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The SPI values for the Bilate catchment in Figure 3(d) range from −14 to 13. The SPI of the study area is divided into five levels according to the flood susceptibility, very high (−14 to (−6.3)), high (−6.2 to (−1.7)), medium (−1.6 to 0.31), low (0.32–0.34), and very low (3.5–13), respectively (Figure 3(d)). The area with a high SPI implies the area of high exposure to the erosive power of runoff and soil erosion (Table 6). The increasing value of the SPI resulted from a high slope and high flow rate of water in a given area. As a result, the amount of water contributed to the upward slope and increased water flow rate, resulting in a high rate of the SPI value and sediment deposition downstream of the catchment. That is why, the downstream of the watershed had a very high flood susceptibility in comparison to the upstream of the watershed. The SPI determines the capacity of a stream to carry sediment. As a result, the SPI values in most flood-affected regions are the lowest (−14 to (−6.3)).
Table 6

The SPI factor of the Bilate catchment

SPIArea (km2)Area (%)
−14 to (−6.3) 2,800.7 49.8 
−6.2 to (−1.7) 387.9 6.9 
−1.6 to 0.31 431.3 7.7 
0.32–3.4 1,777.2 31.6 
3.5–13 227.8 
Total 5,625 100 
SPIArea (km2)Area (%)
−14 to (−6.3) 2,800.7 49.8 
−6.2 to (−1.7) 387.9 6.9 
−1.6 to 0.31 431.3 7.7 
0.32–3.4 1,777.2 31.6 
3.5–13 227.8 
Total 5,625 100 
Figure 4

Flood susceptibility map.

Figure 4

Flood susceptibility map.

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