Subbasin Case Study

RESULTS AND DISCUSSION

4.1 Processed data for simulation

Curve Number The soil and land cover map per subbasin were combined in QGIS and the corresponding data was extracted. After assigning specific curve numbers for each land cover and soil type combination, the weighted curve numbers and %impervious areas of each subbasin were computed and are shown in Table 4-1.

Table 4-1. Curve number and %impervious area of each subbasin.
Subbasin Weighted Curve Number %Impervious
1 69.20 1.18
2 32.98 3.64
3 53.87 1.95
4 71.53 1.23
5 85.71 1.02
6 68.55 2.50
7 70.99 0.67

The highest curve number was observed for Subbasin 4, which is explained by the dominant heavy soil in the area. Though highest in percentage of impervious area, Subbasin 2
Actual and simulated peak discharge and volume of Subbasin 5.
PARAMETER SIMULATED VALUE ACTUAL VALUE %DIFFERENCE
Peak Discharge (m3/s) 118.2517 121.4857 2.66204
Volume (x103 m3) 4,658.9086 4,506.1047 3.39104

Figure 4-2. Actual and simulated hydrograph of Subbasin 5.

The simulated peak discharge and volume were very close to the actual values with only around 2.7 and 3.4% difference, respectively. This shows that the simulations were accurate. Moreover, Figure 4-2 shows that the simulated hydrograph follows the same pattern as the actual hydrograph; including the same time to peak and peak discharge although the rising limb is not that steep as in the actual hydrograph. The lag time between peak rainfall and peak flow was observed to be 2 hrs.
The subbasin model had a root mean square error of 8.3040 m3/s and mean absolute error of 5.1575 m3/s. Values of the Subbasin 5 parameters before and after calibration are shown in Table 4-4. Table 4-4. Subbasin 5 input values before and after calibration.
PARAMETER BEFORE CALIBRATION AFTER CALIBRATION
Curve Number 85.71 86.8
%Impervious Area 1.02 1.02
Lag (min) 102.1 72
Initial Discharge (m3/s) 50 60
Recession Constant 0.5
SCS Runoff Curve

The HEC-HMS generated graph showing the rainfall-runoff relationship is shown in Figure 4-5. From the figure, it is observed that the trend behaves in a semi-parabolic manner, where the runoff gradually increases as rainfall goes higher. The upward concavity of the line means that precipitation is not entirely converted to runoff. This may be attributed to losses due to infiltration and deep percolation, evapotranspiration, and temporary storage.
The composite curve number of the MBRB was computed to be 70.25. Using the tabulated rainfall vs runoff values generated by SCS, the SCS runoff curve for the whole basin was created. This was drawn in the same graph for comparison with the HEC-HMS generated curve.

Figure 4-5. HEC-HMS generated rainfall-runoff relationship of MBRB in comparison with the SCS runoff