3 Case Study

3.1 Study Areas

The research object of this study is three basins, i.e. Xiangxi River, Jinghe River and Zhongzhou River. They are tributaries of three state-level basins in central, northwestern and southeastern China, i.e. Yangtze River, Yellow River and Pearl River, respectively.
Xiangxi River (110°25′~116°06′E, 30°57′~31°34′N, Fig. 3a), also known as Zhaojun River, is located in the western part of Hubei Province, China. It is a first tributary of Yangtze River. Its waterway is 97.3 kilometers and catchment area is about 3,100 square kilometers. It originated from Shennongjia and has two sources. The east source is in the Shenmadian, Shennongjia, and the river turns from east to west and then to southeast. Since the west is from the south of the Great Shennongjia Mountain, the river is from the northwest to the southeast. And the Xiangxi River Basin is located in subtropical monsoon climate with four distinct seasons.
Jinghe River (106°14′~108°42′E, 34°46′~37°19′N, Fig. 3b) is a secondary tributary in Yellow River and one of the first tributary in Weihe River. It is located in the middle of the Loess Plateau. Its waterway is 455.1 kilometers and catchment is 45,421 square kilometers. The river flows from northwest to southeast. The flood and dry in Jinghe River Basin vary greatly, and the flood peak period is from July to August. According to the hydrological data from 1960 to 1987, the average monthly runoff is more than 160 times the difference between flood and dry. In addition, due to its location in the Loess Plateau, the soil erosion is serious. The sediment concentration of Jinghe River is the highest in the three basins.
Zhongzhou River (107°55′~108°22′E, 24°42′~25°34′N, Fig. 3c), also known as Xiaohuan River, is located in Huanjiang Autonomous County in Guangxi province, China. It is a tributary of Pearl River. With a total length of 136.5 kilometers and drainage area of 2,328 square kilometers, Zhongzhou River is the second largest river in Huanjiang County. It starts from the Zaibian District in Congjiang County, Guizhou Province, and flows into Longjiang River from north to south. The flood peak in Zhongzhou River usually appeares in June and July, and the runoff was greatly affected by precipitation.

3.2 Hydro-Climatic Features

According to historical observation data, the climatic and hydrologic characteristics of the three basins are analyzed. It can be seen from the monthly average results (Fig. 4) that annual distribution of precipitation in Xiangxi River and Jinghe River is relatively uniform, and the two basins have basically the same tendency. The peak precipitation in Zhongzhou river is obvious, from April to June. The annual temperature distribution in three basins are similar, and the overall temperature decreases with the increase of latitude where the basin in. In terms of runoff, no significant difference between Xiangxi River Basin and Zhongzhou River Basin, with an average annual runoff of about 30 m3/s. However, the scale of Jinghe River Basin is small, and the average annual runoff is only 1.9m3/s. The flood periods of the three basins are very close, July and August in Xiangxi and Jinghe River, June in Zhongzhou river.
The interannual variations of climate factors in the three basins are all not obvious. Interannual coefficient variation (CV) values of temperature are less than 0.1, and that of precipitation are less than 0.3. However, the variation of runoff has different performance. The interannual CV of Xiangxi River is 0.30, Jinghe River is 0.62, and Zhongzhou river is 0.42. One important reason for the large value of runoff in Jinghe River is the runoff is not plenty in itself. It is worth mentioning that the correlation coefficient between precipitation and runoff in Zhongzhou River basin reaches 0.69. In the monthly data, the correlation coefficient can reach 0.85 which is the highest in the three basins.

3.3 Data

Hydrological models need to be calibrated and verified firstly. Observation data is used as the training sample. The above 2/3 of the training sample is used as the calibrated sample, and the remaining 1/3 is used for verification. Observed precipitation, temperature and runoff from 1991/1/1 to 2002/12/31, 1960/1/1 to 1978/3/31 and 2009/1/1 to 2012/4/30 are used for model calibration in three basins, i.e. Xiangxi River, Jinghe River and Zhongzhou River, respectively. And the same data during 2002/1/1-2008/12/31, 1978/4/1-1987/12/31 and 2012/5/1-2013/12/31 are used for verifying, respectively.