SCS-CN Method
Development of Hydrologic Soil Map
The soil map for the Koraiyar basin is extracted from the Harmonized World Soil
Database. The soil map is rectified and mosaicked so that the study area was
extracted from the harmonized world soil database by subsetting it from the full
map. The soil in the basin is classified according to HSG A, B, C, and D as per the
US Natural Resource Conservation Service (NRCS) (USDA 1974). The A-type
soil covers 16.05% with alluvium sandy loam and loamy soil. The spread of
B-type soil is noted in the basin, which has a moderate runoff rate in the basin.
The C-type soil with subtle texture, grey soil, and black soil of 30.93% is distributed
throughout the basin. The D-type of soil such as clay and red ferruginous soils are
also observed in the basin. The characteristics of soil A have low runoff potential and
a high water transmission rate. The soil group C indicates moderate infiltration, and
D-type soil has high runoff potential and low water rate transmission.
SCS-CN Approach for Runoff Calculations
In the early 1950s, the United States Department of Agriculture, Natural Resources
Conservation Service, named Soil Conservation Service (SCS) developed a method
for assessing the volume of direct runoff from rainfall (Pancholi et al. 2015). This
method is often referred to as the CN method empirically developed for the basins.
The standard SCS-CN method is based on the following relationship between
rainfall, P (mm), and runoff, Q (mm). The CN factor is a conversion of potential
maximum retention (Chow et al. 1988), where Q is denoted as collected runoff
(in mm) (Eq. (3.4)). The I a is the initial abstraction (from infiltration, evaporation,
and interception) (mm) as shown in Eq. (3.5). The I a is substituted in Eq. (3.6) to give
Eq. (3.7).
Q ¼
P À I a
ð
Þ
2
P À I a þ S
ð3:4Þ
I a ¼ 0:2 Ã S
ð3:5Þ
S ¼
254000
CN
À 254
ð3:6Þ
where S ispotential maximum retention in (mm); CN is known as a curve number
value
3 Flood Risk Assessment for a Medium Size City Using Geospatial Techniques with. . .
61
Development of Hydrologic Soil Map
The soil map for the Koraiyar basin is extracted from the Harmonized World Soil
Database. The soil map is rectified and mosaicked so that the study area was
extracted from the harmonized world soil database by subsetting it from the full
map. The soil in the basin is classified according to HSG A, B, C, and D as per the
US Natural Resource Conservation Service (NRCS) (USDA 1974). The A-type
soil covers 16.05% with alluvium sandy loam and loamy soil. The spread of
B-type soil is noted in the basin, which has a moderate runoff rate in the basin.
The C-type soil with subtle texture, grey soil, and black soil of 30.93% is distributed
throughout the basin. The D-type of soil such as clay and red ferruginous soils are
also observed in the basin. The characteristics of soil A have low runoff potential and
a high water transmission rate. The soil group C indicates moderate infiltration, and
D-type soil has high runoff potential and low water rate transmission.
SCS-CN Approach for Runoff Calculations
In the early 1950s, the United States Department of Agriculture, Natural Resources
Conservation Service, named Soil Conservation Service (SCS) developed a method
for assessing the volume of direct runoff from rainfall (Pancholi et al. 2015). This
method is often referred to as the CN method empirically developed for the basins.
The standard SCS-CN method is based on the following relationship between
rainfall, P (mm), and runoff, Q (mm). The CN factor is a conversion of potential
maximum retention (Chow et al. 1988), where Q is denoted as collected runoff
(in mm) (Eq. (3.4)). The I a is the initial abstraction (from infiltration, evaporation,
and interception) (mm) as shown in Eq. (3.5). The I a is substituted in Eq. (3.6) to give
Eq. (3.7).
Q ¼
P À I a
ð
Þ
2
P À I a þ S
ð3:4Þ
I a ¼ 0:2 Ã S
ð3:5Þ
S ¼
254000
CN
À 254
ð3:6Þ
where S ispotential maximum retention in (mm); CN is known as a curve number
value
3 Flood Risk Assessment for a Medium Size City Using Geospatial Techniques with. . .
61