Figure 4. Generated file by data logger
3.3.2.1. Throughfall
Throughfall at each sampling plot was measured using five troughs made of PVC
pipe with the dimension of 200 x 15 cm and placed randomly beneath the canopy in
a radial base. The troughs were installed in parallel or near to flat angle to the ground
surface in order to keep its catchments area approximately 200 cm x 15 cm using
stanchion of bamboo. Each of the troughs was then connected to a TBT by a 0.5 inch
plastic tube.
At 1
st
sampling plot, the main tree was an Agathis with its conical canopy, surrounded
by other higher canopy. Some gaps between those canopies were exist that would allow
large portion of free throughfall. Canopy drip at the main tree would be long enough
after the gross rainfall begun as the canopy was thick.
At 2
nd
sampling plot, the main tree was the top canopy with large gaps from
surrounding trees. Smaller and lower trees were exists but the main tree canopy was the
largest. It was a thin canopy so large amount of drip might flow to the ground surface.
Large portion of free throughfall was also possible because other canopies were
smaller and lower that did not have large canopy size.
Densest canopy with some multi-storey existence was at 3
rd
sampling plot. Almost no rainfall could reach the ground surface
without striking the canopy first; canopy drip was seemed contribute to most of
throughfall measured.
3.3.2.2. Stemflow
For stemflow measurement, a part of the tree trunk near to breast height was chosen
at each sampling plot. First, the trunk’s surface was slivered in a circumference
order, and then a half-section of 1 inch plastic tube was nailed following the sliver
pattern to trap water flow on the trunk surface. Water trapped by this plastic tube
then channeled down to a TBS by a 0.5 inch plastic tube.
Figure 5. Plastic tube to collect stemflow
3.4. Interception and canopy parameters estimations
Interception is calculated as the difference between gross and net rainfall
while canopy parameters canopy capacity and canopy porosity are estimated using
gross rainfall and throughfall data gained from the measurement. Canopy capacity is
estimated by plotting gross rainfall and throughfall data in a scatter plot diagram
using Leyton method 1967. A line then drawn to connect the upper most points but
exclude the points where gross rainfall could not saturate the canopy. This line then
extrapolated until it crosses the negative y- axis. The absolute value crossed by the line
is the estimated canopy capacity value. For estimating the canopy porosity, a linear
regression equation is made from the scatter plot diagram. The coefficient of the equation
then identified as the canopy porosity value.
8
IV. RESULTS AND DISCUSSIONS
4.1. Typical distribution of gross and net rainfall
Even though gross rainfall at the AWS and the sampling plots were assumed to be
in the same rainfall regime, 12 rain days of total measurement period seems to have
some differences. The data logger was record net rainfall earlier than the AWS
which indicates that the sampling plots receive gross rainfall earlier and possibly
different quantity than the AWS.
Figure 7 shows a typical distribution of gross and net rainfall in the heavy rainfall
class. There is a time lag between gross and net rainfall where net rainfall occur
approximately after 18 minutes of intermittent gross rainfall. It means that for
the rainfall at 11 December, 18 minutes was required to saturate the forest canopy before
net rainfall occurs. The net rainfall from each sampling plot occurs within the same
time interval with similar quantity but as rainfall intensity increases, the three plots
received different quantity of net rainfall. After the first rainfall stops, small quantity
of net rainfall still occurs which indicates a canopy
drainage process
was in
progress. In the 2 The measured Throughfall in this study
is 71 of gross rainfall 286.6 mm with variability found among sampling plots
61 - 78 of gross rainfall. By the troughs arrangement which were placed on
radial base and random beneath the canopy at each sampling plot it was hard to identify
whether throughfall was large near the tree stem such as found by Ford and Deans
1978 or large as distance increase from the tree stem such as found by Leyton and
Carlisle 1959 because throughfall measured would be an average from those troughs. As
mentioned by Jarvis 2000, some epiphytes did also exist within the forest canopy but it
was difficult to differentiate between the actual trees leaves and the epiphytes, so we
can not estimate its proportion and its contribution to the throughfall.
nd
rainfall event at 11 December, the three sampling plots require 6
minutes to saturate the canopy and after that net rainfall occurred with different quantity.
The 2
nd
plot received the largest quantity of net rainfall which possibly because the
canopy has the largest portion of open space compared to the other plots while the 3
rd
plot receives the least which possibly because it
has a multi-storey canopy so that rain water is distributed through the canopies.
As net rainfall assumed to occur after the canopy gets saturated, it indicates that
canopy was in potential evaporation state at the moment where the canopy was able to
evaporate water retained on its surface at the maximum degree, influenced only by the
weather factor at the moment. The potential evaporation condition would continue until
the gross rainfall stops, and then any net rainfall occurred after that was assumed as
canopy drainage process.
Sometimes net rainfall was higher than the rainfall occurred in the open space
within the same time intervals, this could be caused by the water stored in canopy surface
from previous rainfall that was flow down to the ground surface together with current
rainfall drip. High wind speed or turbulence within the canopy gaps might be the source
for this condition. Another possibility for a higher net rainfall measured was because
actually a higher gross rainfall occurs at that sampling plot. This was in the same
agreement with Ford and Dean 1978 who mentioned that in some condition, different
gross rainfall might be measured between two adjacent places. This difference for the
studied area might be caused by some wind occurred during rainfall event or the
topography factor where the sampling plots are in a higher altitude than the AWS.
Presented in appendices 2 – 4, net rainfall at every rainfall classes occur in
various time lag with the gross rainfall, depends on the intensity at the beginning of
the rainfall. Small rainfall intensity and an intermittent rainfall would make net rainfall
occur in a high time lag with the gross rainfall while high intensity of rainfall
would reduce the time lag between gross and net rainfall.
4.2. Throughfall
The 3
rd
sampling plot received the least throughfall 61 of gross rainfall which
possibly caused by its multi-storey canopy that could retain a large quantity of
rainwater while the largest throughfall measured in 2
nd
sampling plot that might be caused by large portion of open space.
9
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