Surface Run-off, Soil Erosion and Slope Instability - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 125

4.2.1 Determination of Runoff Curve Number (RCN)

The Runoff Curve Number (RCN) is a quantitative descriptor of the land and soil
complex and is commonly assigned based on information acquired from eld

surveys and interpretation of Aerial photograph or Satellite Imageries. The RCN of
a Soil

c antecedent moisture
condition (AMC) takes on values from 0 to 100. This number is derived from the
character of the soil, vegetation, including crops and the land use of that soil as well
as intensity of use. When CN equals to 100, S becomes zero (In water-logged areas
or in wet paddy

— Vegetation — Land (SVL) complex in a speci

field). This leads to Q (Runoff) = P (Precipitation). In other cases,

when S 0, this gives Q =0.

The CN value is derived on the basis of Soil

, CN →

→ ∞

Land (SVL) and
Antecedent Moisture Condition complex. The soils are grouped into four types
according to their hydrologic character. The land use, with respect to their hydro-
logic i.e. draining condition is to be analyzed with due importance. The determi-
nation of CN value requires the following data input

— Vegetation —

1. Land use Land cover class.

2. Hydrologic Soil groups.
3. Hydrologic (Draining) condition.
4. Antecedent Moisture Condition (AMC).

—

4.2.1.1 Land Use and Land Treatment

Land use is the watershed cover and it includes every kind of vegetation, litter and
mulch and fallow as well as non agricultural uses such as water surfaces (lakes,
swamps etc.) and impervious surfaces (roads, roofs etc.). Forest area includes all
lands classed as forest under any legal enactment dealing with forest or adminis-
tered as forest whether state owned or private. Tree crops include woody perennial
plants that reach a mature height of at least 8 ft and have well de

ned stems and a

de nite crown shape. Plantation is an important land use of the study area. Tea is

the single plantation crop of this region and occupies a considerable section of land,
and exerts a great influence on the economy and culture of the concerned society.
Open space includes the parks, golf course, cemeteries, lawns etc. where the grass
or trees cover ranges from greater than 50 % to less than 75 %. Urban district
category reflects the commercial, business and industrial sectors of the study area
where 70 85 % areas is impervious in nature. Residential district means the urban

residential houses, either reflects the compact settlement area or the scattered houses
of tea gardens, forest etc. and here 65 to 12 % area is impervious in nature. Land
use and land cover map of the Shivkhola watershed is prepared with the help of
LISS-III Satellite Image, Google Earth Image in consultation with Survey of India
(SOI Topo-sheet. Later on, with proper ground truth veri

-

cation with GPS a land
use and land cover map is developed. In the present study broadly the Shivkhola