Since the mid-1980s, with the exploitation and operation of the “two dam and eight cascade” program, which takes Xiaowan dam and Nuozhadu dam as its core component, the ecosystem health of the Lancnag River has been seriously affected by the construction and the operation of those dams. And the ecological water requirements, the most important basis to maintain the ecological health of the Lancang River, have attracted most of our attention. According to our field investigation of the Lancang River and analysis of the data collected during the investigation, the ecological water requirement of the Lancang river mainly includes the following several respects: the basic flow which is used to maintain the basic form of river; the environmental (purifying) flow which Keeps water of the river in a good quality to maintain its basic function; the transporting sediment flow which is used to maintain and regulate water sediment balance and the ecological flow which is used to provide enough water in order to guarantee the survival of living beings in the river and their habitat. Considering the supporting information data, the present paper adopts the ecological runoff——grade coefficient of ecological water requirements methods.
Firstly, based on the investigation of hydrological, ecological and human activity condition of the Lancang River, this paper divides the Lancang River into three regions, nine reaches and five branches, and the present study choose up reach (Jiuzhou), the middle reach (Gajiu) and the lower reach (Yunjinghong) as study targets. Secondly, Built on the annual measure runoff, the present paper uses the ecological characteristic comparison and statistical methods including variation and concentration analysis to calculate the grade coefficient of ecological water requirements in the three different regions of the Lancang River, and uses the frequency method to calculate the ecological runoff based on principle of low runoff with high frequency.
At last, this paper calculates the ecological water requirements in the Lancang River. During this process, this paper combines the grade coefficients of EWR with ecological runoff to calculate the Lancang River ecological water requirement, by using the frequency and Coefficient of variation methods. Considering the big difference in coefficient of variation of runoff in flood and non-flood season, this paper calculates the ecological water requirement respectively.
By the systematic analysis of the outcome, we can get the minimum, satisfying and optimal ecological water requirements in the Lancang River, and they are 161.77×108m3、278.22×108m3 and 358.82×108m3.The three ecological water requirements respectively occupy 28.49%、49.00%、63.20% of the average measure run-off (567.75×108m³) of Lancang River, and they also respectively occupy 21.15%、36.37%、46.90% of the natural run-off (765×108m³)of Lancang River.