Water Yield Literature Review
53
2 Transform Model
Unit Hydrograph SCS method is employed in this research for transform model. Recapitulation of the result from transform model parameter calculation of
each sub-basin is shown in Table 15. Input parameter needed for SCS method also develops synthetic unit hydrograph that is based on dimensionless hydrograph. In
HEC-HMS, SCS method only needs time lag parameter as its input. Based on the calculation, average SCS time lag for each sub-watershed is 250.839 minutes.
Table 15. SCS Hydrograph Parameters for Transform Model Input
Sub Basin Lag Time
Minutes W510
295.590 W480
184.150 W500
289.030 W470
208.263 W420
194.950 W410
261.237 W460
209.080 W430
308.360 W490
282.320 W450
8.267 W380
131.580 W390
79.071 W400
422.230 W370
291.050 W350
383.180 W340
268.010 W310
499.682 W300
159.715 W280
290.180
3 Baseflow Model
Initial discharge parameter, recession constants and threshold flow in base flow model are determined by observation hydrograph flow in SPAS Empang. The
contribution of base flow and recession constants in each sub-watershed is assumed to be proportional with the width of each sub-watershed. The equation needed for
recession constants, k is:
54 � = ��
�� �
�
− �� �
�
�
Qt is base flow in t period, and Qo is initial discharge in t=0. Based on the observation hydrograph in SPAS Empang during the observation period, average k
value is obtained, i.e. 0,72. Threshold flow is the flow in the beginning of recession curve in declining side of a hydrograph. In HEC-HMS, threshold flow is determined
as a ratio to peak. The ratio to peak of observation hydrograph, in Empang ranges from 0.25
– 0.5; or 0.42 in average.
Table 16. Baseflow Model Parameter
Sub Basin
Initial Type
Initial Discharge
m
3
s Recession
Constant Threshold Type
Threshold Flow
m
3
s Ratio to
Peak
W510 Discharge
0.03 0.9
Ratio to Peak 0.4
W480 Discharge
0.03 0.8
Ratio to Peak 0.4
W500 Discharge
0.03 0.9
Ratio to Peak 0.4
W470 Discharge
0.03 0.8
Ratio to Peak 0.5
W420 Discharge
0.025 0.9
Ratio to Peak 0.4
W410 Discharge
0.03 0.9
Ratio to Peak 0.5
W460 Discharge
0.03 0.7
Ratio to Peak 0.5
W430 Discharge
0.025 0.8
Ratio to Peak 0.5
W490 Discharge
0.03 0.8
Ratio to Peak 0.4
W450 Discharge
0.025 0.9
Ratio to Peak 0.5
W380 Discharge
0.03 0.8
Ratio to Peak 0.5
W390 Discharge
8.5 0.9
Ratio to Peak 0.4
W400 Discharge
0.03 0.1
Ratio to Peak 0.25
W370 Discharge
0.03 0.75
Ratio to Peak 0.4
W350 Discharge
0.03 0.1
Threshold Discharge 11.33
W340 Discharge
0.03 0.8
Ratio to Peak 0.4
W310 Discharge
8.5 0.1
Threshold Discharge 5.66
W300 Discharge
0.03 0.8
Ratio to Peak 0.4
W280 Discharge
0.03 0.9
Ratio to Peak 0.3
4 Routing Model
The calculation of river flow routing in HEC-HMS is shown in routing model channel flow model. The research employs Lag method. The parameter needed is
flow travel time from inlet to outlet, determined by correlation between the flow rate and the river length. Based on the configuration of Upstream Cisadane