Environmental Engineering Reference
In-Depth Information
Plateau is thick and the
c
value is small. In a hilly area with slope debris in the suburbs of Beijing and the
in Xiaojiang watershed, one ton of soil erosion may cause a larger area of vegetation-damage, and the
c
value is large. Parameter
b
represents the effect of increasing erosion by removing topsoil in the process
of erosion. The surface of the hilly area with slope debris near Beijing is composed of various particles
with a wide range of size and erosion cannot penetrate down, therefore, the value of
b
is small. In the
Xiaojiang watershed and on the Loess Plateau, the vegetation protects the soil from erosion. Once the top
layer is eroded the erosion may further penetrate down to the erodible soil. Therefore, the value of
b
is
large. Parameter
f
represents the effect of vegetation controlling erosion, which is large for the Loess
Plateau.
A correlation analysis is performed with the data in Table 2.2. The result indicates that parameter
a
depends mainly on the precipitation and annual average temperature; and parameter
c
depends mainly on
the slope and the median diameter of the soil. Figure 2.41 shows parameter
a
as a function of the
precipitation and annual average temperature, and parameter
c
as a function of the slope and the median
diameter of the soil. In the varying ranges of the precipitation, temperature, slope, and soil size, the two
parameters can be preliminarily determined with the diagrams. The parameters
b
and
f
cannot be clearly
expressed as functions of the climatic, morphological, and soil conditions of the watershed from the
correlation analysis with the available data. More work is needed to establish empirical formulas for
parameters
b
and
f
. If, however, parameters
a
and
c
are predetermined for a watershed according to the
precipitation, temperature, slope, and soil size, parameters
b
and
f
can be easily determined with data.
Fig. 2.41
For the loess plateau (a) Parameter
a
as a function of the annual precipitation and annual average temperature;
and (b) Parameter
c
as a function of the slope and the median diameter of the soil
2.3.4
Application of Vegetation-Erosion Chart
Effective strategies for restoration of watershed vegetation and erosion control can be found by analyzing
the vegetation erosion chart.
2.3.4.1
Loess Plateau
The geographical range of the Loess Plateau is N 33°43'-N 41°16'
ǃ
E 100°54'-E 114°33'. It extends from
Taihang Mountain in the east to the Helan Mountain in the west and from the Qingling Mountain in the
south to the Great Wall in the north. The total area is about 480,000 km
2
. The annual precipitation varies in
the range of 200-700 mm, with the highest precipitation in the southeastern part and lowest precipitation
in the northwestern part. More than 60% of the rainfall occurs in June, July, August and September (Tang,
2004). The Loess Plateau is divided into four areas according to climate, topography, soil, morphology,
and soil erosion: Area I is a sandy and hilly area along the Great Wall, as shown in Fig. 2.42; Area II is
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