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3.4.2.2 Sub-Basin Lag Time and Peaking Coefficient
This research was use the Snyder transform method to transform the rainfall into unit hydrograph. The Snyder transform method requires the lag time
and peaking coefficient information to determine the peak time. Lag time is defined as the time difference between the peak of the rain event and the peak
discharge. Based on the Snyder theory, the rainfall transformations into unit hydrograph are determined by some basin parameter, in which those parameters
can be measured based on the physical form of the watershed Hartanto, N. 2009.
Figure 3.13 Hydrograph element; lag time illustration. Basin lag time can be obtained by using some lag time equation. This
study is using Tulsa rural lag time equation, in which the equation was developed by US Army Corps of Engineers for Tusla district.
= … … … … … … … … … … … … … … … … … … … … … … . 6
Where Ct is a coefficient for natural watershed with value 1.420, Tp is lag time, L is sub-basin length, Lca is length to sub-basin centroid, S is maximum river slope,
and m is a power coefficient with default value of 0.33. The Snyder peaking coefficient can be calculated by using following
relationship for the peak flow rate that can be used solve for Snyder’s peaking coefficient.
= 640
… … … … … … … … … … … … … … … … … … … … … … … … … … . 7
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Where Cp is Snyder peaking coefficient, Qp = 380Tp
-98
and Tp is sub-basin lag time which is calculated previously. Actually the peaking coefficient can be
adjusted manually to find the best correlation between the observed data and simulated product.
3.4.2.3 Curve Number Determination
Curve number is a transformation among land cover, soil type, and slope to represent the amount of surface runoff in a watershed. Curve number method
can use widely and it’s the most efficient method to calculate surface runoff for single rain event in a watershed. Although the method is designed for a single
storm event, it can be scaled to find average annual runoff values. The stat requirements for this method are very low, rainfall amount and curve number. The
curve number is based on the areas hydrologic soil group, land use, treatment and hydrologic condition.
In this research, the curve numbers are obtained by overlaying the land cover maps, and soil type maps, then the intersection among those three
parameters are gives a numbers based on the curve number look up table which is developed by Soil Conservation Services NRCS – United States Department of
Agricultural USDA. The calculation of curve numbers were performed using the equation 4, this described in the literature review section.
The NRCS-CN method does not take into account the effect of slope on runoff yield. However, there are few models that incorporate a slope factor to CN
method to improve estimation of surface runoff depth and volume Ebrahimian, 2009. Because of the characteristics of Palu catchment is hilly, slope adjustments
to the curve number was performed by employing the equation as below: =
322.79 + 15.63 + 323.52
… … … … … … … … … … … … … … … … … … . 8 Where K is curve number constant, and is slope.
The next steps is calculate the sub-basin averages curve number by employing the weighted averages equation as below:
= … … … … … … … … … … … … . . … … … … … … 9
Where is sub-basin average curve number.
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3.4.2.4 Areal Rainfall Estimation
The HEC-HMS model requires daily time series precipitation data as the input. The total of precipitation data from all meteorological stations were located
inside and outside the catchment area is used to estimate the rainfall amount. A daily areal rainfall of catchment will calculated from daily point measurement of
meteorological stations using Thiessen Polygon Methods. The Thiessen Polygon is one way of calculating areal precipitation. This
method gives weight to station data in proportion to the space between the stations. The area of each polygon inside the sub-basin, as a percentage of the
total sub-basin area, is calculated. This factor is then used as the weight of the station situated within that polygon. The Thiessen weight for each station was
calculated for each sub-basin in Table 3.4. The precipitation for the whole area is then calculated as follows:
= 1
… … … … … … … … … … … … … … … … … … … … … … … … … 7
Where: P
= Averages areal Rainfall Pj
= Rainfall measured at each station Aj
= Area of each polygon inside the basin Atot = Total sub-basin area
Table 3.4 Metrological station weight calculated using theissen polygon method
Rainfall Station Sub‐basin
1 2
3 4
5 6
7
Bangga Atas 0.192
0.042 0.047
Bangga Bawah 0.056
0.104 Bora
0.109 0.341
0.186 Kalawara
0.002 0.316
0.456 0.039
mantikole 0.285
0.002 0.162
Mutiara 0.009
0.652 Palolo
0.038 0.733
0.542 0.608
0.001 Tuwa
0.749 0.581
0.006 0.099
Wuasa 0.381
0.261
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