Table 2. Canopy capacity and canopy porosity from others interception study Study Site
Location Canopy Capacity,
S mm Canopy
Porosity, p Source
Unlogged forest Indonesia
1.3 0.1
Asdak et al. 1998b Logged forest
Indonesia 1
0.3 Asdak et al. 1998b
Hardwood stand Canada
1 – 1.1 0.61 – 0.85
Carlyle-Moses et al. 1999
Natural forest Indonesia
1.3 0.7
Anwar 2003 Lowland coastal
rainforest Australia 3.5
0.035 Wallace et al. 2006
Lowland tropical rainforest
Puerto Rico 1.15
- Schellekens et al.
1999 Lower montane
forest Ecuador
1.91 – 2.46 0.42 – 0.63
Fleischbein et al. 2005
III. METHODS
3.1. Study site Cibojong micro-watershed is a 1392 ha
forested catchment in southern slope of mount Salak, part of upper Cicatih
watershed with 50 of the land use was forest, 28.71 rice fields, 7.53 settlement,
6.87 shrubs, 5.79 mixture gardens, 0.94 farm land, and 0.24 grass Pawitan
et al. 2006.
The sampling plots were located at the margin area of the natural forest, near the
camping ground and Guest House area of Wanawisata Cangkuang Sukabumi which
belongs to Perum Perhutani. We choose three sampling plots with different canopy
cover. The 1
st
sampling plot was an Agathis tree which has a conical canopy and small
tree diameter. Canopy gaps were exist between the Agathis tree and the
surrounding trees which were broadleaved canopy. The 2
nd
sampling plot was a broadleaved canopy which has a small
canopy size and large portion of open space between its canopy and the surrounding
trees. The tree diameter was larger and has a coarser surface than the Agathis. Some
epiphytes did grow on the tree canopy but it was difficult to differentiate between the tree
leaves and the epiphyte. The 3
rd
sampling plot was also a broadleaved canopy which
has larger canopy size than the tree at 2
nd
sampling plot. It was a multi-storey canopy where the surrounding canopies were
occupied the upper and lower position of the main tree canopy.
3.2. Initial investigation
This study used sets of instrument developed by Instrumentation workshop of
Department Goephysics and Meteorology, Bogor Agricultural University for measuring
the net rainfall. The instruments used were new, so we need to conduct an initial
investigation to know its performance. The conducted initial investigation was about
dynamic calibration of the tipping buckets used. This investigation aimed to identify
the tipping buckets capability when measuring different flowrate of water.
3.2.1. Tipping bucket calibration
We used two different capacities of tipping buckets, named as tipping bucket for
throughfall TBT which was used to measure throughfall and tipping bucket for
stemflow TBS which was used to measure stemflow Figure 2.
Figure 1. TBT the big ones and TBS the small ones
When a tipping bucket flowed with continuous water, a variable quantity of
water was lost during the time taken for the tipping bucket to move from rest to the
position at which the central bucket division
4
was beneath the inlet stream of water. It causes an underestimation between water
measured by the tipping bucket and the actual water flowed. To avoid this
underestimation, Calder and Kidd 1978 were suggesting that each of the tipping
buckets should be calibrated dynamically.
We carried out the dynamic calibration according to the concept introduced by
Calder and Kidd 1978 by pouring five different flowrate Q to every tipping
bucket. A small water pump was used to produce a constant water flow to the tipping
bucket. For TBT, each flowrate was carried out for approximately 50 tips while for TBS
were 100 tips. From the tips then we get the mean time between successive tips for each
tipping bucket. From each tipping bucket, five different
flowrate values which were given then plotted against its mean time. After that we
got the relationship between inverse of flowrate 1Q at the x-axis and its mean
time T at y-axis in the form of linear regression equation Table 3. Later, the
equation would be used to determine the volume for each bucket tip during field
measurement based on the time between two successive tips.
Table 3. Dynamic calibration equation of the tipping buckets
Sampling plot Bucket ID
Dynamic Calibration Equation
R
2
TBS 9 T = 44.4391Q + 0.3203
0.980 1
st
TBT 10 T = 269.411Q + 1.1012
0.998 TBS 7
T = 44.4391Q + 0.3202 0.980
2
nd
TBT 6 T = 262.261Q + 1.5297
0.978 TBS 8
T = 44.4391Q + 0.3202 0.980
3
rd
TBT 9 T = 279.241Q + 1.6207
0.998
As the equation’s output volume was in milliliter ml unit while throughfall andor
stemflow value should be in millimeter mm unit, then the volume need to be
divided by troughs size for throughfall or canopy size for stemflow.
According to Calder and Kidd 1978, the equation resulted from dynamic
calibration was as follow: y = ax + b
Where: y = time between successive
tipping bucket tips sec a = volume ml
x = 1Q sml b = intercept sec, represents
time required for the tipping bucket to tips when a
minimum flowrate is given
To get volume of a bucket for each tip in a certain time interval:
Q b
y Q
1 b
y a
volume ×
− →
− =
While conversion from volume to depth- equivalent:
2 cm
size nopy
troughsca 10
ml volume
mm depth
× =
Trough size = 200 × 15 cm
2
= 3.000 cm
2
Total of 5 troughs size = 3000 cm
2
× 5 units = 15.000 cm
2
3.2.2. Dynamic calibration result
