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INTRODUCTION
1.1
Introduction
Flooding on China’s largest and most important river, the Yangtze, is frequently reported
in the press due to the almost annual occurrence of monsoon-related floods. In addition,
the Three Gorges Dam being built to harness power from the Yangtze is also a matter of
hot debate. In contrast, China’s second river, the Yellow River (Huanghe) is not often in
the news, yet this river probably poses a greater threat to the people living around it than
the Yangtze does. It has been estimated that major floods on the Yellow River could
threaten the lives of 150 million people: it is not by chance that the Yellow River has
earned the name ‘China’s sorrow’. The threat posed by the Yellow River is caused by a
major peculiarity: its huge sediment content, which has caused rapid sedimentation in its
lower course. This, in turn has in the past led to regular major changes in its course. Since
the early 1950’s, however, the river has been harnessed but continuing sedimentation has
raised the river bed to several metres above the surrounding landscape, so that breaching
of the dikes could result in disaster. The Yellow river basin has been studied intensively
by Chinese scientists for over 50 years and the Chinese government is well aware of the
problems posed by the river and seems committed to combat them. Since the Loess
Plateau is the source of about 90% of all the sediment that enters the Yellow River
(Douglas, 1989; Zhaohui Wan & Zhaoyin Wang, 1994), much attention is being directed
at decreasing the erosion rates in this important part of the Yellow River catchment.
Reducing the erosion rates on the Loess Plateau should decrease downstream
sedimentation problems while at the same time reducing the loss of agricultural land on
the Loess Plateau itself.
1.2
The Loess Plateau
1.2.1 Introduction
Loess is defined by Pye (1987) as a terrestrial windblown silt deposit that forms in semiarid continental climates. It consists mostly of quartz, feldspar, mica, clay minerals and
carbonate grains with the clay minerals and carbonate acting as cementation material. The
proportions of the constituents may vary widely from place to place. Most loess deposits
were formed during the Pleistocene. More than 6 % of China is covered by loess: the
Loess Plateau of central China has an area of about 300,000 km2 (Tan, 1988; Muxart et
al., 1994). According to Derbyshire et al. (1991), this is the area having a minimum loess
thickness of 10 metres. Other authors therefore mention larger areas. The maximum loess
thickness is about 300 metres. The Loess Plateau is situated in the Yellow River basin, in
northern China and covers large parts of the Gansu, Ningxia, Shaanxi and Shanxi
provinces. Figure 1.1 shows the location of loess in China, while figure 1.2 shows a map
of the Yellow River basin.
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Ürümqi
Loess
Plateau
Harbin
Beijing
Yellow
River
Lhasa
Wuhan
Chengdu
N
Yangtze
River
Guangzhou
1000 kilometres
Figure 1.1 Distribution of loess in China. Adapted from Pye (1987)
1.2.2 Loess erosion
Basin-wide erosion
The Loess Plateau has some of the highest erosion rates on the entire planet. Some of the
table lands of the Loess Plateau are very dissected by gullies, but the region with the
highest erosion rates is generally considered to be the hilly part of the Loess Plateau,
which is also very dissected by gullies. This region is mostly located in the northern part
of Shanxi and Shaanxi Provinces. Figure 1.3 shows a typical landscape for the hilly part
of the Loess Plateau. Jiang Deqi et al. (1981) estimated that erosion rates may be as much
as 18,000 tonnes per square kilometre per year for the hilly loess region of the Wuding
catchment, which is one of the main Loess Plateau tributaries of the Yellow River (figure
1.2). Sediment concentrations in runoff on the Loess Plateau of over 1000 g/l have been
recorded regularly. There are several reasons for these very high erosion rates:
•
•
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First, the loess is very erodible, especially when wet.
Second, the area’s rainfall is characterized by heavy storms in summer (mainly
July and August). Single storms can produce 10% of yearly precipitation and 40%
of erosion (Gong Shiyang & Jiang Deqi, 1979, Zhang et al., 1990). Though the
saturated conductivity of the loess is generally higher than rainfall intensity,
crusting prevents that all water infiltrates (Douglas, 1989). Muxart et al. (1994)
found that as much as 95% of rainfall can become runoff due to crusting.
• Third, the area has considerable relief. Continuing uplift is an important factor in
causing this.
• Finally, vegetation cover is generally sparse. This is partly caused by a semi-arid
climate with cold winters, but also by deforestation and grazing (Jiang Deqi et al.,
1981).
Yellow
River
n xi
Suide
Sha
a
g xi
Yan R. Ansai
Lanzhou
Gansu
BO
HAI
Wuding R.
Nin
Yinchuan
Beijing
Yan’an
Shaanxi
Xi’an
Zhengzhou
N
500 kilometres
Figure 1.2 Map of the Yellow River basin. Adapted from Xu Jiongxin (1999a) and Pye (1987)
Erosion rates have not always been so high. Rem Mei-e & Zhu Xianmo (1994) showed
how different kinds of information (written records, Yellow River delta volumes) indicate
that the serious soil erosion on the Loess Plateau started at about 1000 AD. Xu Jiongxin
(2001) found that bank breaching of the Yellow River increased in frequency from the
10th century AD. According to him, breaching frequency depends on sediment load,
which apparently increased because erosion on the Loess Plateau was increased by
destruction of the natural vegetation. Such destruction greatly reduces the high natural
permeability of loess as well as the resistance to erosion. Rainfall experiments reported
by Rem Mei-e & Zhu Xianmo (1994) also show the much higher erosion rates of bare
soils, which have resulted in a large extension of the gullied area and also increased the
relative relief of the area. Headcut retreat rates are at present sometimes as high as 3
meters per year. On the other hand, Long Yuqian & Xiong Guishu (1981) reported that
historic literature from the Eastern Han Dynasty (25-220 AD) already recorded very high
sediment contents: ‘the silt occupied six tenths of the volume in one barrel of water
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sampled’. Nevertheless, such observations seem to have been exception rather than rule
before about 1000 AD.
