Attribution Analysis Of Runoff Change And Future Flood Risk Assessment In The Xiliu Songhua River

Climate change and changes in substrate elements have severely affected the regional water cycle,leading to changes in runoff processes in many of the world’s rivers.Changes in water resources due to altered river runoff processes and changes in the risk of catastrophic events such as floods pose great challenges to watershed water resources planning,management,and flood control.Therefore,it is of great importance to quantitatively study the contribution of climate change and changes in substratum elements to runoff changes and to assess the flood risk in watersheds under changing environments for rational planning and utilization of water resources and disaster prevention and mitigation.Although scholars have carried out a lot of research on the attribution analysis of runoff change and flood risk assessment,there are still some deficiencies.In the aspect of attribution analysis of runoff change,most of the existing studies were carried out on the basis of water balance at the multi-year scale,and few studies on the change of runoff at the seasonal scale.In terms of flood risk assessment,most of the previous studies were on the analysis of historical scenarios and current flood events,and there was little work on the assessment of flood risk in the future watershed.For this reason,the Xiliu Songhua River basin of the study was used to analyze the evolution characteristics of hydrometeorological elements in the study area by using various mathematical statistics methods.Based on the unsteady Budyko framework,the influences of climate change and underlying surface factors on annual and seasonal runoff in the study area were quantitatively analyzed.A watershed flood risk assessment framework including analytic hierarchy process(AHP),entropy weight method and GCM downscaling method was established to evaluate the flood risk distribution in the Xiliu Songhua River under different future scenarios.The main results of this study are as follows:In the Songhua River Basin of the West flow,the inconsistency test of hydrometeorological elements can effectively judge the variation trend of hydrometeorological elements.The hydrothermal coupling equation of the basin can be established by the Budyko framework to determine the degree of runoff changes by climate factors and underlying surface factors.The flood risk map established by MCDA can determine the intensity of flood risk in the Xiliu Songhua River basin in the future.(1)Based on the Mann-Kendall trend test and linear trend test,it can be seen that the sequence of hydrometeorological elements in Xiliu Songhua River Basin showed an insignificant change trend during 1989-2018.The mean values of precipitation,runoff and potential evaporation were 600-700 mm,150-250 mm and 800mm-850 mm,respectively.Based on Pettitt mutation test,it can be seen that the sequence of hydrometeorological elements in Xiliu Songhua River Basin showed an insignificant increasing trend,and the mutation of hydroclimatic elements was concentrated in 2009.Based on this,the research period can be divided into the baseline period(1989-2009)and the change period(2010-2018).On the annual scale,runoff in the changing period showed an upward trend compared with that in the base period.On the seasonal scale,spring and summer are the seasons with obvious runoff changes,and the spring runoff in the upper reaches increases obviously,while the summer runoff in the lower reaches increases obviously.(2)Based on the water balance relationship,the unsteady Budyko relationship,which can describe the coupling of water and heat at the annual scale in the Xiliu Songhua River Basin,is derived,and based on this,the degree of change of climate change and underlying surface change on runoff change in the Xiliu Songhua River Basin is quantitatively evaluated.Based on the attribution analysis,it is shown that climate activity is the main factor causing the runoff change in the Xiliu Songhua River at the annual scale,and its contribution rate is more than 80.3% on average.The runoff in spring and autumn was significantly affected by the underlying surface change,and its contribution rate was 59.5% and 73.0% on average.The main reasons for the annual runoff change in Xiliu Songhua River Basin are the increase of human production and living water,the increase of spring meltwater,the delay of river freezing time and the change of annual frozen soil activity.(3)Based on historical flood disaster events and MCDA method,this study draws a flood risk assessment map reflecting flood disaster characteristics in Xiliu Songhua River Basin,and finds that the flood risk in the central region of Xiliu Songhua River Basin is higher under historical scenarios.In addition,the climate and human economic data of four future scenarios(SSP1-2.6,SSP2-4.5,SSP3-7.0,SSP5-8.5)are brought into the Xiliu Songhua River flood risk assessment framework to get the flood risk assessment map of Xiliu Songhua River basin under future scenarios.In general,the near-term(2026-2050)flood risk showed a decreasing trend,while the medium-term(2051-2075)and long-term(2076-2100)flood risk showed a significant increase.