Decreased water and sediment fluxes

During the last 40-50 years, water and sediment discharges by the Yellow River decreased rapidly (Ye 1994; Wang et al. 1997; Pang et al. 1999; Xu and Sun 2003). Figure 15.2 shows the water and sediment discharges at the upper, middle, and lower reaches of the Yellow River from 1950 to 2002. A clear decline in discharge since the 1970s occurred in the middle, and lower reaches, especially at Lijin station. The Lijin station is the most downstream sampling point on the main stream of the Yellow River and the water and sediment transports measured at this station are usually regarded as the water and sediment fluxes into the sea.

Between 1950 and 2000, the water flux into the Bohai Sea decreased from 480 to 46 x 108 m3 yr-1, and the sediment discharge decreased from 13 to 0.3 x 1081 yr_1. The decline in water volume was so great that the river failed to flow downstream. Although there were some very dry years, such as 1875-1878, and 1922-1932, there was no record of the Yellow river failing to flow downstream during these periods. The first flow-cut-off of the Yellow River at Lijin station occurred in 1972, but it happened regularly after that and became more and more severe. The flow-cut-off of the Yellow river downstream occurred in 21 of the years from 1972 to 1998. The longest

Fig. 15.2. (a) Water and (b) sediment discharge at upper, middle, and lower reaches of the Yellow River.

no-flow period was for 256 days in 1997 (Li et al. 1997). Since then the Yellow River had become a seasonal river. The maximum course length with no-flow was 683 km in 1995, which is 97% of the whole downstream course length (Pang et al. 1999). Since 2000, when a modulated water policy was applied, the Yellow River has not dried up. However, the water discharge is limited, being about 40 m3 in spring.

2.2.1. Water consumption

Most of the drainage area of the Yellow River is located in a semiarid zone, and water resources are far from meeting water demand. In the past 30-40 years, water consumption from the river increased, especially in t 140 "a 120

t 140 "a 120

1950s 1960s 1970s 1980s 1990s

[D upper stream @ mid-stream 0 down stream

Fig. 15.3. Human consumption of water in the Yellow River basin in different decades (Pang et al. 1999).

1950s 1960s 1970s 1980s 1990s

[D upper stream @ mid-stream 0 down stream

Fig. 15.3. Human consumption of water in the Yellow River basin in different decades (Pang et al. 1999).

the lower reaches of the Yellow River (Fig. 15.3). From 1950s to 1970s the volume of water consumed downstream increased from about 19 x 108 m3 yr_1 to about 84 x 108 m3 yr_1 which was greater than that used in any other period (Pang et al. 1999). In the 1990s, the quantity of water consumed amounted to more than 60% of the total "natural" runoff. Reduction in runoff by water consumption lowers the river's ability to transport sediment to the sea. Consequently, there is a negative correlation between the annual water and sediment fluxes and annual water consumption.

Pang et al. (1999) pointed out that flow-cut-off of the Yellow downstream did not happen even during the continuously dry years from 1922 to 1932. Therefore, the main factor responsible for the river flow-cut-off must be the rapid increase in consumption of water by human activities in the river basin.

2.2.2. Water/Soil conservation

Since 1960, water/soil conservation has been practiced in the drainage basin. By 1998, conservation had been applied to about 17.1 x 104 km2 in the loess plateau (IRTCES 2001), which led to a decrease in water and sediment fluxes into the sea. Land terracing, and tree- and grass-planting also significantly reduced erosion on hillslopes. Check dams trapped eroded sediments in gullies and increased sediment storage. As a result of the above practices, transport of sediment into the Yellow River and then into the sea has been considerably reduced. However, the practice of water/soil conservation is relatively small compared to other human activities along the Yellow River. Increasing the total area of land terracing, and tree- and grass-planting by 102 km2, would decrease the sediment and water flux to the sea by 0.19 million t yr_1, and 0.02 billion m3 yr_1, respectively (Xu and Sun 2003; Xu 2003).

2.2.3. Dam construction

An investigation of sedimentation in reservoirs along the whole basin (19901992) found that there were eight reservoirs along the main stream of the Yellow River, with a total capacity of 41.3 billion m3, and that 8.0 billion m3 of sediment had been deposited in these reservoirs. Along the main tributaries, there are 483 reservoirs with a capacity larger than 105 m3, and their total capacity is nearly 7.6 billion m3. Up to 1997, the total cumulative volume of sediment within these reservoirs was 3.3 billion m3 (Xu 2003). A large amount of sediment, together with water, had been trapped by these reservoirs, which led to a sharp reduction in sediment and water fluxes to the sea at the beginning of water storage of the reservoirs. Sediment trapped in reservoirs is very important, but not the crucial factor responsible for the continuous decline of sediment flux to the sea.

The Sanmenxia Reservoir is one of the most important reservoirs along the Yellow River. In terms of operation mode, the use of this reservoir can be divided into three stages. From 1960 to 1964, the reservoir was used for water storage, trapping almost all sediment in the reservoir. The total volume of sediment was 4.5 billion m3. From 1964 to 1973, the reservoir was used for flood retention, and after flooding, sediment can be released through the dam. During this period, the total volume of sediment deposited was 1.2 billion m3. After 1973, the reservoir was used under the mode of "storing clear water and releasing sediment", which significantly reduced sedimentation in the reservoir. During the 18-year period from 1973 to 1990, only 0.4 billion m3 of sediment was trapped in the reservoir.

The Xiaolangdi Reservoir located between Sanmenxia and Huayuankou was built in 1999. The reservoir was designed to hold 12.7 billion m3 water initially, and after sedimentation to hold 5.1 billion m3 water. That is to say, the Xiaolangdi reservoir is expected to trap as much as 7.6 billion m3 sediment from the Yellow River within 20-30 years. From 1999 to 2002, 0.9 billion m3 of sediment has been trapped in this reservoir (IRTCES 2000, 2001, 2002).

2.2.4. Fluctuations of water and sediment discharges with climate

Spectral analyses were made with a long-time series (1950-2002) of water/sediment discharges at Lijin station. The most protruding peaks corresponded to the time of El Niño events suggesting that water and hence sediment discharges from the Yellow River are strongly correlated with El Nino events through precipitation, as suggested by Hu et al. (1998). Another peak in both water and sediment spectra, appears to be a quasi-biennial oscillation signal associated with the monsoon. It is suggested that climate change associated with El Niño and the monsoon can have a significant effect upon the water and sediment discharges of the Yellow River.

On the other hand, a tendency of climate change in the last 40-50 years in northern China is for the area covering the Yellow River basin to be continuously dry. For example, according to Xu (2003) and Xu and Sun (2003)'s regression equations, the annual precipitation in the area upstream of Huayuankou decreased by about 70 mm from the 1960s to 1990s, and sediment and water discharge to the sea would have decreased by more than 414.4 million t yr_1 and 12.3 billion m3 yr_1, respectively.

3. Yangtze River

The Yangtze River (Changjiang; Fig. 15.4) discharges almost the same amount of water to the sea as the Ganges River, but has a greater basin, a much larger drainage area, and contains more paddy fields with high

Fig. 15.4. The Yangtze River basin.

fertilizer inputs. Because of the extensive, high input agriculture in the basin, it provides a suitable example for demonstrating the impact of human activities on nutrient input to the coastal zone.

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