Reasons for site selection
The natural condition of Yanhe River Basin is poor. It is situated in an arid to semi-arid region, with low precipitation and major rainfall (about 70%) in the summer and early autumn as heavy storms.
The soil erosion is very severe in the Yanhe River Basin amounts to 4829.20 kmē, about 62.90% of the whole basin. The average annual runoff is 0.22 billion m3, and the runoff modulus accounting for 4,776.36 m3/kmē/yr. The average annual sediment flow is 4.776 million tons, and the sediment flow modulus accounting for 8,100 t/kmē/yr. The coarse sediment (> 0.05 mm particle size) flow modulus is 2,430 t/kmē/yr at the Ganguyi Hydrology Station. The gully density (the length of channel/kmē) amounts to 2.1 to 4.6 km/kmē.
The high soil erosion rates lower the fertility of the land and also greatly decrease the available soil water thus inducing lower yield of grain and biomass.
The sediment concentration of Yanhe River is very high (Figure 3). The data from Yanhe hydrology Station shows that the mean sediment concentration is 227,227 kg/m3, and the highest one was 1300 kg/m3 recorded in June 1963. The sediment would silt in the reservoirs and riverbed of the downstream.
Figure 3: The Muddy flow of Yanhe River
