Vegetation-Erosion Dynamics - River Dynamics and Integrated River Management - page 55

2.1.2.2 The Principal Factors Affecting Water Erosion
Principal factors affecting erosion are climate, soil characteristics, topography and ground cover.
Climate —Climate affects erosion potential both directly and indirectly. In the direct relation, rain is
the driving force of erosion. Raindrops dislodge soil particles, and runoff carries the particles away. The
erosive power of rain is determined by rainfall intensity (inches or millimeters of rain per hour) and
droplet size. For example, Meyer (1971) reported that where annual rainfall is 760 mm, raindrop impact
energy over a 2.6 km 2 area is equivalent to nearly 10,000 tons of TNT annually. Table 2.1 shows the kinetic
energy of rainfall of various intensities. The table shows that 6 mm raindrops falling from a cloudburst
have over 2,000 times as much kinetic energy per unit time as a drizzle with l-mm raindrops. A highly
intense rainfall of relatively short duration can produce far more erosion than a long-duration storm of
low intensity. Also, storms with large raindrops are much more erosive than misty rains with small droplets.
Though yearly rainfalls over 2,540 mm commonly occur in the Pacific northwest of the U.S., storms in
that area tend to have low intensity with a very fine droplet size, and, thus, erosion is not severe (Gray
and Leiser, 1982; Lull, 1959).

Table 2.1 Kinetic energy of rainfalls of various intensities and droplet sizes (after Gray and Leiser, 1982; Lull, 1959)

Median
diameter
(mm)

Drops per area
per time
drops/ (m 2 ·s)

Intensity
(mm/hr)

Fall velocity
(m/s)

Kinetic energy
J/ (m 2 ·hr)

Rainfall

5.896*10 -7

Fog 0.127 0.01 0.003 67,425,135

1.159*10 -3 Mist 0.051 0.1 0.213 27017

Drizzle 0.254 0.96 4.115 151 2.160

Light rain 1.016 1.24 4.785 280 11.632

Moderate rain 3.81 1.60 5.700 495 61.896

Heavy rain 15.24 2.05 6.706 495 342.537

Excessive rain 40.64 2.40 7.315 818 1,087.011

Cloudburst 101.6 2.85 7.894 1,216 3,165.584

Cloudburst 101.6 4.00 8.900 441 4,025.210

Cloudburst 101.6 6.00 9.296 129 4,388.617

The indirect relation between climate and erosion is subtle. The yearly pattern of rainfall and temperature
by and large determines both the extent and the growth rate of vegetation. As will be seen later, vegetation
is the most important form of erosion control. Climates with relatively mild year-round temperatures and
frequent, regular rainfall (as in the southeastern the U.S. and the British Isles) are highly favorable to
plant growth. Vegetation grows rapidly and provides a complete ground cover, which protects the soil
from erosion. A cleared land can be greened easily if the revegetation is properly done.
In erosive areas the cycle of soil freeze and thaw exacerbates soil erosion. Soil particles and rocks may
be detached because water in the pores and interstices of rocks swell as it is frozen. Figure 2.7 shows the
soil erosion caused by soil freeze and thaw in the Xizhao Gully on the Loess Plateau in China. The rocks in
the area are formed in Tertiary material and are loosely bonded. Water penetrates into the rock interstices
and breaks the rocks when it is frozen. The erosion rate of rocks in the area is as high as 2-10 mm per year.
The Sierra Nevada, Cascade Range, and dry climates, such as the vast desert areas of the southwestern
United States and the Loess Plateau in the northwestern China, are far less favorable for plant growth,
and, thus, are much more susceptible to erosion. In each of those climatic extremes, the natural vegetation
requires a very long period of time to become established. It lives in a fragile balance with its environment.
Because the climate is so harsh, it is very difficult to reestablish any plant cover that is disturbed. Rainfall is