Surface Run-off, Soil Erosion and Slope Instability - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 124

help to predict the periodic slope susceptibility to soil saturation, soil erosion, and
soil slip and on the basis of which we can infer a rational management procedure
to reduce slope vulnerability.

4.2 Run-off Estimation

The runoff equation for small mountain watershed can be written as:

Qt ¼ fPt ð S Ia Þ ð 4 : 1 Þ

; ;

(Hand book of hydrology 1972 ).
Qt is the depth of runoff over the catchment during selective time period “ t ”

Pt is the depth of rainfall over the catchment during time period “ t ”

S is the potential maximum retention of water by the soil in equivalent depth

over the catchment

Ia is the initial abstraction during the period between the beginnings of rainfall

Pt and runoff Qt in equivalent depth over the catchment.

The relation between S and Ia is very important to understand as a part of S is
retained in the soil in the form of interception, in ltration, depression storage and

absorption initially in the form of Ia. Ia varies according to soil condition and
Antecedent Moisture Condition (AMC) which is the existing moisture condition of
soil and land and expressed as the amount of rainfall received in past 5 days.

2

¼ ð

P 0 2S Þ

:

Q ð 4 : 2 Þ

2

ð P þ 0 8S Þ

:

(Equation applicable to small hilly watershed. For a given storm 20 % (0.2S) of
the potential maximum retention is the initial abstraction before runoff begins)
where,

25400
CN

S ¼ 254 ð 4 : 3 Þ

Q Actual direct runoff (in mm)
P Total Rainfall (in mm)
S Potential maximum retention (in mm)
CN SCS Runoff Curve Number.