Model Calibration and Verification

56 Figure 4.14 Simulated hydrograph using 2007 rainfall data for different land cover scenarios. Simulation using these three different scenarios, the peak flow increased 13.37 during wet season when the first scenario compared with the second scenarios. For the second and third scenario, the peak flow also increased about 3.24. The comparison between scenario-based hydrograph and simulated hydrograph using existing condition of 2009 data, the first scenario using RTRW has decreasing the peak flow significantly from 288 m 3 s on 2009 become 258 m 3 s. This because of the forest cover area is much bigger on the RTRW than the existing condition on 2009, which is on 2009 the total forest covers is 1793.2 km 2 or 58.79 and 2024 km 2 or 66.36 by using the first scenario. In addition, with the addition of agroforestry system on the RTRW scenario also gives significant impact to reduce the surface runoff compared with the traditional existing agricultural system; where agroforestry system covered about 527.25 km 2 17.29 and agricultural land covered about 278.8 km 2 9.14 of total catchment area. For the second scenario by using projected land cover area on 2020 and 2030, the simulated hydrograph shows the increased peak flow compared with the existing condition on 2009 from 288 m 3 s to 292 m 3 s on 2020 and 302 m 3 s on 2030. This because of the forest cover was decreased by 4 on 2020 and 9 on 2030 and also the increased of agricultural land from 26 on 2009 become 33 5 10 15 20 25 30 35 40 50 100 150 200 250 300 350 1 31 61 91 121 151 181 211 241 271 301 331 361 Preci p it a Uo n mm Di sch a rges m 3 s Days Precipita\on RTRW LC_2020 LC_2030 57 on 2020 and 41 on 2030. The decreased of forest cover and increased of agricultural land on 2020 and 2030 makes the curve numbers are increased on each sub-basin. Increasing curve number means the surface runoff is increased during the rain event, where the volume of infiltrated water is decreased. Based on all of those three scenarios, the first scenario using Provincial Spatial Development Plan RTRW is produce the lowest peak flow values, while the last scenario produce the highest peak flow. This is because of the assumption made on first scenarios, where all the forest areas are kept maintained and the agricultural lands are well managed by adding agroforestry system for the agricultural land, which is exist inside of the protected forest. The simulated hydrographs from those three scenarios prove that the RTRW has developed with a good approach in terms of hydrological perspective, although the actual condition shows that the catchment has been disturbed by the human activities in the catchment. This is indicated in the simulated hydrograph that by using existing condition has produced much higher discharges and peak flows. In addition, physically the water flow in Palu River shows high sedimentation that indicates the large volume of erosion may have occurred in upper side of the catchment. 58 This page intentionally left blank