Page 83 - Soil Degradation, Conservation and Remediation

70 3 Soil Erosion by Water

and carried by river systems to the Gangetic Delta through geological time. The
process brings about changes in landforms of both the places. The Himalayas are
weathering away and the deltas are accumulating alluviums, and the land level rises
above the sea level. In many cases, frost and high temperature separate pieces of
weathered rock, and the loose material moves downhill to form piles of hillside
waste, debris cones, outwash fans, and other formations (Zachar 1982 ). This is geo-
logical erosion occurring in almost all natural surfaces through natural forces.
However, erosion may be of different types, such as rock erosion, land erosion, and
soil erosion, depending on the surface concerned. Erosion may be divided into natu-
ral or geologic and human-induced or accelerated erosion.
Soil erosion involves two processes: detachment of particles from soil aggregates
and transport of the particles by water or wind. Soil erosion in undisturbed landscapes
by natural forces is called normal erosion, natural erosion, or geological erosion which
occurs almost silently and often leaves no signs. Natural erosion is not of much concern
because the amount of soil loss is readily compensated by the natural processes of soil
formation. But human actions for the exploitation of land, water, vegetation, and soil
resources tremendously accelerated the extent and intensity of soil erosion. This is
known as accelerated soil erosion. Generally accelerated erosion is considered to be
soil erosion proper. Accelerated erosion occurs usually at an alarming rate that reduces
soil quality and crop yield on-site and damages land, water, and installations off-site.

3.2 Causes of Water Erosion

Causes of soil erosion include deforestation, overgrazing, biomass burning, shifting
cultivation, mechanized logging, cultivation up- and downslope, continuous mono
cropping, and compaction of soil by stock and machinery. Deforestation, overgraz-
ing, shifting cultivation, and mono cropping have been discussed in connection with

causes of soil degradation in Chap. 1 . Compaction was discussed in detail in Chap. 2 .
The reader may extract a good account of the causes of soil erosion from these
chapters. It is worth mentioning here that water erosion is a prominent problem of
the sloping lands. Disturbing the slopes for seedbed preparation and cropping is
very risky. General soil conservation practices such as minimum tillage and mulch-
ing help reduce the risks. But slopes steeper than 20° should be left under natural
conditions. Satisfactory forests and grasses can develop there if left undisturbed.
Such slopes are very susceptible to water erosion. Cultivating up and down that is
along the slope produces severe water erosion.

3.3 Types of Water Erosion

Water erosion is caused by water – water that comes in rain and runs off the land as

overland flow or streamflow. At the initial stage, soil particles are detached from

aggregates by the impact of falling raindrops or flowing water, which is followed by