Table 3. Default and Final Values of SWAT calibration parameters for flow and sediment. 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
Variable Parameter File iMet* | Range Default Value Final Value 
Flow Alpha_Bf .gw 1 [0, 1] 0.048 0.26 
Ch K2 rte 1 0-150 0 25 
Cn2 .mgt 3 [-25, 25] varied by LU 1.23745 
Esco .hru 1 [0, 1] 0 1 
Gwqmn ‚ZW 2 [0,1000] 0 -263.22 
Sediment | Ch Cov rte 1 [0, 1] 0 0.601 
Ch Erod rte 1 [0, 1] 0 0.400 
Spcon .bsn 1 [0.0001, 0.01] 0.0001 0.003 
Spexp .bsn 1 [1. 2] 1 1.420 
Usle P .mgt 1 [0, 1] 1 0.981 
0.25 0.233** 
Usle_C crop.dat | 3 [-25, 25] 0.3 ().270 x 
0.01 0.000 * ev» 
  
  
  
  
  
  
* variation method: 1 = replacement of initial parameter by value, 2 = adding value to the initial parameter, 
3 = multiplying initial parameter by a value in percentage 
** for AGRL ; *** for AGRR; **** for FRST 
of data. For model calibration, the 1984 daily stream flow (in 
liters/sec) data and 2002-2005 monthly sediment (in ppm) data 
are used. Meanwhile, the 1985-1986 daily stream flow data 
and 2006-2007 monthly sediment data are used for model 
validation. Daily stream flow data for years 1984 and 1985 are 
used because these are the only period with almost complete 
records. BRS do not have daily records for sediments. Only 
monthly records are available for sediment data and the period 
with the most number of records are 2002-2007. No sediment 
data are available for years earlier than 2002 and data for years 
later than 2007 are fragmental (i.e., more than six months are 
without data). The final values of sediment parameters and five 
most sensitive flow parameters are shown in Table 3. Plots of 
the observed and simulated flow and sediment yields are shown 
in Figures 4 and 5. 
Table 4. Model Performance during calibration. 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
Variable Calibration 
Period | TimeStep | NSE |. R^. | RSR | PBIAS 
Daily 0.89 | 0.74 | 034 | 17.64 
Pow 1954 Monthly | 0.47 | 0.83 | 0.73 | 17.75 
Semon] 2000 Monthly | 0.96 | 0.93 | 0.20 | -7.30 
2005 Annual | 0.99 | 0.97 | 0.11 | -12.31 
Table 5. Model Performance during validation. 
Variable Validation 
Period | Time Step | NSE | RZ | RSR | PBIAS 
Daily 0.89 | 0.74 | 034 | 17.64 
Flow Jose Monthly | 047 | 0.83 | 0.73 | 17.75 
 Sediment | 2002: Monthly | 0.96 | 0.93 | 020 | -7.30 
2005 Annual [| 099 | 097 | 0.11 | -12.31 
  
  
  
  
  
  
  
5. RESULTS AND DISSCUSSION 
5.1. Base Scenario 
The soil loss rate map derived from sediment yield for the base 
scenario is shown in Figure 3. The minimum and maximum 
values for this scenario are 0.45 ton ha'! yr! and 12.05 ton hz'! 
yr! which correspond to subbasins 28 and 21, respectively. The 
simulated maximum value is beyond the upper limit of 
tolerable soil loss of 11.2 t ha'! yr! according to Hudson (1995) 
as cited by Alibuyog et. a/ (2009). Subbasin 28 is characterized 
by a relatively flat terrain with the whole area having slope less 
than or equal to 17% and predominantly an agricultural area 
(AGRR) while most of the subbasin 21 area have steep slopes 
(>17%) and with almost equal distribution of RNGE, AGRR 
and FRST areas. 
     
   
Legend 
  
Reach 
Uhalyr 
1 045-177 
Figure 3. Soil loss rate map of the basin under the base scenario. 
5.2. Model Simulation incorporating Climate Change 
Climate change data are incorporated in the model by inputting 
the projected seasonal change in rainfall and temperature for 
each subbasin. After manipulating the subbasin parameters for 
climate change analysis (RFINC and TMPINC), the calibrated 
model was rerun for two scenarios (A1B and A2) each under 
two time slices centered at year 2020 and 2050. These two 
scenarios have been the focus of climate change model inter- 
comparison studies according to IPCC (2007). The average 
total generated sediment yield of the whole basin for this run is 
shown in Figure 6a. 
5.3. Model Simulation incorporating Land Use/Land Cover 
Change 
The projected land cover change rates derived from subtracting 
the sets of Landsat images are inputted to the model by 
modifying the individual HRU files (with file extension *.hru) 
for each subbasin. The calibrated model is rerun with the 
modified HRU files and the average sediment yield for the 
simulation period is evaluated. The result of this run is also 
shown in Figure 6a. 
5.4. Model Simulation incorporating Climate Change and 
Land Use/ Land Cover Change 
Using all the modified files in Sections 5.2 and 5.3, a rerun of 
  
     
    
    
     
   
  
    
   
  
   
    
    
  
    
   
    
    
   
    
      
    
     
     
     
       
  
    
   
     
  
   
  
  
    
    
    
   
    
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