9
IV. RESULTS AND DISCUSSIONS
To fully describe the performance of monthly spatial runoff, the result had been
calibrated with the observation data using the Nash-Sucliffe criterion R
2
and the Xiong and Guo relative error RE. The value of R
2
for Cicatih watershed was 66.15 and the RE
was 1.4 . The result was capable of describing the
monthly spatial runoff. So it is possible to utilize the result for learning the runoff
processes in the Cicatih watershed. 4. 1 Water balance components
The water balance components of the Cicatih are summarized as follows: Rainfall
ranged between 77 mm in July and 378 mm in January. Peak of rainfall was 378 mm in
January. Maximum AET was 113 mm in May due to the highest PET occurance. Minimum
AET was 65 mm in September due to the lowest SWC occurance. PET varied from 96
mm in February to 113 mm in May. PET increased as the increased of temperature T.
Mean values of SWC was 291 mm. SWC varied from 152 mm in September to 437 mm
in January depended on the rainfall. SMS occurred during the high rainfall period
between Oktober and June. SMS ranged from 19 mm in June and 241 mm in January. SMS
increased as the increased of rainfall. SMD occurred during the low rainfall period
between July and September. SMD had a small ranged from 19 mm in July to 38 mm in
September due to small difference between PET and AET. Mean, maximum, and
minimum runoff are 135 mm, 261 mm, and 36mm, respectively. Maximum runoff was
260 mm occurred in January and minimum runoff was 35 mm occurred in September.
Rainfall, AET, PET, and runoff of each subwatersheds are summarized in Table 4.
AET varied from 59 mm in Cicatih hulu subwatersheds to 130 mm in Cikembar
subwatershed. Maximum PET was 130 mm found in Cikembar subwatershed and
minimum PET was 88 mm found in Ciheulang subwatershed. SWC varied from
120 mm in Cikembar subwatershed to 453 mm in Ciheulang subwatershed. SMS ranged
from 3 mm in Ciheulang subwatershed 274 mm in Cikembar subwatershed. SMD ranged
from 18 mm in Cipalasari, Cileuleuy, and Ciheulang subwatershed to 46 mm in
Cikembar subwatershed. The highest and lowest values of mean rainfall were 244 mm
occurred in Cipalasari subwatershed and 220 mm occurred in Cikembar subwatershed.
Maximum runoff was 263 mm found in Ciheulang subwatershed in January. Minimum
runoff was 26 mm found in Cikembar subwatershed during the lowest rainfall
occurance in September. 4. 1. 1 Cicatih Hulu subwatershed
Mean AET, PET, rainfall, and runoff were 91 mm, 99 mm, 228 mm, and 133 mm,
respectively. Maximum AET was 109 mm occured in May. Maximum PET was 109 mm
occured in May. SWC varied from 144 mm in September to 428 mm in January. Peak of
rainfall took places in January was 456 mm. SMS ranged from 28 mm in June to 250 mm
in January. SMD ranged from 21 mm in July to 40 mm in September. Runoff varied from
33 mm in September to 260 mm in January.
4.1. 2 Cipalasari subwatershed Maximum AET was 115 mm occured in
May and minimum AET was 69 mm occured in September. Mean values of AET was 98
mm. PET had its maximum value 115 mm in May. Maximum SWC was 438 mm occurred
in January and minimum SWC was 152 mm occurred in September. Maximum rainfall,
which was the highest rainfall occurance among the subwatersheds, occured in October
with the value of 503 mm. Mean values of rainfall was 244 mm. SMS ranged from 17
mm in June to 239 mm in January. SMD ranged from 18 mm in July to 37 mm in
September. Mean values of runoff was 133 mm. Maximum runoff was 259 mm in
January, followed by February 231 mm. 4. 1. 3 Cileuleuy subwatershed
AET varied from 76 mm in September to 126 mm in May. Mean values of AET was 107
mm. Maximum PET was 126 mm occured in May. SWC varied from 141 mm in September
to 427 mm in January. Mean value of rainfall was 231 mm. Mean value of SMS was 109
mm. SMS ranged from 28 mm in June to 250 mm in January. Mean Value of SMD was 7
mm. SMD ranged from 18 mm in July to 41 mm in September. Mean values of runoff was
132 mm. Runoff was well varied. Maximum runoff was 259 mm occured in January.
