Catchment Area and Watershed management Hydrology Model
52 and previous soil humidity. In SCS method, Ia value is calculated based on potential
maximum retention and the curve number. Determination of curve number and width of impervious area is based on calculation in section 3.2.5.
Land use in upstream Cisadane watershed consists of 5 types of land uses, i.e.: water, grassland, estate, settlement, and forest area. Land hydrological
condition is determined by hydrology soil group HSG, which is based on soil type. Curve number at each sub-basin is calculated based on area weight of each land use
according to its HSG. Based on the calculation, average of curve number in Upstream Cisadane Watershed in 2010 is 87.757. Beside curve number, the weight
of impervious area also affect run off volume in each watershed. Table 14 shows curve number, imperviousness and initial abstraction in each sub-basin in upstream
Cisadane watershed.
Table 14. Curve Number Percent Impervious at each sub-basin in Upstream
Cisadane Watershed for Simulation Year 2030
Sub Basin Curve Number
CN Imperviousness
IMP Initial Abstraction
Ia
W280 0.445
0.444 0.440
W300 0.472
0.482 0.476
W310 0.629
0.610 0.593
W340 0.333
0.333 0.321
W350 0.525
0.531 0.523
W370 0.525
0.539 0.534
W380 0.420
0.421 0.421
W390 0.492
0.495 0.495
W400 0.452
0.448 0.443
W410 0.576
0.582 0.576
W420 0.494
0.501 0.501
W430 0.586
0.597 0.593
W450 0.398
0.409 0.409
W460 0.469
0.462 0.457
W470 0.583
0.571 0.566
W480 0.470
0.498 0.600
W490 0.629
0.618 0.615
W500 0.589
0.615 0.637
W510 0.596
0.602 0.631
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: