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