPa observed for groundnut Ong et al., 1987, a C
3
crop like tea. In agreement with theoretical analyses of Bier-
huizen and Slatyer 1965; Monteith 1986 and Jones 1992, the value for tea is lower than the range of
8.4–10.6 g kg
− 1
kPa observed for a C
4
crop, pearl millet Squire, 1990.
5. Conclusions