10
Table 4. Water balance of each subwatershed mm
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Cicatih
Hulu P
AET PET
SWC SMS
SMD Ro
456 94
94 428
250 260
402 93
93 399
221 232
239 99
99 330
152 167
284 106
106 303
126
143 189
109 109
252
75 101
160 101
101 206
28 66
90 75
96 174
21 48
82 64
96 157
32 39
97 59
100 144
40 33
336 101
101 328
150
166 199
99 99
332 155
169 207
95 95
331 154
168
Cipalasari P
AET PET
SWC SMS
SMD Ro
395 100
100 438
239
259 398
99 99
409 210
231 270
105 105
340 141
166 215
113 113
314 115
144 155
115 115
263
64 103
184 107
107 216
17 69
86 85
102 185
18 51
123 74
102 167
28 42
107 69
106 152
37 35
503 107
107 337
139
164 235
105 105
343 144
169 260
101 101
342 143
168
Cileuleuy P
AET PET
SWC SMS
SMD Ro
449 109
109 427
250
259 389
108 108
398 222
230 261
115 115
329 153
164 323
123 123
303 126
141 178
126 126
252
75 100
147 117
117 205
28 66
80 94
112 173
18 47
74 82
112 156
30 38
107 76
116 141
41 32
281 117
117 326
150
162 242
115 115
332 155
167 241
111 111
331 154
166
Ciheulang P
AET PET
SWC SMS
SMD Ro
267 89
89 453
225 263
253 88
88 424
196 234
323 93
93 355
127 171
217 101
101 329
100
148 235
103 103
278
49 108
94 96
96 231
3 74
65 73
91 199
18 55
81 64
91 182
27 46
85 60
95 167
35 39
255 96
96 352
124 169
379 93
93 358
129 173
347 90
90 357
129 172
Cikembar P
AET PET
SWC SMS
SMD Ro
464 113
113 403
274
255 337
112 112
374 245
225 215
119 119
305 176
160 284
127 127
279 150
136 277
130 130
228
99 94
140 122
122 181
52 59
45 92
116 150
24 41
74 80
116 133
36 32
49 74
120 120
46 26
229 122
122 304
174
159 276
119 119
308 179
162 249
115 115
307 178
161 4. 1. 4 Ciheulang subwatershed
AET of each months had a small range. Mean values of AET was 87 mm. Maximum
values of PET was 103 mm occurred in Mei and minimum value of PET was 88 mm
occurred in February. SWC varied from 167 mm in September to 453 mm in January. The
rainfall was well varied. Mean values of rainfall was 217 mm. Peak of rainfall was 379
mm occured in November. SMS ranged from 3 mm in June to 225 mm in January. SMD
ranged from 18 mm in July to 35 mm in September. Mean values of runoff was 138
mm. Maximum runoff was 263 mm occured in January.
4. 1. 5 Cikembar subwatershed
Mean value of AET was 110 mm. Maximum AET was 130 mm occured in May.
PET had its highest value 130 mm in May
11
and the lowest value of PET was 112 mm in February. Maximum SWC was 403 mm
occurred in January and minimum SWC was 120 mm occurred in September. Mean values
of rainfall was 220 mm. Rainfall peak of 464 mm took places in January. SMS ranged from
52 mm in June to 274 mm in January. SMD ranged from 24 mm in July to 46 mm in
September. Mean values of runoff was 126 mm. During the low rainfall period runoff
values were less of than 50 mm.
4. 2 Runoff distributions of Cicatih watershed
Figure 5 shows the comparison between the observed and calculated monthly runoff
hydrograph in 1999 at the outlet of the Cicatih watershed. Mean values of observed runoff
and calculated runoff were 1157 mm and 1140 mm, respectively. The error of calculated
runoff varied from its maximum in January to the minimum in September. The highest
amount of observed runoff was 1892 mm occurred in March. Different from the
observed runoff, the highest amount of calculated runoff was 2296 mm occured in
January.
Flow directions Figure 6 affected the runoff accumulation. Flow directions was
estimated using PC Raster. DEM data was used as the only input. It was generated by
considering the 8 point pour algorithm with flow directions from each cell to its steepest
downslope neighbour. Moreover, runoff accumulation was generated using the flow
directions. Runoff accumulation calculated the amount of runoff that flowed out of the cell
into its neighbouring downstream cell. The runoff accumulation amount was the amount
of runoff in the cell itself plus the amount of runoff in upstream cells of the cell. In Cicatih
watershed, runoff accumulation had its maximum at the outlet.
