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
To simulate how much water would be measured by the tipping buckets within a
variable tips interval, we gave a range of T values to every equation resulted from the
dynamic calibration Tables 4 and 5 .
5
Table 4. Depth per tip of TBT resulted from the dynamic calibration equation for a given T range values
TBT 10 TBT 6
TBT 9 Given T
sec Q
mls V ml
Depth mm
Q mls
V ml
Depth mm
Q mls
V ml
Depth mm
4 92.9 372
0.2 106.2
425 0.3 117.4 469 0.3
5 69.1 346
0.2 75.6
378 0.3 82.6 413 0.3
6 55.0 330
0.2 58.7
352 0.2 63.8 383 0.3
7 45.7 320
0.2 47.9
336 0.2 51.9 363 0.2
8 39.1 312
0.2 40.5
324 0.2 43.8 350 0.2
9 34.1 307
0.2 35.1
316 0.2 37.8 341 0.2
10 30.3 303 0.2
31.0 310 0.2 33.3
333 0.2 15 19.4
291 0.2 19.5
292 0.2 20.9 313 0.2
20 14.3 285 0.2
14.2 284 0.2 15.2
304 0.2 30 9.3
280 0.2
9.2 276 0.2 9.8 295 0.2
50 5.5 275
0.2 5.4
271 0.2 5.8 289 0.2 100 2.7
272 0.2 2.7 266 0.2 2.8 284 0.2
200 1.4 271 0.2 1.3
264 0.2 1.4 282 0.2 300 0.9
270 0.2 0.9 264 0.2 0.9 281 0.2
400 0.7 270 0.2 0.7
263 0.2 0.7 280 0.2 500 0.5
270 0.2 0.5 263 0.2 0.6 280 0.2
1.000 0.3 270 0.2 0.3 263 0.2 0.3 280 0.2
Table 5. Depth per tip of TBS resulted from the dynamic calibration equation for a given T range values
TBS 9 TBS 7
TBS 8 Given T
sec Q
mls V ml
Depth mm
Q mls
V ml
Depth mm
Q mls
V ml
Depth mm
2 26.5 53
0.006 26.5
53 0.001 26.5 53 0.001 3 16.6
50 0.005
16.6 50 0.001 16.6 50 0.001
4 12.1 48
0.005 12.1
48 0.001 12.1 48 0.001 5 9.5
47 0.005
9.5 47 0.001 9.5 47 0.001
6 7.8 47
0.005 7.8
47 0.001 7.8 47 0.001 7 6.7
47 0.005
6.7 47 0.001 6.7 47 0.001
8 5.8 46
0.005 5.8
46 0.001 5.8 46 0.001 9 5.1
46 0.005
5.1 46 0.001 5.1 46 0.001
10 4.6 46
0.005 4.6
46 0.001 4.6 46 0.001 15 3.0
45 0.005
3.0 45 0.001 3.0 45 0.001
20 2.3 45
0.005 2.3
45 0.001 2.3 45 0.001 30 1.5
45 0.005
1.5 45 0.001 1.5 45 0.001
100 0.4 45 0.005
0.4 45 0.001 0.4 45 0.001
300 0.1 44 0.005
0.1 44 0.001 0.1 44 0.001
500 0.1 44 0.005
0.1 44 0.001 0.1 44 0.001
1.000 0.0 44 0.005 0.0
44 0.001 0.0 44 0.001
For the TBS, when the volumes were calculated into depth unit, tree with the
smallest canopy size would have the largest depth 0.005 mm per tip while for other
trees even though its have different canopy size but have the same depth 0.001 mm per
tip. On the other side, all TBT were having a similar value 0.2 mm per tip. Several
tipping buckets both TBS and TBT did have higher depth value at its top flow
range. This was in the same agreement as mentioned by Calder and Kidd 1978 that
dynamic calibration was only significant at the top of flow range.
From the result, even though the tipping buckets did have different volume capacity
at each side, it could produce an equal depth value for it’s both sides and were able to
measure a very low throughfall and stemflow.
3.3. Measurement principles
Interception can not be measured directly, it is calculated as the difference
between gross and net rainfall.
6
3.3.1. Gross Rainfall
