Surface Run-off, Soil Erosion and Slope Instability - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 132

5 days antecedent rainfall is considered as important to calculate its cumulative

effects (Schwab et al. 2002 ).
The hydrological condition for the

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first day of a long duration storm is different
to that of 5th day, even if the rainfall in each day remains same. The long time for
accumulation of water to the channel is also an important factor of consideration
(Handbook of Hydrology 1972 ). The authors studied 6 years catastrophic average

rainfall for the month of May, June, July, August. It revealed that last storm
occurred in the month of September and October. July is the maximum catastrophic
rainfall month for all the years. But, 5 days

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' antecedent average rainfall of 90.5 mm

(Table 4.7 ) for the month of Sept 2013 is taken into account and on the basis of

which weighted curve number is converted from AMC-II to AMC-III following
Schwab et al. ( 2002 ) conversion table (Table 4.8 ) to calculate surface run-off from
the Shivkhola Watershed.

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4.2.3 Estimation of Discharge (in m 3 ) from Individual
Sub-watershed

The effectiveness of Remote sensing Data using sophisticated technique of SCS
Curve Number for determining runoff and discharge of watersheds arising out of a
certain rainfall is studied by Tiwari et al. ( 1991 ) and Schwab et al. ( 2002 ). In the
present study the possibilities for conservation and treatment of surface water
draining down the six sub watersheds are analyzed. The calculation of surface run-
off from the individual sub-watersheds is associated with the following steps.

cation and marking up of the sub-watersheds draining the
water from its upslope contributing area up to the point of junction with
main stream.
Step-II The division of the main basin into 6 sub-watersheds according to
convenience of further calculation.
Step-III Analysis of SOI Topographical map (1987), LISS III Satellite Imagery-
2002, Google Earth Image and intensive

Step-I The identi

field investigation for sub-

cation (Tables 4.1 and 4.9 ).
Step-IV Determination of Curve Number under different hydrological condition
for different hydrological soil group and for individual land use in AMC-
II following empirical curve number table.
Step-V The Curve Number in AMC II condition for each land use category is
then applied in order to estimate Weighted Curve Number (Ragan et al.
1980 ) for each sub watershed applying the following formula.

watershed wise land use classi

CN 1 A 1 þ CN 2 A 2 ......... CN n A n

Weighted CN ¼ ð 4 : 4 Þ

A 1 þ A 2 ............... A n