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710 Results of the SWAT model is expected to help to determine management actions of land and
water resources in supporting watershed management, so that the upstream and downstream subzone able to function sustainably which produces water optimally.
One of the methods that is used to estimate the value of streamflow and sedimentation ie approach hydrological model of SWAT. This model has been used as an official reference for
estimate Total Maximum Daily Load in the United States or The maximum load of pollutants allowed in a watershed Borah et al., 2006. Europe and Asia have been many publications and
applications SWAT models with satisfactory results, there are currently more than 250 papers related to SWAT for watershed modelling worldwide Gassman et al., 2007. SWAT model is
the most widely model used around the current world related to the hydrological modelling. This paper estimates the streamflow and sedimentation on Sawangan Sub Watershed in each
land unit based on soil type, slope and land use.
2. EXPERIMENTAL METHOD 2.1. Materials and Tools
Materials required in this research are DEM Digital Elevation Model Map with a resolution of 90 m derived from Satelite Imagery Raster, land use map, and soil types map. Climate data
used were daily precipitation data mm for four years 2008 –2011, the maximum and
minimum temperature ºC, solar radiation MJm2hari and wind speed mdt. Precipitation data was obtained from three graduated rainfall stations namely Sawangan
station, Tikala station and BMKG Station in Sam Ratulangi Airport. Other data used were the daily and monthly streamflow actual on Sawangan outlet. While the tools used were a
computer with Arcview software, SWAT software, GPS and stationery.
2.2. Work Procedure
Figures 1: The work procedures of research
2.3. Data Analysis
Comparison simulation streamflow with actual streamflow was conducted by SWAT CUP using the coeficient of determination R
2
. Coeficient approaching one signifying that the the value of model results approaching the true value, as for the determination coeficient
determination is:
climate data TRMM and Qmorph
Soil Map Land Use Map
DEM Map
The model structure data
Land Use Data Soil Data
Delineation of the research area and Establishment of HRU
Model SWAT Calibration R
2
Output Models
Search watershed problematic
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711 √
3. RESULT AND DISCUSION
Sawangan Sub Watershed geographically located at latitude 1 21’00’’ LS - 1
28’00’’ N and longitude 124
51’00’’ BB - 124 56’58’’ BT. Administratively Sawangan is in the area of
Minahasa Regency, North Sulawesi Province.
3.1 Characteristic of Sawangan Sub Watershed Sawangan Sub Watershed are dominated by grayish brown regosol soil with an area 2534.00
ha 35.08 and andosol fawn with an area 2446.16 ha 33.86. Based on the topography, Sawangan sub watershed divided into the category of rather steep with an area of 2306.51 ha
31.93, ramps 24.51 1770.71 ha and steep with an area of 1568.89 21.72. So it can be stated that the sawangan sub watershed have grade slope is relatively large. While the slope is
very steep only 8.60 of the total area ie 621.55 ha. Despite having the smallest extents this area should get an important concern in the determination of land use pattern with consider
the soil type. If in this condition land use applied incorectly it will cause impact damage that cause harm downstream area.
There are seven land use pattern ie primery forest, secondary forest, mixed plantation, dry land venture, settlements, farmfield and plantations crops. Sawangan sub watershed are
dominated by plantation crops of 6184.75 ha 85.61 and mixed plantations covering an area of 644.65 ha 8.92 . The smallest land area are field of 5.70 ha 0.08 and secondary
forest area of 1.05 ha 0.01. Primery forests are in the upstream area 259.53 ha 3.59 , it shows that the forested areas was only 3.6 of the total Sawangan sub watershed areas. The
main cultivated commodities is coconut Cocos nucifera, an excellent and preferably product for The North Sulawesi people even since the dutch colonial era. Plants on mixed plantation are
generally combination of coconuts and cloves while on dry land venture are short-term and seasonal crops. Commodity in plantation crops generally consists of coconut and clove with
monoculture systems. 3.2 Calibration of SWAT Model
Wide models from the DEM delineation is 7223.93 ha of the Sawangan sub watershed total area. 73 land unit HRU are formed by SWAT models that are the results of overlaying soil
types map, land use map and slope map. Based on calibration using SWAT CUP showed that the model feasible to use. Calibration results showed a p factor value of 0.03 and r factor value
of 0.40 while the feasibility value of the model indicated by R
2
value of 0.76. Calibration is the process of verifying the data results of the model with one of the actual data field.
Table 1 shows that the calibration value still within the tolerance limits are allowed feasible to use. Based on the research results of Andronita 2011 in predicting Citarum watershed
produce R
2
value of 0.712. The same thing happened in the research of Cisadane Watershed, whereas the feasibility of model was measured by R
2
value ie 0.76 Junaidi and Tarigan, 2012. Table 1: The result of the model calibration
Variable p_factor
r-factor R
2
NS br
2
MSE SSQR
Flow Out 0.03
0.40 0.76
-2.63 0.2401
11.6187 11.4591
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3.3 The Search of Land Unit Potentially Causing Problems on the Sub Wathershed 3.3.1
Streamflow Results of the model analysis showed that the average number of monthly streamflow which
occurs at the outlet Sawangan Sub Waterdhed between 2.5 –3.5 m
3
dt. This is the highest streamflow because it is the accumulation result of each streamflow within each land unit
above it. The implications of high streamflow on the outlet area seen from river bank erosion meandering so the river body in this region becomes wider. Large streamflow concentration
is generally located in the downstream and river branches with a flat topography. The high streamflow in the downstream caused by land use factors in the upstream.
