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
10
11 Dec 07
3 6
9
3 6
9 12
15 18
21
Pg 72.4 mm Pn 3 31.9 mm
3 6
9
3 6
9 12
15 18
21
Pg 72.4 mm Pn 2 61.9 mm
3 6
9
3 6
9 12
15 18
21
Pg 72.4 mm Pn 1 53.8 m
Time hrs
m
Interception 1st plot = 25.7
Interception 3rd plot = 55.9 Interception 2nd plot = 14.5
Figure 6. Typical gross and net ranfall distribution of the heavy-rainfall showing a various proportion of net rainfall at each sampling plot in a 6-min time interval
D e
pth mm
11
10 20
30 40
50 60
70
10 20
30 40
50 60
70 80
S = 1 mm a
10 20
30 40
50 60
70
10 20
30 40
50 60
70 80
S = 0.5 mm b
10 20
30 40
50 60
70
10 20
30 40
50 60
70 80
S = 1 mm c
Figure 7. The scatter of P
g
againts T
f
. Following Leyton 1976 the line envelopes the scatters and intercept y-axis indicating the value of canopy capacity, S at 1
st
plot a, 2
nd
plot b, and 3
rd
plot c
y = 0.792x ‐ 0.817
R² = 0.972
10 20
30 40
50 60
70
20 40
60 8
T f
mm
Pg mm
a
y = 0.834x ‐ 1.068
R² = 0.980
10 20
30 40
50 60
70
20 40
60 8
Tf m
m
Pg mm
b
y = 0.521x + 1.902
R² = 0.867
10 20
30 40
50 60
70
20 40
60 8
T f
mm
Pg mm
c
Figure 8. Relationship between P
g
and T
f
showing the slopes that indicates canopy porosity, p at 1
st
plot a, 2
nd
plot b, and 3
rd
plot c
12
The averaged canopy porosity in this study is 0.7, indicates 70 of canopy gap
existence within the sampling plots which allow large portion of free throughfall
occurrence. The effect of this high porosity is a short time of delay between gross and
net rainfall at the beginning of a rainfall event, besides it increases the quantity of net
rainfall.
The averaged canopy capacity for this study is 0.8 mm that indicates when the
canopy was in a dry condition, 0.8 mm of rainfall was required to saturate the canopy
but wind speed or any turbulence occurred above the canopy could reduces water stored
on the canopy surface.
There is a tendency that larger canopy porosity leads to a lower canopy capacity.
This can be understood that in a canopy with large gaps area, the canopy size is reduced
and result in a small canopy size that would store less water on its surface. In 2
nd
sampling plot, the canopy porosity is large and the canopy capacity is low while at the
1
st
sampling plot, even though the canopy porosity is not much differs than the 2
nd
sampling plot, it has a higher canopy capacity. It seems that the conical canopy at
1
st
sampling plot retain more water than a broadleaved canopy at 2
nd
sampling plot. At 3
rd
sampling plot, even though is a broadleaved canopy, but multi-storey
canopies were exist so that large quantity could also be stored on the canopy surface.
As mentioned by Hall 2003 that at a single canopy layer, raindrop size has
significant influence to the canopy capacity during rainfall event. This type of canopy
was sensitive to the changes of raindrop sizes, large drops would make canopy to
store less water because the energy brought by the drops when hitting the canopy often
was higher than the canopy ability to retain water on its surface. In contrast, small
raindrops might retained on canopy surface until the canopy was saturated because it did
not have enough energy to bounce and splash out right after it hit the canopy. For
other canopy with some multi-storey or conical canopy such as Agathis tree, canopy
wetness is influenced by the canopy drips. During rainfall event, water from the top of
the canopy flows to the 2
nd
layer, after the 2
nd
layer saturated, additional water flow to the 3
rd
layer, and so on. Within this canopy, net rainfall did have longer time lag with
gross rainfall. As this type of canopy has small canopy size but have multi layer
canopy, interception loss was lower than the single layer canopy while net rainfall was
high with some time delay to reach ground surface.
4.3. Stemflow
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