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Based on the hydrologic soil groups, the CN values were calculated using an unsupervised classification followed by a supervised classification of a 2010 SPOT-5 image (before urbanization) and a 2015 Landsat-8 image (after urbanization). These values were validated using field check activities. Reasonable results were obtained from the classification, where the Kappa coefficients of Congalton (1991) showed an average commission of 92% and 93% for the urban, soil, rock, and vegetation areas. Based on this classification, the infiltration rates into the catchment were assigned to groups B and C (Table 4 and Figures 2, 8(a) and 8(b)). The percent of imperviousness for each sub-basin is estimated based on current and proposed land-use types (Figures 8(a) and 8(b)). As per the NRCS CN tables and other relevant data, a composite CN of 81.59 is assumed for the 2010 land-use map. However, Harrat (basalt rocks), sand and gravel, vegetation, and fine barren sandy and silty soils cover the area (Table 4). Urbanization is taking place rapidly in the southwestern part of the studied watershed. Accelerated urban expansion in 2015 is causing changes in catchment land-use, increasing the composite CN to 83.71. Comparison of the two successive land-use maps (2010–2015) shows that urbanization increased by 8% of the total area (Table 4 and Figures 8(a) and 8(b)).
Table 4

Calculations of the weighted CN in the study area (2010 and 2015)

  Land-use/coverArea (A) km2Curve number (CN)A*CNW.CN
2010 Harrat (basalt) 17.12 86 1472.32  
Sand and gravel 9.12 76 693.12  
Vegetated area 7.17 78 559.26  
Fine sand and silty soil 3.29 82 269.78  
 Σ 36.7  2,994.48 81.59 
2015 Harrat (basalt) 16.64 86.00 1,431.04  
Sand and gravel 6.32 76 480.32  
Vegetated area 6.14 78 478.92  
Fine sand and silty soil 3.08 82 252.56  
Urban-up area 4.52 95 429.40  
 Σ 36.70  3,072.24 83.71 
  Land-use/coverArea (A) km2Curve number (CN)A*CNW.CN
2010 Harrat (basalt) 17.12 86 1472.32  
Sand and gravel 9.12 76 693.12  
Vegetated area 7.17 78 559.26  
Fine sand and silty soil 3.29 82 269.78  
 Σ 36.7  2,994.48 81.59 
2015 Harrat (basalt) 16.64 86.00 1,431.04  
Sand and gravel 6.32 76 480.32  
Vegetated area 6.14 78 478.92  
Fine sand and silty soil 3.08 82 252.56  
Urban-up area 4.52 95 429.40  
 Σ 36.70  3,072.24 83.71 
Figure 8

(a) Land-use map of the study area classified according to: (a) SPOT-5 Satellite image (2010); (b) Landsat-8 Satellite image (2015).

Figure 8

(a) Land-use map of the study area classified according to: (a) SPOT-5 Satellite image (2010); (b) Landsat-8 Satellite image (2015).

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