Figure 2: Calibration of Sawangan Sub Watershed Model
Figure 3: Map of streamflow spread in the land unit Based on the observation, region with high streamflow is dominated by land use of palm
plantation and mixed plantation coconut and clove. The lack of woodland resulted the rainwater that falls on the lands surface will be runoff. These conditions will impact on
downstream because water submissions will be faster because there is no vegetation that holds and reduce the flow rate. Vegetation that has layered canopy structure can decrease the
terminal velocity of rainwater and reduce the diameter of raindrops. Asdak, 2010.
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713 3.3.2
Sedimentation Model outcomes indicate that sedimentation in Sawangan Subwatershed vary widely. Monthly
average sedimentation on the upstream of between 4.42 – 8.85 m
3
ha occured on land units number 61, 62, 68 and 73. Small sedimentation in this area caused by woodland vegetation
condition are still good. According Asdak 2010 vegetation influence on erotion and sedimentation, namely : 1 protecting the soil ssurface from the rain blows; 2 lowering the
speed of flowing water; 3 withstand the soil particle in their place and 4 maintining stability of soil capacity in absorbing water.
Table 2: Several of land unit that has the highest and lowest streamflow
Land unit
Large ha
Land use Soil Type
Slope Streamflow
m
3
dt Streamflow
Range m
3
dt
1 26.71
Plantation crops Andosol fawn
0-8 2.53
2.5 – 3.5
3 11.31
Plantation crops Andosol fawn
8-15 2.40
1.5 – 2.5
6 81.66
Mixed plantations Latosols reddish brown
15-25 2.35
1.5 – 2.5
8 15.12
Plantation crops Latosols reddish brown
8-15 1.21
1 – 1.5
11 12.26
Plantation crops Latosols reddish brown
15-25 1.11
1 – 1.5
5 62.27
Plantation crops Grayish brown regosol
15-25 0.01
– 0.5 16
77.86 Plantation crops
Andosol fawn 25-40
0.01 – 0.5
22 61.51
Plantation crops Andosol fawn
25-40 0.01
– 0.5 56.18
Plantation crops Dark black andisol
25-40 0.01
– 0.5 35
63.60 Plantation crops
Grayissh brown regosol 15-25
0.01 – 0.5
The soil type on that land unit is grayish brown regosol on the land unit No. 61 and 73 while on land unit number 62 and 68 is dark black andisol. Regosol is soil with high category against
vulnerability to erosion whereas andisol soil including category rather high Paimin et al, 2006. This soil type highly erodible Foth, 1994, so that the forested areas should not be converted
for other uses because would be very difficult to recover it.
Figure 4: Sedimentation map of Sawangan subwatershed
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714 The highest monthly average sedimentation of 17.70
– 22.13 tonha on the land unit 29, based on observation that land unit is mix plantations and most of the land has been opened
for the settlements. The soil type on that land unit is grayish brown regosol with a steep slope. Most of the land with very steep slope potentially resulted criticality, especially without
vegetation cover. Addition to enlarging the amount of surface runoff slopes also increases runoff. Increasingly skewed of slope, the number of grains of soil that splashed down by the
collision of rain more and more. If the ground slopes become steeper twice then the number of erosion per unit area becomes 2
–2.5 times more Arsyad, 2006. Table 3: Sedimentation on Sawangan Subwatershed
Land unit
Large Land use
Soil Type Slope
Sedimentation Sedimentation
Range tonha
5 62.27
Plantation crops Grayish brown
regosol 15-25
2.06 – 4.42
14 145.36
Plantation crops Andosol fawn
15-25 2.44
– 4.42 22
61.51 Plantation crops
Andosol fawn 25-40
2.49 – 4.42
45 142.70
Settlements Andosol fawn
15-25 2.84
– 4.42 33
56.18 Plantation crops
Dark black Andisol
25-40 2.86
– 4.42 23
71.78 Plantation crops
Redish brown Latosol
25-40 14.61
13.28 – 17.70
18 60.75
Plantation crops Grayish brown
regosol 25-40
14.96 13.28
– 17.70 40
233.67 Plantation crops
Andosol fawn 25-40
15.90 13.28
– 17.70 17
46.68 Plantation crops
Redish brown Latosol
25-40 16.61
13.28 – 17.70
29 61.79
Settlements Grayish brown
regosol 25-40
18.57 17.70
– 22.13
Region with high monthly average sedimentation of 13.28 – 17.70 tonha namely on land
unit 17,18,23,40 and 29. The land use on that unit is plantation crops and mixed plantations. Although this land use have annual plant but the vegetation is not able to significant effect on
minimization of erosion and sedimentation. This is because plants on that land uses tend to be uniform and don’t have stratum as vegetation on the forest. The second land units located on
the downstream, the high sedimentation caused by soil type andisol which is high category vulnerable to erosion and slope is rather steep. Andisol are soils which has a layer of 36 cm
with andik properties at depth 60 cm, this land formerly known as andosol Hardjowigeno, 2007.
Sedimentation is also caused by land clearing condusted by community in upstream area. Based on observations community follow with land clearance on Rumengkor village, Koka
village and Kembes village area. There has also been an avalanche on plantations land and residential community in Rumengkor village land unit 60.
4. CONCLUSION
