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1servation
IAPRS & SIS, Vol.34, Part 7, “Resource and Environmental Monitoring”, Hyderabad, India, 2002
3.2 Sediment Yield Index (SYD) for watershed
prioritization
Watershed was prioritized following sediment yield index
(SYI) approach of All India Soil and Land Use Survey (AIS
LUS, 1991) described by Equation 7.
SYI- » (EI*Aie*d/ Aw) * 100 (7)
Where SYI = Sediment Yield Index
El = Weight value of erosion
Aie = Area of the erosion intensity / composite unit
d = Delivary ratio in percentage
Aw = Total area of watershed
Physiography map, slope map derived from DEM and land use
/ land cover map were combined to generate composite map
with classes of various combination of physiography, land use
and slope class. These units were assigned weightage values
ranging 13 to 19 and were added to attribute table. The delivary
ratio values were assigned to different sub-watershed varied
from 0.60 to 0.85. The SYI was computed for each sub-
watershed using equation 7. The SYI values were grouped into
priority category codes following the standards adopted by AIS
LUS.
3.3 Land capability classification
Treatment-oriented land capability classification (Sheng, 1972)
was used to assess the capability of land for suggesting
conservation measures. The land capability classification
consider the terrain slope and soil depth to assess the capability
class.
4. RESULT AND DISCUSSION
Umkhen watershed was divided into 29 sub-watershed for
prioritization on the basis of average soil loss and silt yield
index. Nine physiographic units were identified in the
watershed to study soils. The soils are shallow to deep in depth
and well to moderately well drained. Soils belongs to loamy
skeletal, coarse loamy and fine loamy textural class. Slope map
was generated from DEM reveals that 7.25 per cent land are
nearly leveled whereas 29.60, 35.63 and 27.52 percent land are
undulating/ rolling, moderately sloping and steep sloping,
respectively. Land use / land cover map prepared from merged
LISS III + PAN data on scale 1:25,000 show that 67.32 percent
area of watershed is under pine forest with varying density
classes viz., moderate forest (30.79 %), open forest (28.38 %)
and dense forest (8.15%) whereas 26.26 percent area falls under
agriculture comprising areal extent under fallow (upland),
shifting fallow, bun agriculture and fallow (valley fills) are
11.44, 8.32, 4.90 and 1.60 percent, respectively.
Figure 1. Land use / land cover in Umkhen watershed
Erosion hazard was assessed following USLE model and
grouped into five classes ranging from very slight to very
severe as presented in Table 1 and Figure 2. Erosion hazard
analysis reveals that 33.83 percent area experience very severe
erosion hazard followed by severe ( 22.95 %), moderate (19.91
%) and slight (13.95 %) erosion hazard in the watershed.
Soil loss Erosion Area in ha. Area in %
t/ha/year hazard
class
<3 Very slight 1567 9.37
3-10 Slight 2336 13.94
10-25 Moderate 3337 19.91
25-50 Severe 3847 22.95
>50 Very 5670 33.83
severe
Table 1. Erosion hazard classes based on USLE model
HEC
*
ii
iru
LL LLLA
H]
Figure 2. Soil erosion hazard in the Umkhen watershed
Land use land cover map was intersected with erosion hazard
(soil loss) map to predict average soil loss in various land use /
land cover. Among agriculture land, valley fills area has slight
erosion hazard (average 6.16 t/ha/yr) due to nearly level
topography and well management practices. Fallow upland and
shifting fallow land witness severe erosion hazard (average loss
54.48 to 83.05 U/ha/yr) because of moderate to steep sloping
topography with poor management practices, Highest average
soil loss was noticed (average loss 124.55 t/ha/yr) in barren /
scrub lands due to absence of adequate vegetal cover associated
with moderate to steep sloping terrain. Area under dense pine
forest has moderate erosion hazard ( average 23.33 t/ha/yr)
whereas moderate pine forest and open pine forest area have
severe erosion hazard (average loss 39.98 to 45.79 t/ha/yr).
Erosion hazard map was intersected with sub-watershed map to
compute weighted average soil loss in each sub-watershed to
determine priority of sub-watershed as high, medium and low
priority class. Analysis reveals that 6 sub-watershed belongs to
high , 15 sub-watershed to medium and 8 sub-watershed to
low priority category.
Sediment Yield Index ( SYI) was computed to prioritize the
sub-watershed for conservation planning. Composite map was
generated by integrating physiography, land-use, soil texture
and slope resulting 16 different Erosion Intensity Units (EIU).
Weightage values and delivery ratio values for each EIU were
assigned. Sediment Yield Index (SYI) of the sub-watersheds
ranging from 849 to 1194 (Table 2). The study reveals that 11
sub-watershed belong to high, 11 sub-watershed to medium
and 5 sub-watershed to low priority class, respectively.