Surface Run-off, Soil Erosion and Slope Instability - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 131

Table 4.6 Seasonal rainfall limits for the antecedent soil moisture conditions (AMC)
Condition General description 5 days antecedent

rainfall (mm)
Dormant
season

'

Growing
season

I Optimum soil condition from about lower plastic

limit to wilting point

13 36

II Average value for annual flood 13 28 36 53

- -

III Heavy rainfall to light rainfall and low temperature

within 5 days prior to the given storm

28 53

Source Soil Conservation Service ( 1972 )

AMC I: Soils are dry but not to the wilting point; satisfactory cultivation has
taken place.
AMC II: Average condition.
AMC III: Heavy rainfall to light rainfall and low temperature have occurred
within the last 5 days; Saturated soil (Table 4.6 ).

ltration and surface
storage, all of which occur before runoff begins. An exact determination of Ia is
very dif

Initial Abstraction (Ia) consists mainly of interception, in

cult. However, for practical purpose, Ia can be related to S. Based on
analysis of data from a large number of small watersheds, the Soil Conservation
Service (1969) found Ia to be roughly equal to 0.2S (Table 4.4 ). It can also be
estimated by relating to the antecedent soil-moisture index. Potential maximum
retention (S) includes Ia and the in

ltration occurring after runoff begins. This later

in ltration is controlled by the rate of in ltration at the soil surface or by the rate of

transmission in the soil pro le (Hand Book of Hydrology 1972 ). The magnitude of

' will be reduced during a successive period of storm (such as one day per week).

The maximum limit of

S '

' S ' depends on the character of soil and soil cover complex.

The factor is also related with antecedent moisture condition (AMC) deter-

mined by the total rainfall in the 5 days period preceding a storm.

' S '

4.2.2 Calculation for Antecedent Moisture Condition
or Cumulative Effects of Rainfall in Last Storm
of the Year

4.2.2.1 Antecedent Rainfall Conditions and Curve Numbers
(for Ia = 0.2S)

The rain water does not completely drains down slope as an immediate effect, but
has an effect for next 5 days, on an average. So for any hydrological calculation