Environmental Engineering Reference
In-Depth Information
an area, they can be directly applied to the sub-watersheds or neighboring areas with the same climate,
topography, soil, and vegetation composition.
Fig. 2.36
Comparison of the theoretical curves calculated with the same parameters as for the parent Xiaojiang
watershed with the real development process of (a) vegetation cover and (b) erosion rate for the Shengou watershed
(after Wang et al., 2003a)
2.3.3
Vegetation-Erosion Chart
The vegetation-erosion chart is used to discuss the development trends of vegetation and erosion in the
case of no human-caused stresses. If the stress terms are zero, Eqs. (2.14) and (2.15) can be rewritten as:
'
'
V V E
E EfV
¾
¿
(2.24)
d
V
d
E
in which
V
'
,
E
'
, which are expressed as functions of
V
and
E
.
V
and
E
may be positive
d
t
d
t
or negative, therefore, the
V-E
plane:
V
[0,1],
E
f can be divided into three zones by the two
[0,
)
lines
VE
'
,'
, or
0
;
a
f
EVE V
c
(2.25)
b
If the values of
a
,
c
,
b
, and
f
are known
ˈ
the two lines are given by Eq. (2.25) and the vegetation-erosion
chart can be developed.
Figure 2.37 shows the vegetation-erosion chart for the Xiaojiang watershed, in which the values of
a
,
c
,
b
, and
f
are from Eq. (2.19). The three zones are:
Zone A: d
V
/d
t <
0, d
E
/d
t
> 0
.
In this zone the vegetation cover is deteriorating and the erosion rate is
increasing. The larger the zone is, the more difficult for the vegetation to develop. The size of Zone A
-
®
¯ ¿
, the larger the value, the smaller the zone.
Zone C: d
V
/d
t
>0
,
d
E
/d
t
< 0. In this zone the ecological system is developing toward complete
vegetation cover and zero erosion rate. A large Zone C favors stable vegetation. The size of the zone
af
cb
depends on the value of max
af
cb
-
®
¯ ¿
, the larger the value, the larger
the zone. For a watershed in this
zone, a forest may be logged to a certain extent (not bringing the system over the lower line) and the
vegetation may recover after a period. The period of recovery depends on the ecological resilience.
depends on the value of min
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