Assessing the impact of Danjiangkou Reservoir on the hydrologic alterations in the middle reach of the Hanjiang River using the histogram comparison approach

The Histogram Comparison Approach was used to assess the hydrologic alteration of the Hanjiang River using daily streamflow data collected from the Huangzhuang hydrometric station, which monitors the hydrologic processes in the middle reach of the river, for the period 1951-2014. The Danjiangkou Reservoir, which is the water source for the middle route of South-to-North Water Diversion Project in China, was constructed from 1967-1974, with 1951-1966 considered the pre-impact period and 1976-2014 as the post-impact period. 32 indicators were calculated to quantify the alteration of the hydrologic regime of the Hanjiang. The overall degree of alteration was valued at 47% and was classified as "moderate". The alterations of all 32 parameters ranged from 21%-83%; of these, 2, 25, and 5 parameters were classified as "high", "moderate" and "low", respectively. Significant increases were observed from the number of reversals and low-flow relevant parameters, including the monthly flow from December to March, the annual flow minima, and the base flow index. The results of the present research have important scientific significance for maintaining the health of the riverine ecosystem in the middle reach of the Hanjiang River.