2. STUDY AREA
The Indravati is one of the northern tributaries of the Godavari
in its lower reach. The Indravati catchment lies between
latitudes 18 27' N to 20 41' N and longitudes 80° 05" E to 83°
07° E (Figure 1). The river Indravati rises at an altitude of about
914 m near Thuamal Rampur village in the Kalahandi district
of Orissa on the western slopes of the Eastern Ghats and joins
Godavari at an altitude of about 125 m. The main river flows
for a length of about 477 Km. The Indravati basin with a
catchment area of 41285 Km? constitutes 13.32 % of the total
Godavari basin. The basin has high hills, deep valleys and large
plateaus. The mean annual rainfall in of this area is about 1288
mm, most of which occurs between May and September.
Average potential evaporation rates are 6.5 mm per day, while
average minimum and maximum temperature are 13°C and 39
‘C respectively. There are no major irrigation projects existing
in the study area. The major land covers in the catchment are
forest (68 %), followed by agriculture (22 96). Agriculture is the
main occupation of the people in the area.
Basin-sub basin map of India
Godavari Basin
XC NS
60 km
Figure 1. Location map of Indravati Catchment
3. METHODOLOGY
3.1 Soil Erosion Model - USLE
Techniques for prediction of soil loss have evolved over the
years. The most widely used equation for soil loss prediction of
the catchment is the Universal Soil Loss Equation (USLE). The
USLE equation computes average annual soil loss (A) which is
a product of five different factors that affect soil loss, and is
given by:
A-RKLSCP (1)
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B8, 2012
XXII ISPRS Congress, 25 August - 01 September 2012, Melbourne, Australia
Where, À = average annual soil loss in tons per hectare, R =
rainfall-runoff erosivity factor (MJ/ha.mm/h), K = soil
erodibility factor (t.ha.h/ha/MJ/mm), LS = topographic or slope
length/steepness factor, C = cover and cropping-management
factor, P = supporting practices (land use) factor. All of the
factors are dimensionless, with the exception of R and K. The
preparation of spatial data base for this model is explained
below.
3.1.1 Rainfall Erosivity Factor (R): The erosivity factor
R is often determined from rainfall intensity if such data are
available. In majority of cases rainfall intensity data are very
rare, consequently attempts have been made to determine
erosivity from daily rainfall data (Jain er al., 2001). In River
Indravati catchment, no station has rainfall intensity data.
Therefore R is determined using mean annual rainfall as
recommended by Morgan and Davidson (1991). The expression
is given below.
R=P*05 Q)
Where, P = mean annual rainfall in mm and R = rainfall
erosivity factor in MJ/ha.mm/h. A 20-year time series of
monthly girded average precipitation dataset from the Climatic
Research Unit -Average Climatology 2.0 (CRU-CL 2.0)
(http-//Www.cru.uea.ac.uk/cru/data/tmc.htm) from 1982 to 2002
is used in preparing R factor layer. Inverse distance method,
which is very fast and efficient weighted average interpolation
method in ILWIS, is used to show spatial distribution of mean
R factor values in Indravati catchment.
3.1.2 Soil Erodibility Factor (K): The soil erodibility
factor (K) represents both susceptibility of soil to erosion and
the amount and rate of runoff, as measured under standard plot
condition. In the study area no detailed soil map in the large
scale is available. The soil map prepared by National Atlas and
Thematic Mapping Organization, Department of Science and
Technology, Government of India on 1: 2 Million scale is used
to prepare K factor.
3.1.3 Slope length and steepness factor (LS): The
topography affects the runoff characteristics and transport
processes of sediment on a watershed scale. A 90 m resolution
DEM from the Shuttle Radar Topography Mission (SRTM) is
downloaded from ftp;//eOmss21u.ecs.nasa.gov/srtm/ , and gaps
of no data is filled with coarser Gtopo 30 DEM
(http/pdaac.usgs.gov/etopo30/hvdro/index.asp).This rectified
90 m resolution DEM is used to prepare the LS factor as
discussed below.
Slope Length Factor (L); Mc Cool et al. (1987) presented
the following relationship to compute the slope length or L
factor:
L = (W22.1)" (3)
where L = slope length factor; À = field slope length (m); m =
dimensionless exponent that depends on slope steepness, being
0.5 for slopes exceeding 5 percent, 0.4 for 4 percent slopes and
0.3 for slopes less than 3 percent. A grid size of 100 m is used
as field slope length (4). Similar assumption of field slope
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