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Malaysia Water Research Journal
study, after the initial calibration, LZSN was set at 0.16 m and held constant for all
land uses. The upper zone nominal storage UZSN, a user specified parameter was
set according to EPA BASINS Technical Note 6 (USEPA, 2000). Donigian et al. (1978)
assumes UZSN as 0.06 of LZSN for steep slopes, limited vegetation, low depression
storage; 0.08 of LZSN for moderate slopes, moderate vegetation, and moderate
depression storage; 0.14 LZSN for heavy vegetal or forest cover, soils subject to
cracking, high depression storage and very mild slopes and final value is set to
be 0.55 inch. For Skudai Watershed, 0.08 of LZSN is used since it has moderate
slope (0.5%) and moderate vegetation and depression storage. Since surface
runoff and baseflow indicate direct contribution of water quality simulation, the
fraction of groundwater inflow DEEPFR was set at 0.2 to ensure model stability.
For the mean soil infiltration rate parameter, INFILT possible value was set to be
0.24 in/hr and may decrease to produce more upper zone and interflow storage
water. Excessive value of INFILT can lead to a greater overland flow and interflow
in Skudai river. To have a better picture of these four parameters sensitivity, Table
4 summarizes the detail analysis of these assessment.
Table 4: Parameters sensitivity analysis for Skudai River
Parameters unit in (inch) Runoff in (%)
INFILT Increased 0.05 Increased 6.78
DEEPFR Increased 0.1 Decreased 14.96
UZSN Increased 0.3 Decreased 20.52
LZSN Decreased 1.0 Increased 5.94
Based on the sensitivity analysis performed, it is concluded that runoff
volume was highly sensitive to INFILT then to DEEPFR, UZSN and LZSN. High INFILT
caused less water to flow out as runoff and low DEEPFR increased runoff, resulting
reduced fraction of groundwater inflow.
Model Calibration
Calibration of the model was conducted between simulated and observed
flow monitoring data from Kampung Pertanian station. During the calibration
process, each of parameters were adjusted to match the observed and
simulated flows. Initial value parameters were adjusted one by one within typical
range and keeping others constant. The simulated output values were then
compared with the initial values to check its sensitivity with total runoff volumes.
A time-series plot of the observed and simulated hourly flows (cfs) in Figure 3
shows simulated monthly yields generally were within 10.09% of recorded values,
although there were some unbalanced between first and third months. As shown
in Table 5, simulated runoff volumes of November are consistently underestimated
compared to observed runoff volumes may be due to low rainfall at Kampung
Pertanian gauging station. The coefficient of determination (R2) indicates that
the model gives almost 74% of total variability in the observed data at an hourly
Institut Penyelidikan Hidraulik Kebangsaan Malaysia (NAHRIM)
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National Hydraulic Institute of Malaysia (NAHRIM)