The Runoff Responses By The Operation Of Cascade Hydropower Generations In The Mainstream Of The Lhasa River

As one of the most effective means for human beings to change the process of surface runoff and water resource allocation,reservoir dam construction not only alleviates water resource shortage and promotes economic development,but also has a negative impact on river ecological environment.Quantitative evaluation of runoff variation caused by reservoir construction and operation is of great significance for rational reservoir operation and reduction of ecological impact caused by reservoir construction.In this paper,the main stream of Lhasa River in Tibet is taken as the research object to simulate the influence of cascade hydropower stations under different numbers and different operation modes on the river situations such as monthly average flow,extreme flow,flood fluctuation frequency and frequency.Then,the multi-series contribution rate segmentation is used to analyze the contribution rate of the variation of natural runoff,water diversion and cascade reservoir operation with different operation modes to the average monthly runoff.The main results of this study are as follows:(1)Based on the daily runoff from 1956 to 2016 of Pando,tangja and Lhasa,the characters of daily runoff was analyzed using Mann-Kendall mutation test and cumulative anomaly slip t-test.The annual runoff of Lhasa River fluctuates and has periodic characteristics,with a change period of about 30 years.Among them,the annual runoff from 1950s to 1970s,1990s to the beginning of the 21st century shows an increasing trend;from 1970s to 1990s.the runoff from the end of the 21st century to the present has a decreasing trendThe abrupt change years of runoff are 1995 and 2005.(2)Modsim-dss is used to build the operation model of cascade hydropower stations in the main stream of Lhasa River,which can simulate the daily runoff of each section under different number of cascade hydropower stations.IHA-RVA mothed is used to analyze the change of river hydrological situation before and after the operation of cascade hydropower stations.Under the influence of the operation of cascade hydropower stations on Lhasa River,the hydrological situation has changed significantly.The maximum change degree of single index is March and October,which belongs to severe change.Under the current condition,the comprehensive change degree of hydrological situation of Tangjia station is 0.21,and the heavy change index accounts for 8%of the total index.The comprehensive change degree of hydrological situation of Tangjia station in the planning period is 0.41,which is moderate change,and severe change variable indicators accounted for 28%of the total.(3)The influence of cascade hydropower station operation on hydrological situation under dispatching and power dispatching is analyzed quantitatively.Under the conventional operation mode,the hydrological situation of Lhasa station has slightly changed,with the comprehensive change degree of 0.20;under the power operation mode,the comprehensive change degree of Lhasa station is 0.48,which is moderate.The contribution rate of the operation of cascade hydropower stations to the change degree of monthly average discharge of Lhasa hydrological station is 0.17 in conventional operation mode,and 0.38 in power operation mode.The contribution rate of cascade power station operation to the change of monthly average flow of Lhasa station under power dispatching is 124%higher than that of conventional dispatching.The above research results provide technical support for the next step of ecological operation of cascade hydropower stations in Lhasa River and have positive significance for protecting the ecological environment of Lhasa River.