Simulation of Surface Water Retention using HEC-GeoHMS Model (Case Study: Upper Ciliwung Watershed, West Java)

Surface water retention is the ability of the soil in retaining the water (water storage). One of the ways that use to be as surface water retention is waterworks through the reservoir. The reservoir is expected to accommodate water and increase infiltration. The optimizing of the reservoir is a part of runoff management, which in the process, the rainwater is retained as long as possible in somewhere before entering the sewer so that the amount of the flow can be lowered in order to reduce the flood in a drainage area or a basin river. Flooding can occur due to the inability of a watershed in retaining and absorbing the water that falls to the surface of the earth, which in turn, the overload of water turns to be runoff and then flowing through the river to a downstream or a sea. One of the watersheds that undergo the environmental damage is the Upper Ciliwung watershed. The analytical approach in the study of the hydrological watershed system is a theoretical basis for integrating the components information of a watershed system into a hydrological model. One of hydrological models application is HEC-GeoHMS Model (Hydrologic Engineering Centre-Geospatial Hydrologic Modeling) which is effective in reducing the runoff. The model proved the effectiveness from the data calibration which was seen by the value of R2 from 0.73-0.87 and value of NSE from 0.52-0.85. Meanwhile, validation data value showed by the R2 from 0.69-0.97 and value of NSE from 0.80-0.88. Simulations with the implementation of Ciawi and Sukamahi Reservoirs reduced runoff by 70 % from the actual condition.