UNDERSTANDING THE PROCESS OF SOIL EROSION
AND WATER INFILTRATION
by Rolf Derpsch
Soil erosion by water and runoff is often accepted as an unavoidable phenomenon associated with
agriculture on sloping
UNDERSTANDING THE PROCESS OF SOIL EROSION
AND WATER INFILTRATION
by Rolf Derpsch
Soil erosion by water and runoff is often accepted as an unavoidable phenomenon associated with
agriculture on sloping land. But soil loss by erosion or runoff is not an unavoidable process.
According to Lal (1982), occurrence of erosion damage on cultivated land is merely a symptom of
land misuse for that ecological environment. In other words inappropriate farming practices have
been used. It is not nature (slope and rainfall intensity), but rather irrational farming methods
used by man, which are responsible for erosion and its negative consequences. The farmer can,
through the utilization of site specific and adapted farming systems and management practices,
effectively control erosion, reduce runoff and increase water infiltration on his land. Runoff
water is lost to crops, while infiltrated water can be effectively used by plants, which is very
important in dryer climates.
Conventional farming practices utilized in many parts of the world have had negative consequences
in terms of soil and water preservation as well as on the conservation of the environment as a whole.
This is due to improper soil use, monoculture and the use of tillage tools that leave the soil
bare and pulverize it excessively, leaving it in such a condition that it can be carried away by
heavy rains. The utilization of inadequate technologies that are not adapted to site specific
conditions (slope, rainfall intensities) results in runoff, soil erosion and degradation. Thus,
the consequence of traditional cultivation methods is the gradual loss of soil and fertility
until the land becomes unproductive.
Failure of landowners and agricultural managers to comprehend the significance of erosion, as
well as intensive weathering under hot, humid conditions, has brought about the widespread
distribution of poor, badly eroded, infertile soils all over the tropics and subtropics
(Ochse, et al., 1961). But the same process has also happened in more temperate climates (e.g.
United States, Russia, etc.). Eroded, unproductive and abandoned land as well as advanced signs
of desertification are silent testimonies of this phenomenon all over the world.
Besides making agricultural soil unproductive, erosion of agricultural land and runoff results
in the deposition of soil particles in unwanted areas (sedimentation of roads, creeks, rivers,
lakes, dams, etc.) with all its negative consequences for traffic, the generation of electric power,
the delivery of drinking water, leisure areas, etc., resulting in important expenditures for the
government as well as for society as a whole.
Furthermore, the importance of erosion control is not restricted to the maintenance of the
productive potential and fertility of soils for future generations; it is also an effective means
to ensure employment in rural areas and reduce rural exodus. Efficient erosion control is therefore
very advantageous from the ecologic and social perspectives, besides being highly significant from
an economic point of view.
THE EROSION PROCESS
Soil erosion is caused by non infiltrated water that runs of a field. It is astonishing that often
the process of soil erosion and water infiltration into the soil is not well understood by farmers,
but also by extension workers and scientists as well. Pictures showing the raindrop impact on a bare
soil surface and information explaining the mechanisms of water infiltration into the soil goes back
to the 1940s. Despite scientific and empirical evidence explaining these processes, many people still
think that the soil has to be loosened by tillage to increase water infiltration and reduce runoff.
Figure 1: The impact of
raindrops on a bare soil surface. When it rains, drops up to 6 mm (0.24 inch) in diameter
bombard the soil surface at impact velocities of up to 32 km per hour (20 mph). This force
throws soil particles and water in all directions on a distance of up to 1 m (3.3 feet).
(Pictures made by USDA in the 1940s).
Runoff and erosion start with raindrop impact on bare soil surface. Soil splash seen on fence
posts, or on walls in a field or plot of bare soil, is evidence of the force of large raindrops
striking bare soil (Harrold, 1972). Meyer and Mannering (1967) reported that in one year, raindrops
deliver to an acre of land an impact energy equivalent to 20 tons of TNT (50 t/ha dinamite).
The impact of falling raindrops disaggregates the soil into very fine particles, which clog soil
pores and create a surface seal that impedes rapid water infiltration (Figure 2).
Due to surface sealing, only a
small portion of rainwater can infiltrate into the soil; most of it runs off over the soil surface,
therefore is lost to plants and causes erosion damage when flowing down the slopes. On the other
hand, when the soil is cov