Applicability of the Hydrological Simulation Program-FORTRAN (HSPF) for Modeling Runoff and Sediment in HawaI‘i WatershedsAbstract
The goal of this project was to assess the applicability of the watershed model HSPF for simulating the hydrological and sedimentation processes occurring in Hawai‘i’s watersheds. Specifically, the study determined if the model could be calibrated and validated at various temporal resolutions. A sensitivity analysis was also conducted to identify parameters that were most influential. The results of the project evaluated the potential use of the model in Hawai‘i by identifying its strengths and weaknesses.
The model parameters were initially estimated based on values found in reviewed literature for both HSPF parameters and Hawai‘i specific data. The parameters were then adjusted to minimize the difference between observed and simulated values. First, the water flow calibration was conducted by comparing the simulated and observed streamflow. Second, a hydraulic calibration was performed using the hydraulic parameters such as channel width, cross-sectional area, and velocity. Third, the sediment calibration was conducted using the suspended sediment discharge (SSD) data. The model’s performance was evaluated based on relative percent errors, regression analysis, and other selected statistical parameters. Finally a sensitivity analysis was conducted to determine which parameters the model outputs (runoff and sediment) are most responsive to.
Flow data for water years 2002, 2003,and 2004 were used for calibration while flow data for water years 1996, 1997, and 1998 were used for validation of the hydrology. For suspended sediment, water years 1995 and 1996 were used for calibration and water years 1993 and 1994 were used for validation.
Based on both the graphical and statistical analyses for the results, the hydrology calibration and validation were acceptable for annual, monthly, and weekly, yet was not acceptable for daily flows. Further, graphical and statistical results for the sediment calibration and validation showed that the annual SSD calibrations were acceptable, yet the monthly, weekly, and daily calibrations were not acceptable. In addition, the sediment validation was acceptable for annual SSD only if the two most extreme events excluded, yet it was not acceptable for monthly, weekly, and daily SSD.
Based on a sensitivity index, several of the parameters were found to have a greater influence on the flow and sediment. However, such a prediction is not accurate, considering that the approach does not consider the interactions and interrelationships between various parameters.
Given these results, HSPF may be used under typical Hawai‘i conditions (i.e., steep slopes, temporally and spatially varied rainfall, and diverse land use) to simulate hydrology for a temporal resolution as fine as weekly. However, errors are still likely due to model and data limitations. Furthermore, based on the results, HSPF may be used to predict annual sediment values under typical Hawai‘i conditions. Yet the model may also give insights on estimating the monthly sediment loading. The model was not validated for the monthly sediment values. It is likely that this was due to the model’s inability to account for mass wasting.
Simulated and observed mean weekly flow hydrograph for the hydrology calibration period