Figure 1.3 Typical landscape of the hilly part of the Loess Plateau, northern Shaanxi
Discharging all the sediment delivered to the Yellow River requires substantial river
flow, which puts limits on the amount of water that can be used for irrigation, even
though irrigation with dirty water has been successfully applied in places. Aggradation of
the river bed has already caused the river to flow 5-10 meters above the surrounding area
along its lower reaches (Douglas, 1989; Zhang et al., 1990; Zhu et al., 1997). Long
Yuqian & Xiong Guishu reported annual sedimentation rates of 4 to 7 centimetres over
the period 1951-1977, which causes, as already noted, a major flooding risk. Before the
Yellow River was harnessed (from 1946 on), it changed course once every century and
flooded every 2 out of 3 years (Zhang et al., 1990).
Combating erosion
Reducing the flooding risk and using the Yellow River water for agriculture and industry
requires a large reduction in the sediment content. The most effective way of doing this is
to decrease the erosion rate on the Loess Plateau because this is the major sediment
source in the Yellow River basin. A major project has therefore been started to reduce
erosion on the Loess Plateau, mainly by check-dams and terrace building. According to
Jiang Deqi et al. (1981) the sediment discharge of the Wuding catchment decreased by
28% between 1957 and 1978. However, most of this decrease was due to reservoirs and
dams, which have limited capacity. Afforestation and terracing should result in more
permanent decreases in sediment production, while grasses such as Jiji grass might be
used to stabilise gullies. Fang Zhengsan et al. (1981) reported that terracing can decrease
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erosion by as much as 95%. They also described several methods that have been used to
create terraces. Terrace building had already started several hundred years ago and is now
widespread on the Loess Plateau. Terraces are effective against erosion because they have
low slope angles, which reduce water velocity and increase infiltration. However, they
require a high level of maintenance and are prone to gullying when they are not properly
constructed. Other measures that reduce water velocity and increase infiltration should
also be effective in combating erosion. Removal of the soil crust seems to be a good
option, though Muxart et al. (1994) found that cracks in soil crusts caused by drying out
of the soil did not disappear due to swelling on rewetting, but instead had to filled with
sediment before runoff across the cracks could occur. Therefore, the net effect of crusts
might not always be as clear as expected.
Despite all efforts to reduce erosion rates, the Loess Plateau is likely to remain an area
having considerable erosion. It will remain a high-relief, low vegetation-cover area with
heavy storms on erodible soils. Since the gully erosion has very markedly increased local
relief, it is unrealistic to think that proper conservation methods will reduce erosion rates
to pre-deforestation levels. Nevertheless, such conservation methods could achieve large
reductions of current erosion rates. The best place to implement conservation measures is
at the sediment source.
1.3
The EROCHINA Project
In 1998, a European project called EROCHINA started, in which several European and
several Chinese partners participated. Its aim was to find ways to decrease erosion rates
in a small catchment on the Loess Plateau. The project used a participatory approach in
the sense that farmers were involved in the process of identification and design of
solutions to erosion related problems. All farmers were interviewed to find out their
opinions on soil erosion, on economical problems and on possible solutions. The results
of the participatory approach have been discussed elsewhere (Messing & Hoang
Fagerström, 2001; Hoang Fagerström et al., in press). Based on data obtained from the
farmers, on government policy and on data collected in the catchment a number of land
use scenarios were developed. The effects of these scenarios in terms of soil erosion were
investigated using the process based erosion model LISEM (Limburg Soil Erosion
Model, De Roo et al., 1996a; Jetten & De Roo, 2001). The research described in this
thesis was part of the EROCHINA project and focused on process based erosion
modelling in the selected catchment.
1.4
The aims of this thesis
Soil erosion modelling is potentially a powerful tool for combating soil erosion. It helps
us better to understand erosion, better to locate erosion hotspots, to predict erosion and to
evaluate the effect of different soil and water conservation methods. Even though
research on the Loess Plateau has been intense for the past 50 years, process based
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erosion models have not often been applied. Instead, more attention has been given to
monitoring.
The presence of loess on steep slopes requires special attention in erosion modelling, and
the Loess Plateau has several characteristics that specifically need to be addressed:
• Slopes in the erodible loess can be very steep, which may have consequences for
flow velocity and transport capacity of the flow.
• Sediment concentrations in runoff may be extremely high. At such concentrations
the fluid properties might differ from those of clear water.
• The area is heavily dissected by gullies. Thus, erosion models should be able to
cope with gully erosion, or at least with gullies as a source of sediment.
The aims of this thesis are:
1) To evaluate what are the effects of these particular characteristics of the Loess
Plateau on soil erosion processes.
2) To evaluate whether or not process based erosion models in general, and LISEM
in particular, can deal with those characteristics.
3) To adapt the LISEM model to Loess Plateau conditions if this proves necessary.
4) To calibrate and validate the LISEM model for a small catchment on the Chinese
Loess Plateau.
5) To simulate the effect that different soil and water conservation methods have on
soil erosion.
Chapter 2 examines the abilities of current erosion models to deal with the characteristics
of the Loess Plateau. Chapter 3 describes the study area, a small catchment on the Loess
Plateau, in more detail. Chapter 4 lists the methods used in the field as well as the
measurement results. The Loess Plateau characteristics of steep slopes, high
concentrations and presence of gullies are discussed one by one in chapters 5 to 8. The
effects of these changes on the LISEM simulations are evaluated in chapter 9. Finally,
Lisem is calibrated in chapter 10 and used to simulate the effect of soil and water
conservation methods in chapter 11.
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