Flow directions to west and southwest were common directions from the right side of
the Cicatih watershed. On the other side, the flow directions to southeast was common
direction from the left side of Cicatih watershed. Kurnianto 2004 described that
the flow directions was affected by the slope directions of Salak mountain in left side and
Pangrango mountain in right side of the Cicatih watershed. It caused the runoff
accumulation from left and righ side met at the midle part of Cicatih watershed. Finally,
the runoff accumulation flowed toward the watershed outlet.
Runoff accumulation varied in the watershed. It led to higher accumulation in the
most center part, and to lower accumulation in the mountain and at high altitudes. Maximum
runoff accumulation was 400000 mm occured in the center part of Cicatih watershed bottom
area because of their low altitude. Areas with 0 mm accumulation indicated that all of runoff
from that areas flowed out into its neighbouring downstream areas.
The result of the first stream order from the estimation using PC Raster Figure 7 was
similar with the observed first stream order Figure 8. In addition, some of second, third,
and fourth stream orders from the estimation using PC Raster were also similar with the
observed stream orders. However, runoff accumulation of the first stream order did not
appear in the estimation result.
500 1000
1500 2000
2500
jan feb
mar may
jun jul
aug sep
oct dec
Month m
3
s
Observed Ro Calculated Ro
Figure 5. The observed and calculated monthly runoff hydrograph of Cicatih watershed, Sukabumi in 1999.
12
Figure 6. Predicted flow directions using PC Raster.
Figure 7. Predicted runoff accumulation in January using PC Raster and their stream orders.
13
Source: Bakosurtanal 1999 Figure 8. Stream orders of Cicatih watershed, Sukabumi.
The results obtained from the calculation using the input 1999 data field
were monthly spatial variability of runoff distribution. Spatial distribution of runoff was
affected by the rainfall, land cover types, and WHC. However, monthly runoff distribution
was only affected by the rainfall.
The monthly runoff distributions showed that minimum runoff occurred
between May and September. September 35 mm was the month of minimum runoff. And
maximum runoff period occured in the period of October until April. January 260 mm was
the maximum runoff month because of the occurance of maximum rainfall in January.
The pattern of rainfall-runoff relationship was in agreement with result report by Xu 1997
in six watersheds from the humid region in Southern China and one catchment from the
semi-arid and semi-humid region in Nothern China. Putty and Prasad 2002 showed that
runoff increased as the significant increased of rainfall. Furthermore, The areaaverage values
of rainfall distributions and runoff distributions in Cicatih watershed described
the rainfall-runoff relationship as well see Table. 5.
The spatial distribution of runoff from the images see: Figure 9, Figure 10, and Figure
11 reflect that the contributions from the garden areas was the highest, because this was
the most common land cover in the Cicatih watershed. And the lowest contiribution of
runoff portion was obtained from the grass areas because of the their small areas. The
pattern of major runoff portion from the major land cover areas was also found by Karvonen
1999 in the Lestijoki watershed, Finland but with the different major land cover
contribution. The most common land cover in the Lestijoki watershed, Finland was forest.
During the high rainfall period, maximum runoff could be found in the forest areas 286
mm, followed by cropland areas 269 mm. And minimum runoff was 251 mm occurred
in settlement areas. In the low rainfall period, maximum runoff 109 mm was found in
forest areas. And minimum runoff was 0 mm occured in the settlement areas, followed by
cropland areas 5 mm around the mountain Salak and mountain Pangrango. It concluded
that the land use types-runoff relationship was widely affected by the rainfall. The high
runoff occurred in forest areas due to their high rainfall. Runoff increased with
significant incresing rainfall, following a positive linear function.
Table 5. Monthly runoff distributions in Cicatih watershed, Sukabumi
Month Rainfall mm
Runoff mm
Jan Feb
Mar Apr
May Jun
Jul Aug
Sep Oct
Nov Dec
378 345
277 255
200 139
77 87
94 325
280 274
260 232
167 144
103
69 50
42 35
166 169
169
14
Figure 9. Calculated monthly spatial runoff distributions in January-April.
15
Figure 10. Calculated monthly spatial runoff distributions in May-August.
16
Figure 11. Calculated monthly spatial runoff distributions in September-December.
17
V. CONCLUSIONS AND SUGGESTION
Kategori