Page 87 - Soil Degradation, Conservation and Remediation

74 3 Soil Erosion by Water

Fig. 3.4 Rill erosion (Photo courtesy of USDA-NRCS)

3.3.3 Rill Erosion

When rainfall exceeds the rate of infiltration, water accumulates on the surface, and
if the land is sloping, it moves along the slope. On gently sloping lands, with stand-

ing crops or in fields that have been recently tilled, moving water concentrates along
tiny channels called rills (Fig. 3.4 ). Rills are less than 30 cm deep. The cutting

action of flowing water detaches soil particles, and runoff water carries them away.
The amount of soil loss may be high, but the small channels do not usually interfere
with tillage implements. The rills may be leveled by normal tillage operations. Rill
erosion is often the initial stage of gully erosion.
Rill erosion is largely caused as a result of large amounts of material that are
released and transported for variable distances in concentrated areas. On the other

hand, the flow of water over the surface has a smaller effect on soil detachment, but

a larger transportation effect. Yet flowing water, especially on tilled land, can
become the agent of transport of particles loosened mechanically, chemically, or by
means other than the water flow itself, and therefore it is a phenomenon of great

importance from the point of view of total soil losses (Zachar 1982 ). Water concen-

trates in places over the field due to reduction in infiltration, increase in precipita-

tion, and surface roughness of the land. Water concentrates along tillage lines, rows
of crops, impedance by exposed roots, around clods, etc. and from shallow and
narrow channels known as rills. As this gathering of water proceeds, the total
amount of water remaining the same, the depth of the water increases, together with
the velocity, kinetic energy, and detaching as well as carrying capacity of the water.
At high precipitation intensities, there is greater clogging of pores, and the