Mery Apple
Applicability
of the Hydrological Simulation Program-FORTRAN (HSPF) for Modeling
Runoff and
Sediment in HawaI‘i Watersheds
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.
