After reading this article you will learn about how rain drops cause soil erosion.
It is now established that collision of rain drops on bare soil and resulting splash is the major cause of soil erosion by water. About 95% of soil is splashed by falling rain drops and runoff water erodes less than 5% of the soil. From the mechanical point of view, work must be done to erode soil and the source of energy to perform this work must be from rain drop impact and runoff in the water erosion process.
We know that the mechanical energy is manifested in two forms: kinetic and potential. Kinetic energy is the energy possessed by a substance by virtue of its motion and is proportional to the product of the moving mass and half of the square of the velocity of the mass, i.e.
E = Â½ mv2 â€¦(2.2)
E = Kinetic energy
m = mass of water or drop in question
v = velocity of the mass of water
Potential energy is the energy possessed by a substance by virtue of its position and is expressed as:
Ep = mgh â€¦(2.3)
Ep = potential energy of the mass of water
m = mass of water in question
g = acceleration due to gravity
h = height of water mass above the reference level.
The ultimate amount of potential energy available for erosion is determined by the vertical distance of the soil mass from the base level of erosion. It represents the lowest elevation to which the soil can be transported.
In general, it is the sea level; however, for individual plots or areas h represents the elevation difference between the original position and the foot of the slope. Sheet erosion, in general, is the result of kinetic energy associated with rain drop impact and runoff. Similarly, mass movement is largely due to the potential energy resulting from the position of the soil above the point to which it can slide or drop.
The erosive capacity of a falling mass of water depends on the energy per unit area of the individual drop. The kinetic energy of the falling drop determines the force of blow that must be absorbed at each point of impact, while the horizontal area of the drop determines the amount of soil that must sustain that blow.
It is the application of this energy in the form of rain drop impact that accounts for the greatest part of the dispersion of soil particles.
When a rain drop strikes bare soil, especially pulverized soil, the force of impact dislodges fine particles, thus making muddy water. As this muddy water sinks into the ground, the fine particles tend to filter out at or near the surface and form a thin muddy film which chokes the soil pores.
Evidence of this phenomenon is seen in the thin layer of fine sediment which tends to crack and curl upon the soil surface as it dries after a heavy rain.
Also, the compacting action of falling raindrop causes direct reduction in the infiltration rate. This is illustrated by the fact that rain falling on the sand decreases its infiltration rate without producing turbid water. When the surface soil is ponded by raindrops, the infiltration rate decreases rapidly as the proportion of large drops and their velocity increases.
In the same rainfall, the decrease in infiltration rate is the greatest on the flat land because the compaction effect of raindrops brings more direct and stronger pressure to bear on the flat land than on slopes.
The falling drops lift the soil into air and splash it back and forth. Rain drops exert three important influences:
(i) It detaches soil particles;
(ii) Destroys granulation; and
(iii) Splashes soil particles.
Standing water that is churned by splashing has been found to contain as much as 20% of soil. The rain drops, by keeping the water turbid, increase the ability of the runoff water to transport soil particles. Tests on bare soil show that runoff starts within 1 to 3 min. from the beginning of rain.
The total energy of rain drops has been calculated as being equal to roughly 100 HP on an acre during rainfall of 0.25 cm/hr and 250 HP at 5.0 cm/hr.
The force of falling rain drops may be 10,000 times the energy of the surface runoff, even on steep slopes. Drop size may vary from 1 to 8 mm and the velocity nearly 7.8-9.0 m/sec. High intensity rains are associated with larger drop diameter. As the drop size increases from 1-5 mm the infiltration rate decreases by as much as 70%.
A 2.5 mm drop diameter is assumed to be representative of erosion-producing rains and is used for analysis of the influence of changes in fall-height on the rainfall erosion index.
The mean drop size ranges from 2.0 to 2.5 mm for rainfall intensities ranging from 1.25 to 5.0 cm/hr. A 2.5 mm drop starting from zero velocity will reach a velocity of 2.96 m/sec, 3.98 m/sec, 5.19 m/sec, and 7.41 m/sec, in falls of 0.5 m, 1 m, 2 m and 20 m or more respectively (Table 2.1).
Relationship between drop fall velocity and distance of fall has also been shown in Fig. 2.4.