XXIX-B8, 2012
per hectare, R =
wh), K = soil
'ographic or slope
ping-management
factor. All of the
of R and K. The
odel is explained
1e erosivity factor
/ if such data are
sity data are very
ide to determine
, 2001). In River
ll intensity data.
nnual rainfall as
L). The expression
Q)
and R - rainfall
ir time series of
from the Climatic
(CRU-CL 2.0)
rom 1982 to 2002
distance method,
rage interpolation
tribution of mean
soil erodibility
il to erosion and
der standard plot
map in the large
[ational Atlas and
t of Science and
lion scale is used
ctor (LS): The
cs and transport
\ 90 m resolution
ission (SRTM) is
dsrtm/ , and gaps
topo 30 DEM
sp).This rectified
the LS factor as
(1987) presented
lope length or L
(3)
' length (m); m —
' steepness, being
ercent slopes and
of 100 m is used
n of field slope
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
length is made by several researchers (Onyando ef al., 2005;
Fistikoglu and Harmancioglu, 2002; Jain et al., 2001).
Slope Steepness Factor (S):For slope length longer than 4 m,
the slope steepness factor is derived using the following
equations (McCool et al., 1987):
S = 10.8 sin 8 + 0.03 (for slope gradient < 9 %) (4a)
S = 16.8 sin 8 — 0.05 (for slope gradient > 9 96) (4b)
where S = slope steepness factor and 0 — slope angle in degree.
The slope steepness factor is dimensionless.
3.1.4 Cover (C) and Conservation practices (P) factors: The
C factor is derived from NDVI distribution obtained from
Landsat images downloaded (via
http//edesns17.cr.uses.gov/FarihExplorer/) on internet. NDVI
is positively correlated with the amount of green biomass, so it
can be used to give an indication for differences in green
vegetation coverage. NDVI-values are scaled to approximate C
values using the following formula, developed by European
Soil Bureau:
where; a, B are the parameters that determine the shape of the
NDVI-C curve. An a-value of 2 and a f-value of 1 are assigned
to give the reasonable results (after Van der Knijff et al., 2000).
P is the conservation practice factor, reflects the impact of
support practices in the average annual erosion rate. It is the
ratio of soil loss with contouring and/or strip cropping to that
with straight row farming up-and-down slope. As there is only
a very small area has conservation practices in the study area, P
factor values are assumed as 1 for the basin
3.2 Sediment Yield Estimation
The ratio of sediment delivered at a given area in the stream
system to the gross erosion is the sediment delivery ratio for
that drainage area. Thus, the annual sediment yield of a
watershed is defined as follows:
SY = (A) (SDR) (6)
Where, A = total gross erosion computed from USLE, SDR =
sediment delivery ratio. A general equation for computing
watershed delivery ratios is not yet available since they depend
on several properties of the watershed like infiltration,
roughness, vegetation cover, hydrograph or runoff drainage,
etc. Since much of the above data are not available for the study
area to derive SDR, some of the simple models given by
different researchers have been tried to estimate sediment yield
at the outlet of the basin, but the one given below by Williams
and Berndt’s (1972) is finally chosen because it gives
reasonable results despite using few catchment characteristics.
SDR = 0.627 SLP 9^9? (7)
Where, SLP = % slope of main stream channel.
3.3 Spatial Distribution of Soil Loss
After completing data input procedure and preparation of the
appropriate maps as data layers, they are analyzed in the GIS,
to provide a estimate of the gross erosion map on 200 m X 200
m pixel size. Average soil loss is calculated as the product of
each pixel value with pixel area then dividing with total area of
the basin. The USLE model is applied for the following two
scenarios.
3.3.1 Estimation of Average Annual Soil Loss
Average annual soil loss is estimated based on 20-year average
rainfall erosivity factor and K, LS, C, P factors.
3.3.2 Prioritization of Sub-Watersheds
The Indravathi basin is divided into 424 sub-
watersheds for prioritization purpose. Derived average annual
soil loss layer is crossed with the sub-watershed map in a GIS
environment to obtain soil loss in each sub-watershed. Average
soil loss values in t/ha/yr for each sub-watershed are obtained
by using aggregation option of ILWIS in table operation.
Prioritization of sub-watershed has been done on the basis of
average annual soil loss. Estimated values of sub-watershed
wise soil loss are classified as follows (Table 1). P1 is the first
priority category followed by P2, P3, P4, P5 and P6.
Sr. No | Priority Class | Soil Loss (t/ha/yr) | Class
1 P6 <5 Slight
2 P5 5-10 Moderate
3 P4 10-20 High
4 P3 20 — 40 Very high
S P2 40 — 80 Severe
6 P1 > 80 Very severe
Table 1. Soil Loss Categories according to Average Annual
Soil Loss
4. RESULTS AND DISCUSSION
The results obtained by analyzing the data are presented and
discussed in this section. The average annual R factor values
vary from 550 to 670 MJ.mm ha'! h'! with a mean value of 602
MJ.mm ha'! h'! and a standard deviation is 25. The K value in
the study area varies from 0.6 to 0.8. DEM of the study area
revealed that 42 % of area between altitude from 500 m to 700
m. The combined spatial distribution of LS factor is derived
using the DEM of the study area. LS factor values in the study
area vary from 0.2 to 587 with a mean value of 4. Areas with
LS value between 0 and 4 cover 78 % of the catchment area,
and only 10 % of the catchment has LS values greater than 11.
Spatial distribution of C factor was derived for the year 1998
and the C value in the study area varies from 0.1 to 0.3.
After completing data input procedure and preparation of the
appropriate maps as data layers, they were multiplied in the
GIS, to provide a estimate of the gross erosion map on 200 m X
200 m pixel size. Gross erosion map was reclassified (Figure 2)
as per the guidelines suggested by Singh et al. (1992) for Indian
conditions (Table 1). From the model output predictions ( Table
2) it is found that on average, 74.11 Million tons of soil are
moved annually per year and average erosion rate predicted is
18 tons/ha/year. Sediment delivery ratio (SDR) for the
catchment is found to be 0.3 using the empirical equation of
Williams and Berndt. By multiplying the gross erosion with
