The downward spiral of land degradation resulting from the feedback loop between soil and vegetation. As the natural vegetation is disturbed, soil becomes exposed to raindrops and wind leading to erosion and loss of soil, including organic matter and nutrients. The now impoverished soil can support only stunted crops or other vegetation, which leaves the soil with even less protective cover and root mass than before. Soil loss becomes severe, such that the soil depth and the capacity to hold water are greatly reduced and vegetation can barely survive, leaving extremely degraded soil. Incapable of providing nutrients and water needed to support healthy growth of natural vegetation or crops, the site continues to erode, polluting rivers with sediment and impoverishing the people who attempt to grow their food on the land. (Diagram courtesy of R. Weil)

2. Nature of water erosion

Raindrop splash
Sheet erosion
Channelized flow (rill and gully) associated with runoff.

3. Factors affecting water erosion and its control

The USLE: Universal Soil Loss Equation

A = R K Ls C P

A: erosion in tons per acre per year
R: rainfall factor (storms vs. mists)
K: soil erodibility (0.01 to 1.0)
Ls: field length or slope factor
C: vegetative cover (types and density of plants/residues)
P: practices used for erosion control (contours, terraces, strip cropping)

• Soil erosion occurs in three phases:

– Particle detachment (by raindrop impact or moving water, or by wind)
– Sediment transportation
– Sediment deposition