This paper presents the simulation of hourly rainfall-runoff relationship with appropriate hydrologic parameters on selected three catchments; Bedup, Batu Gong and Rayu catchments, located at southern region of Sarawak, Malaysia. Hydrologic model selected for this study is Hydrologic Engineering Centre-Hydrologic Modeling System (HEC-HMS). The model parameters were optimized with combination of initial and constant loss, Clark unit hydrograph, recession baseflow and Muskingum routing methods. Each of these hydrologic parameters was calibrated automatically through optimization manager function that built within HEC-HMC model. To ensure the robustness of optimal parameters obtained, each catchment will be validated with three independent storm events. The accuracy of simulation results was measured with coefficient of correlation and peak error. Two methods were applied to obtain time of concentration (T_c) and storage coefficient (R), which are the fundamental parameters for Clark instantaneous unit hydrograph. First method was obtained through optimization manager function that build within HEC-HMS, and second method was through calculation according to Hydrological Procedure No. 5 (1974). The results revealed that the optimized T_c and R are very close with calculated T_c and R. The relationship between R and catchment area (A) was then established through a linear equation. The relationship between R and A for southern region of Sarawak is represented as R = 0.0535A - 1.4093. Establishment of this equation will enable the hydrologists and researchers to estimate the T_c and R values by just determining the catchment area, especially for the ungauged catchments at southern region of Sarawak.

Hydrologic modeling in water resource management is crucial to predict the rainfall-runoff in a watershed. The phenomena of rainfall transformation into runoff remains complex as the consequence of hydrologic cycles and climate characteristics are non-uniform, due to the influence of hydrology parameters including percolation, infiltration, groundwater discharge and storage, surface runoff and evaporation in a catchment. These parameters represent the characteristics of the watershed and may vary for different watersheds.

However, it is really difficult to obtain accurate and suitable parameters for a study catchment. The relationship among parameters for different catchments varies and is even computed with the same hydrologic model. Therefore, this study was carried out using HEC-HMS modeling system to analyze the model parameters for three catchment areas, located in Kuching and Samarahan divisions, southern region of Sarawak, Malaysia. Hourly rainfall and runoff data that obtained from Department of Irrigation and Drainage (DID) Sarawak are fed into HEC-HMS model to obtain the parameters.

The model is built with the combination of initial and constant loss, recession baseflow and Muskingum routing method. Instantaneous unit hydrographs are derived using Clark method. Clark method requires two fundamental parameters, time of concentration (T_c) and storage coefficient (R). Clark's T_c is the time of travel required by last drop of rainfall excess at the hydraulically most remote point in the catchment to reach the channel network (Straub et al., 2000). Jawed (1973) described that R is equal to discharge at point of inflection on observed hydrograph divided by the slope at that point. Calibration process is conducted to obtain the model parameters using optimization manager function that built within HEC-HMS. The optimal parameters obtained will then be validated with three independent storm events for each study catchment. Model performance is evaluated with coefficient of correlation and peak error to justify the accuracy of the calibrated parameters in the modeling.

Soil And Water Sciences Journal, Hydrologic Parameters, Muskingum Routing Methods, Water Resource Management, Hydrologic Modeling, Postfire Response Modeling, Rainfall Gauging Stations, Hydrometric Station, Rayu Watersheds, Model Calibration, Meteorological Model, Dendrite Watershed Systems, Meteorological Data Series, Clark Method, Watershed Storage, Validation Processes.