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3.4.2.4 Areal Rainfall Estimation
The HEC-HMS model requires daily time series precipitation data as the input. The total of precipitation data from all meteorological stations were located
inside and outside the catchment area is used to estimate the rainfall amount. A daily areal rainfall of catchment will calculated from daily point measurement of
meteorological stations using Thiessen Polygon Methods. The Thiessen Polygon is one way of calculating areal precipitation. This
method gives weight to station data in proportion to the space between the stations. The area of each polygon inside the sub-basin, as a percentage of the
total sub-basin area, is calculated. This factor is then used as the weight of the station situated within that polygon. The Thiessen weight for each station was
calculated for each sub-basin in Table 3.4. The precipitation for the whole area is then calculated as follows:
= 1
… … … … … … … … … … … … … … … … … … … … … … … … … 7
Where: P
= Averages areal Rainfall Pj
= Rainfall measured at each station Aj
= Area of each polygon inside the basin Atot = Total sub-basin area
Table 3.4 Metrological station weight calculated using theissen polygon method
Rainfall Station Sub‐basin
1 2
3 4
5 6
7
Bangga Atas 0.192
0.042 0.047
Bangga Bawah 0.056
0.104 Bora
0.109 0.341
0.186 Kalawara
0.002 0.316
0.456 0.039
mantikole 0.285
0.002 0.162
Mutiara 0.009
0.652 Palolo
0.038 0.733
0.542 0.608
0.001 Tuwa
0.749 0.581
0.006 0.099
Wuasa 0.381
0.261
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3.5 Model Calibration and Verification
Model calibration is process for fine-tuning of the input parameter data; the performance of the model will be improved. Hydrological models require the
procedure of adjusting values of the model input parameters to match model output with measured field data for the selected period and situation entered to the
model Rientjes, 2007. To assess the model performances in terms of hydrological responses to
land cover changes, the validation processes are needed. The simulation results of this model by using different rain gauges will compare using statistical criteria.
The statistical criteria are Nash-Sutcliffe coefficient and relative volume error. The efficiency of Nash-Sutcliffe coefficient Nash and Sutcliffe, 1970 is
defined as:
= 1 − −
− … … … … … … … … … … … … … … … … .. 10
Where Qot
i
and Qsti are observed and simulated daily discharge at time step t
i
respectively and is mean observed daily discharge and N is the total
number of time steps. The relative volume error of this research is needed to calculate in order to
examine the model performances. Relative volume error of this model will calculate using equation 11.
= −
100 … … … … … … … … … … … … . . 11
Where RV
E
is relative volume error, Qot
i
and Qst
i
are observed and simulated daily discharge at time step t
i
respectively.
3.6 Responses of River System to Land Cover Changes on the Catchment
Area
The changes of land cover gave indirect impact to the hydrological responses on the river system. Generally, the impacts of land cover changes in
long terms are changes of the river characteristics such the changes of river discharge extremely and the increasing of runoff. Assessing the effect of land
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