| The Lhasa River is a major tributary of the Yarlung Zangbo River on the Qinghai-Tibet Plateau.It is one of the highest braided rivers in the world,with a total length of 568 km and a basin area of 32,896 km2.In recent years,the construction and operation of cascade reservoirs and frequent human activities in the basin have caused changes in river runoff,braided river channels,and riverside wetlands.Based on the measured data of three hydrological stations and meteorological stations in the main stream of the Lhasa River,this research analyzed the variation characteristics of hydrometeorological elements in the Lhasa River Basin,and studied the influence of human activities on the form of braided channel in the middle and lower reaches of the Lhasa River by means of field investigation and remote sensing image analysis.The CASA model was used to calculate the net primary productivity of the Lalu wetland in the lower reaches of the Lhasa River,and the Costanza ecosystem service value formula was used to calculate the ecosystem service value of the Lalu wetland,and study the ecological status of the Lalu wetland.The research results can provide scientific basis for rational development and utilization of water resources in the Lhasa River Basin and ecological protection and restoration of the Lalu Wetland.The main results are as follows:(1)The evolution law of hydrometeorological elements in the Lhasa River Basin was analyzed,and the effects of precipitation and human activities on runoff in flood season were quantitatively evaluated by the double accumulation curve method.1995 and 2005 were the years with abrupt changes in runoff.The years of 1956-1994 were used as the base period,and the quantitative analysis revealed that the human activity contribution proportion to the runoff from 1995 to 2004 is less than 50%,while the contribution proportion of human activity from 2005 to 2016 significantly increases by 71%.The greater impact rate of human activities occurred in the lower reaches of the river,among which the newly built Zhikong-Pangduo Reservoirs are the main controlling factor for regulating the discharge flow.(2)After the operation of Zhikong-Pangduo Reservoirs,the discharge flow was controlled,and the guarantee rate of the ecological base flow of the Lhasa River was reduced.Five hydrological methods were used to calculate the ecological base flow of the Lhasa River,and it was found that the calculation results of the Tennant(30%)method were more consistent with the actual situation.The recommended annual ecological base flow values of Pangduo,Tangjia and Lhasa stations were 58.59 m3/s,73.02 m3/s and 85.64 m3/s.And after the operation of the reservoir,the guarantee level of the ecological base flow in the non-flood season has decreased.(3)Human activities have significantly changed the evolution of braided channels in the middle and lower reaches of the Lhasa River.The braided branching degree of the main channel in the Pangduo-Zhikong reach is not obvious,and the interannual variation of the overall channel morphology is small in the last 10 years,and the branching index is about 1.17.but the sandbar cutting appears in some reach.The activity capacity of braided channel in the ZhikongQushui reach became weak in the past 20 years,and the branching index decreased from 2.92 to 2.03.Compared with remote sensing images over the years,the river channel was obviously invaded by human activities(such as urbanization development,farmland construction,etc.).(4)The Lalu wetland in the lower reaches of Lhasa River has important ecosystem service function and value to Lhasa,while human activities have significantly affected the wetland coverage types.From 1990 to 2010,the average annual vegetation net primary productivity(NPP)of the Lalu Wetland showed an increasing trend,with the fluctuation range of 265.1 to 422.3 gC·m2 a-1.In recent 20 years,the ecosystem service value of the Lalu wetland showed a fluctuating upward trend,increasing by 450,600 yuan,with an annual change rate of 0.12%. |
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