Surface Run-off, Soil Erosion and Slope Instability - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 130

more than 90 % sand or gravel. The saturated hydraulic conductivity of all
soil layers exceeds 40.00 m/s. The depth to water table is greater than

60 cm. Soil group B possesses between 10 and 20 % clay and 50

µ

90 %

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sand. Soil group C is characterized by 20 40 % clay and less than 50 %

sand. Soil group D consists greater than 40 % clay and less than 50 % sand.

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4.2.1.3 Hydrologic Condition

Hydrologic condition indicates the effects of cover type and treatment on in ltration

and runoff and is generally estimated from density of plant and residue cover on
sample areas or refers to the state of vegetation growth. A poor hydrologic con-
dition refers to pasture heavily grazed with sparse vegetation. A fair condition is for
pasture moderately grazed with between half and 3/4th of the basin under plant
cover i.e. the basin has 50 to 25 % plant cover. A good hydrologic condition
indicates that the soil usually has a low runoff potential for the given hydrologic soil
group, land cover type and treatment. Some factors to consider in estimating the
effects of cover on in

i. Canopy or density of lawns, crops
or other vegetation areas; ii. Amount of year-round cover; iii. Amount of grass or
close seeded legumes in rotations; iv. Percent of residue cover; and v. Degree of
surface roughness (Maidment 1972 ).

ltration and runoff are—i. —

4.2.1.4 Antecedent Moisture Condition

Antecedent Moisture Condition (AMC) refers to the water content present in the
soil at a given time. The AMC value is intended to reflect the effect of in

ltration on

both the volume and rate of runoff according to the in ltration curve (Sing 1975).

The amount of rainfall in past few days reflects the condition of Antecedent
Moisture and is as essential indicator of amount of initial abstraction (Ia) and
percentage runoff (Table 4.5 ).
The Soil Conservation Service (SCS) developed three antecedent soil moisture
conditions and labeled them as I, II, III. These AMCs correspond to the following
soil condition:

Table 4.5 Relation between initial abstraction (Ia) and potential retention (S)
Relation between initial abstraction (Ia)
and potential retention (S)

Region/soil type Antecedent moisture

condition (AMC)

Ia = 0.1S Used in black soil region AMC II and AMC III

Ia = 0.2S Small hilly watershed AMC I

Ia = 0.3S All other regions

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