Calibration and validation of a physically distributed hydrological model, MIKE SHE, to predict streamflow at high frequency in a flashy mountainous Hawaii stream

Hawaii streams are short and steep, often producing dangerous flash floods as a result of rainfall events that can be short but intense. The streamflow can change by a factor of 60 in only 15 min. Using streamflow data collected at 15-min intervals, the physically distributed modeling system, MIKE SHE, is applied to the Manoa-Palolo stream system on the island of Oahu, Hawaii, to study the watershed response to storm events. Because of the unavailability of detailed spatially distributed data, a single-valued hydraulic conductivity for the saturated zone is used as the representative of the entire watershed. It is shown that a well-calibrated MIKE SHE with the single-valued hydraulic conductivity is able to produce consistent results with correlation coefficients greater than 0.7. The rainfall distribution along the watershed is the driving factor for the estimation of streamflow. The reciprocal of Manning's roughness coefficient (M) for the watershed and the hydraulic conductivities (vertical and horizontal) of the saturated zone had the most pronounced effects in determining the shape of flood peaks. The peak streamflow is reduced by nearly 1 m³/s for an M value that was changed from 60 to 10. For the upper part of the watershed, which is located in the rainiest and steepest mountainous area, the horizontal hydraulic conductivity value of the saturated zone is insensitive, while the horizontal and vertical hydraulic conductivity values of the saturated zone are sensitive to predict streamflow for the entire watershed. Drainage depth, an average position of phreatic surface above which the water table in one grid starts to drain to the nearest grid or stream, is less sensitive, while drainage time constant, the time required to discharge the drainage water to the nearest grid or stream, is more sensitive for the estimation of base flow. Because calibration for a large basin at small time steps (e.g., every 15 min) takes a long time to complete a year of simulation, splitting the entire watershed into subwatersheds during calibration was useful in examining the effects of key parameters on streamflow estimation before calibrating the parameters for the entire watershed.