Attribution Of Historical Runoff Changes And Prediction Of Future Runoff Changes In The Lancang River Basin

The water cycle in the Lancang River basin has fluctuated due to the global climate change and intensification of human activities,which have resulted in frequent global extreme weather events.In order to better understand the water cycle in the Lancang River basin,it is necessary to analyze the historical and future hydrometeorological elements in the basin.Based on the meteorological and hydrological data of the Lancang River basin from 1961 to 2015,this study uses the Mann–Kendall(MK)trend test and mutation test to analyze the trend of hydro–meteorological variables,as well as which year the runoff changes,respectively.And quantified the impact of climate change and human activities on runoff change.Then a Soil and Water Assessment Tool(SWAT)model of the Lancang River basin was developed based on the basic data of Digital Elevation Model(DEM),land use,soil,and meteorological hydrology.After that,the Coupled Model Intercomparison Project 6(CMIP6)future climate model data were bias corrected using Quantile Mapping(QM)method and the corrected simulation accuracy was better.The bias-corrected future climate data are analyzed,including the spatial and temporal distribution of future climate anomalies and the spatial and temporal trends of future climate change.Finally,the bias-corrected climate model data were input into the Lancang River Basin SWAT model to obtain the Multi-Models Ensemble(MME)predictions of future runoff in the Lancang River Basin,and the changes of future runoff relative to the historical period were also analyzed from different time scales.And finally design and develop a lightweight Web GIS system for presenting multi-source data of the Lancang River basin based on the basic geographic data and the graphical data obtained in this paper.The following conclusions were obtained.(1)The Lancang River basin showed natural fluctuations in precipitation during1961-2015;the potential evapotranspiration showed an extremely significant increasing trend;and the runoff depth showed an extremely significant decreasing trend.(2)Regarding the temporal distribution of future climate anomalies,precipitation and temperature anomalies(relative to 1960-1984)increase year by year in the future 2021-2100 under the SSP1-2.6,SSP2-4.5,and SSP5-8.5 scenarios.The anomaly changes are relatively similar in the early future under the three scenarios,and start to show increasing trends of different degrees in the mid-future,while the anomalies under SSP5-8.5 continue to increase rapidly in the far future,while the anomalies under SSP1-2.6 and SSP2-4.5 have insignificant trends or slightly decreasing trends.(3)In terms of the spatial distribution of future climate anomalies,the temperature anomalies under the three SSP scenarios show positive basin-wide values during the future 2021-2100 period,and the spatial distribution is characterized by greater anomalies in the north than in the south,and the anomalies increase with increasing emission concentrations.The spatial distribution of precipitation anomalies is positive in the north and negative in the south in the near and mid-term future,and positive in the basin-wide range in the far future,but the anomalies in the north are larger than those in the south.(4)In terms of absolute values of future climate change,both precipitation and temperature show increasing trends across the basin during 2021-2100,with increasing concentrations of emissions from the SSP scenario.The growth rate of precipitation is greater in the South than in the north,while the temperature shows the opposite trend,that is,the north is greater than the south.(5)In the human activities period(2005-2015)compared to the base period(1961-2004),both climate change and human activities led to a decrease in runoff,and the decrease in precipitation was the dominant cause of the decrease in runoff with a contribution of 45.64%,followed by human activities with 40.45%,and potential evapotranspiration with the least contribution of 13.91%.(6)The average annual flows in the future 2021-2100 increase on average by 27.33%,25.07% and 31.32%respectively in the three SSP scenarios compared to the base period.And the growth is extremely rapid in the near future and stable in the medium and long term future.(7)The change in future seasonal flows share compared to the base period is more consistent across the three future periods and the three SSP scenarios,i.e.,future spring and winter flows share increase(1.54%,1.67%)and summer and fall flows decrease(-1.08%,-2.13%)compared to the historical period.(8)The intra-annual distribution of monthly mean flows in the Lancang River basin is more uniform during the future2021-2100 period under the SSP1-2.6,SSP2-4.5,and SSP5-8.5 scenarios,decreasing in the rainy season and increasing in the dry season.