Environmental Engineering Reference
In-Depth Information
Fig. 2.37
Vegetation-erosion chart for the Xiaojiang watershed and its sub-watersheds (
ƽ
- Xiaojiang watershed in
the 1990s;
Ʒ
- Heishuihe watershed in the 1998;
Ƶ
- Shengou watershed in the 1996) (after Wang et al., 2003a)
Zone B: d
V
/d
t
> 0, d
E
/d
t
>0
.
In this zone the vegetation cover is in an unstable state. Both vegetation and
erosion are increasing. If erosion increases faster or human caused stresses lead to deforestation and erosion
continues to increase the ecological system may enter Zone A. If vegetation increases faster or human
controls are applied to erosion, such as reforestation of the hills the ecological system may enter Zone C.
The vegetation-erosion chart is exclusively resolved by the parameters
a
,
c
,
b
, and
f
, which are
determined by long-term measurement of the vegetation and erosion variation. The parameters are constant
and universal for the areas of the same climate, topography, soil, and vegetation compositions. If the
values of the parameters
a
and
f
are large, the system in most cases is in the Zone C developing toward
perfect vegetation. If the values of
c
and
b
are large, the system in most cases is in Zone A developing
toward poor vegetation and high rates of erosion.
The three points in Fig. 2.37 indicate the state of the vegetation and erosion for the Xiaojiang watershed
and the Heishuihe and Shengou sub-watersheds. The ecological systems of the Heishuihe and Shengou
watersheds moved from Zone A to Zone C thanks to 20 years of effort of intensified reforestation and
erosion control. Now, they are developing toward perfect vegetation. Nevertheless, the Xiaojiang watershed
as a whole is still in Zone A and exhibits the development trend toward poor vegetation, which can offset,
to a certain extent, the human effort for improvement of the vegetation. The dashed curve in the figure
shows the suggested route for moving the system into Zone C. First, erosion control must be performed
to reduce the erosion rate by 60-80%, thus the system is moved into Zone B. Then the development
trend of
V'>
0 may support reforestation projects moving the system into Zone C. Erosion control is very
important in the area for re-vegetating the hills. Mere planting of trees and shrubs do not work well for
greening land subject to high erosion.
Similarly, the vegetation-erosion chart for the Anjiagou watershed on the Loess Plateau is prepared
using the values of the parameters given by Eq. (2.18), as shown in Fig. 2.38. There is no Zone B but
rather a Zone D, in which
VE
. The ecological system in Zone D also is unstable, because both
vegetation and erosion are reducing. If the vegetation reduces faster than erosion the system may enter
into the Zone A and if erosion reduces faster than vegetation the system may enter into the Zone C.
The Zone C in the chart is very small and the Zone D is large. The tangent of the line
'
,'
0
E
L
s high,
implying high effectiveness of vegetation in controlling erosion. On the other hand, the line
'
0
V
is
almost horizontal, which suggests that erosion control helps little for vegetation development. Thus, in
'
0
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