Hydrological Regime Change And The Estimation Of Ecological Flow In The Lower Mekong River

The natural hydrological regime of a river is a key element that influences the biodiversity of the river and the normal function of the ecosystem.Mekong River is a rare international river in the world that develops vertically.It has significant meaning to maintain the security of ecology in China and Southeast Asia due to its abundant water resources and biodiversity.Under the background of climate change and intensified human activities,the hydrological regime of the Mekong River has changed.That is important to discriminate the changing characteristics of hydrological regime and estimate the ecological flow of the Mekong River under the changing environment for the rational exploit of water resources and the protection of river ecosystem.Based on the observation data of important hydrological stations in the lower Mekong River,this study identified the runoff variation characteristics（including trend,periodicity,mutation,etc.）of hydrological stations in the lower Mekong River under different time scales.The hydrological change extent and the overall change extent are analyzed by applying IHA and RVA from five aspects:discharge,time,frequency,delay and change rate,which reveals the influence scope and extents of hydropower development periods,as well as the differences between upstream and dowmstream.the monthly ecological discharge of each station is estimated by flow duration curve shift method,and the impact of hydrological variation on the ecological flow is analyzed.The main conclusions can be obtained:（1）No obvious variation tendency of annual flow was founded in the lower Mekong River from 1960 to 2018,but there are evident discrepancy between upstream and lower reaches.The regional difference of runoff change is mainly related to the spatial differentiation of rainfall.The mutation test showed that the mutation years of the annual average flow sequence of Luang Prabang,Nakhon Phanom,and Mukdahan stations occurred in 1986,1994,and 1994,respectively.From the scales of monhthly and dry-wet season,the average flow series of 7 stations from January to May and dry season have obvious mutation,while the mutation of average flow series from June to December and wet season only exists in Chiang Saen,Luang Prabang and Nakhon Phanom stations.There is a certain similarity in the runoff periodicity of the 7 stations.The annual average runoff series mainly exist 5-10 years,10-15 years,23-32 years time scale cycles,the main circles are 8 years,13 Years and 28 years.（2）Applying IHA and RVA methods to research the response of hydrological regime of lower Mekong River after the construction of dam in Lancang River cascade,the conclusions indicated that:the discharge from June to November,the maximum value of discharge and its occurrence time,the frequency of high pulses and their duration of lower Mekong River are all low changes degree;the flows from December to May,minimum flow value and its occurrence time,the number of low pulses and their duration are all moderate or above degree of change;the number of flow reversals at most sites are all high changes deree.Environmental changes in this basin mainly affected the number of flow reversals,the minimum flow value,the monthly average flow from February to May,the frequency and duration of low pulses,the mean decrease rate of flow of the lower Mekong River.（3）Taking into account the variability of the distribution characteristics of runoff during the year,the ecological flow of the Lower Mekong station was estimated using the flow duration curve shifting method,and referring to six river environmental management levels from A-F proposed by DWAF（South African Ministry of Water Affairs and Forestry）,the results showed:in order to maintain the management target of optimal river ecological conditions（EMC-A）,the ecological flow of Chiang Saen,Luang Prabang,Nong Khai,Nakhon Phanom,Mukdahan,Pakse and Stung Treng stations are 65647×10~6m~3、93289×10~6m~3、108051×10~6m~3、170606×10~6m~3、179130×10~6m~3、225021×10~6m~3、290360×10~6m~3 which accounted for 78%、77%、76%、75%、74%and 74%of the average annual runoff respectively.Under the context of human activities such as hydropower development in Mekong river basin,in order to achieve the goal of protecting the basic functions of the river ecosystem（EMC-C）,the ecological flows of the seven stations are 40631×10~6m~3、54490×10~6m~3、63242×10~6m~3、90455×10~6m~3、94507×10~6m~3、114956×10~6m~3and 142193×10~6m~3 which accounted for 48%,45%,45%,40%,39%,38%and 36%of the average annual runoff respectively.