Impacts of reservoirs on the streamflow and sediment load of the Hanjiang River, China.

The Hanjiang River is an important tributary of the Yangtze River in China. Long-term observed streamflow and sediment load data that spanned 1951-2014 was collected from the Huangzhuang hydrometric station, which is located at the middle reaches. The data was analyzed to reveal the impacts of reservoirs on streamflow and sediment load of the Hanjiang River. The coefficient of variation (C v) and concentration degree (C d) were applied to describe the intra-annual distribution characteristics. Abrupt changes in the time series of the streamflow and sediment load were detected by the heuristic segmentation algorithm. The annual streamflow significantly decreased from 1561 to 1263 m3/s after 1991, which was mainly caused by climate change. Two significant change points in the annual sediment load series occurred at 1966 and 1985, and the average values of the sub-series were 3198, 952, and 251 kg/s, respectively. Significant change points in the C v and C d series of the streamflow and sediment load occurred around 1967. The C v and C d series decreased dramatically after the change points. Abrupt changes in the time series of the streamflow and sediment load mainly occurred around 1967 when the Danjiangkou reservoir began to impound water, indicating that the Danjiangkou reservoir was an important factor that caused hydrological changes. The reservoir trapped sediment, reduced sediment concentration, mitigated the monthly streamflow and sediment load fluctuations, and reduced the intra-annual variation and concentration. Assessed by the range of variability approaches, the overall alteration degrees of the streamflow and sediment regimes were 36 and 60 %, respectively, which qualified as a moderate degree. The reservoir exerted greater influence on the sediment regime than on the streamflow regime.