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: