Vegetation-Erosion Dynamics - River Dynamics and Integrated River Management - page 86

in western China. The Anjiagou watershed is on the Loess Plateau with elevation in the range of 1,900-
2,250 m above the sea level, and has a drainage area of 9.06 km 2 . There are many gullies in the watershed
with a distribution density of 3.14 km/km 2 . The gullies are 30-50 m deep. The annual precipitation is
427 mm, of which 60% occurs in July, August, and September. The annual pan evaporation is 1,526 mm
and annual average temperature is 6.3 ć . The soil is composed of gray desert soil, spodosol, and loamy
soil. The gully bed and gully slope are covered with halogenic soil. The rate of soil erosion was as high
as 10,000 t/(km 2 ·yr) and the vegetation cover was only 5.7% before the 1950s, when erosion control
projects were launched. The erosion control and reforestation of the watershed were performed in three
phases (Ye, 1986; Li, 1986; Zhang et al., 1986): ķ From the mid 1950s to the mid 1960s, the local people
terraced the sloping farmland, and planted trees and grass on the bare hills and slopes. They constructed
4 sediment-trap dams in the late 1950s. ĸ From the mid 1960s to the late 1970s, the reforestation effort
was interrupted but the construction of terraced farm fields continued. Ĺ From the late 1970s to the
1980s, reforestation was intensified and the construction of terraced fields continued. In the meantime, a
project of comprehensive watershed management was conducted to speed up the erosion control and
greening of the landscape. In the late 1970s, other 3 sediment-trap dams were completed.
Land-use change in the watershed was mainly to farmland, forest, and wasteland, among them farmland
was changed from sloped land to forest and terraced fields and the wasteland was greened into forest and
grassland. Data on land-use change, reforestation, rate of erosion, and the sediment amount trapped by
the dams during the period 1950-1990 were collected. The sediment trap dams raised the gully bed and
reduced the bed slope and bank slope. Even if the dams have been filled with sediment they still are
effective in stabilizing the slopes and promoting vegetation development. The area of reforestation by
planning trees, shrubs, and grass per year is divided over the total area of the watershed, yielding the
value of V W . The value of E W consists of two parts: the reduction in erosion resulting from by changing
the sloped land into terraced fields and from sediment trap dams. The first part is calculated as the ratio
of the area of terraced fields each year to the total area of the watershed multiplied by the sediment-yield
per year. And the second part is obtained by taking the differential of the cumulative sedimentation curve
of the dams divided by the total area.
Because V W and E W are not constants and cannot be described with a simple mathematical function, the
solution, Eqs. (2.14) and (2.15) cannot simply be applied, but rather Eq. (2.13) must be solved numerically.
The four parameters for the Anjiagou watershed were determined as follows:
a = 0.001(1/yr), c = 0.0000018(km 2 /yr), b = 0.01(1/yr), f = 400(t/(km 2 ·yr 2 )) (2.18)
For each parameter, a trial-and-error method was used to compare the computed result with the data.
The trial-and-error method was performed many times for every adjustment of each parameter until the
best-fitting value of the parameter was obtained. Because the vegetation cover V is defined in the domain
[0, 1], and the erosion rate E is in the domain [0, Ğ ), the value of vegetation cover is taken as V = 0 if the
calculation yields V < 0; and V = 1 if the calculation yields V > 1; and the erosion rate is taken as E = 0 if
the calculation yields E < 0.
Figure 2.33(a) shows the computed and measured development process of vegetation cover and
Fig. 2.33(b) shows the variation of the rate of erosion for the Anjiagou watershed. The computed curves
closely follow the real processes, implying that the theoretic model is able to simulate the complex and
dynamic evolution processes of vegetation and erosion.
The Xiaojiang River is a tributary of the Yangtze River that begins on the northeastern Yunnan-Guizhou
Plateau of China. The elevation is in the range of 1,100-4,344 m above the sea level. Dongchuan City is
located in the middle and lower reaches of the river with a drainage area of 1,881 km 2 . It is located in the
semi-tropical zone with the average temperature over 20 ć . The annual precipitation is 688 mm in the
middle and lower reaches and over 1,000 mm in the upper reaches. The drainage area has active tectonic