Based on the data of long-term daily discharge, this paper took the mainstreams of the Hanjiang River and Yangtze River as study areas. The study focused on analyzing the hydrology processes change in the critical sections in the mainstreams of the study rivers ,taking the method of the Indicators of Hydrologic Alteration (IHA).And the study selected the least value of monthly discharge as the minimum ecological discharge(MED), and the appropriate ecological discharge(AED) was computed by frequency analysis method. At last , taking the Hanjiang River for instance , the ecological discharge were calculated in two conditions with interannual variation and without.
The purpose of this research can provide an effective way to evaluate hydrological regime change in damed rivers, also can offer a reference for re-operation of hydropower projects. The main results are as follows:
1. In Xiangyang section of the Hanjiang River , there were 4 parameters belonging to high alteration, 18 parameters belonging to moderate alteration and 10 parameters belonging to low alteration in all IHA parameters. The obvious hydrology parameters alterations were as follows: in dry season, the discharge increased,at the sanme time, the counts and duration of low pulse reduced; in flood season , the discharge had no visible change, the counts and duration of high pulse reduced as well; the average and interannual variation of rise rate diminished; the number of reversals increased. The results of overall alteration showed the hydrology change of this site was between moderate and high alteration. It was suggested that the hydrology regime in Xiangyang was seriously affected by the construction and operation of the Danjiangkou Reservoir.
2. In Xiangyang,when the mean flow is more than 1710m3/s, the year is flood year; when the mean flow is less than 1040m3/s, the year is dry year; the remaining year is level year.This study classifyed the years by using this rule .And the water periods during one year were classifyed ,too. On this basis, the ecological discharges were calculated in two situations with interannual variability and without. It was evident that the MED and AED emendated by Tennant method satisfyed the ecological demands, except very few situation.
3. On the whole, the hydrological alteration of critical sections in the mainstreams of the Yangtze River mainly refered to: the discharge from January to April reduced, while the discharge from June to July increased; the date of discharge reduction in Cuntan and Yichang sections was mostly between August and November, whereas the date of discharge reduction in Hankou and Datong sections was postponed, mostly between October and November; the average discharges of minimum flow group increased , and the date of annual minimum flow advanced; the consecutive daily rise rate showed an downward tendency, while the consecutive daily fall rate showed an upward tendency. It was evident that the MED and AED emendated by Tennant method satisfyed the ecological demands.