Surface Run-off, Soil Erosion and Slope Instability - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 127

Table 4.1 Land use/land cover and their hydrologic conditions
Land use/land cover Intensity and characters Hydrologic

conditions

Bared
surface

Steep slope Without any residue cover Poor

Gentle slope Without any residue cover Poor

Agricultural land Contoured and terrace Poor

Jungle Upslope 50 - 70 % ground is covered by plant Fair

Down slope <50 % ground is covered by plant Poor

Roads Concrete Average 65 % area is impervious area Poor

Metal Average 25 % area is impervious Fair

Settlement Compact 65 % average impervious area Poor

Disperse 25 % average impervious area Fair

Tea garden Upslope More than 75 % area is covered by tea

plantation

Poor

Down slope More than 75 % area is covered by tea

plantation

Good

Open forest Upslope Grazed but not burned Fair

Down slope Grazed but not burned Fair

Degraded
forest

Upslope Forest and tree crops heavily grazed

or burned

Poor

Down slope Forest and tree crops heavily grazed

or burned

Poor

Mixed
forest

Upslope Wood and grass combination 50 and

50 %

Fair

Down slope Wood and grass combination 50 and

50 %

Good

Dense forest >90 % ground is covered by plant Good

4.2.1.2 Hydrologic Soil Groups

Soils are originally assigned to hydrologic soil groups based on measured rainfall,
run-off, and in ltrometer data (Musgrave 1955 ). Most of the soil groupings are

based on the premise that soils found within a climatic region that are similar in
depth to a restrictive layer or water table, transmission rate of water, texture,
structure, and degree of swelling when saturated, will have the similar run-off
responses (National Engineering Handbook, USDA 1993 ). The given four hydro-
logical soil groups are used in determining hydrologic soil cover complexes, which
are used in the method for estimating runoff from rainfall. The soil properties play
an important role in the estimation of runoff from the rainfall. In this concern the
properties can be represented by hydrologic parameters that influence the minimum
rate of in

ltration obtained from a bare soil after prolonged wetting. Hydrologic soil
group-A, having high in ltration rates and low run-off potential even when thor-

oughly wetted and consisting chiefly of deep, well to excessively drained sands or
gravels, having high rate of water transmission (Tables 4.2 and 4.3 ). The USDA