The Impacts Identification Of Climate Change And Human Activities On Hydrological Processes In The Upper Basin Of Pi River

Water,driven by both natural and human activities,is the most active factor in the earth's surface system.Under the background of global climate change and dramatic increase of human activities,the challenge of "water" is even more serious.On one hand,the frequency,intensity and occurrence range of extreme precipitation events caused by climate change are increasing day by day.On the other hand,the risk of flood disaster in watersheds caused by high-intensity human activities is more serious,which has brought tremendous impact on the healthy development of social economy and the construction of ecological civilization.In order to clarify the two core issues of "contribution of climate change and human activities to hydrological processes in watersheds" and "how to make analysis and prediction of hydrological processes under future climate change",the study is based on the theory of "natural-social" dual water cycle and takes "mechanism identification-scenario simulation-quantitative separationfuture prediction" as the main line to construct a suitable research area.Distributed hydrological model quantitatively identifies the impacts of climate change and human activities on hydrological processes by simulating historical processes;and predicts the future runoff changes by setting different climate scenarios,so as to provide scientific support for the sustainable development of regional water resources and flood control and disaster reduction in the future.Firstly,the upper basin of Pi river with high vegetation coverage and frequent flood disasters was selected as the study area.The mutability,trend,significance and periodicity of precipitation and runoff series data in 56 years(1960-2015)of the study area were analyzed by Mann-Kendall test,sliding t-test,Sen's slope estimation,wavelet analysis,and Spearman rank correlation test was used.The correlation between precipitation and runoff was analyzed,and the spatial distribution characteristics of precipitation in flood season and annual were analyzed by Arc GIS,and the abrupt change and trend significance of temperature series data were analyzed.Secondly,the land use change characteristics from 1985 to 2014 were identified by Arc GIS,and the NDVI change trend from 2000 to 2014 was analyzed,so as to clarify the land use and vegetation cover evolution characteristics in the study area.Finally,a distributed hydrological model(WEP model)for the study area was constructed.And by changing the meteorological data and land use data in the model,the contribution of historical climate change and human activities to the runoff variation characteristics of the basin was quantitatively identified.By setting different RCP scenarios,the evolution characteristics of future temperature,precipitation and runoff were predicted and analyzed.The main conclusions are as follows:(1)Spatial and temporal evolution characteristics of Hydrometeorological factors: the main hydrometeorological factors(precipitation,runoff,temperature)mutation occurred in the early 1990s(up to 0.05 significant level),and the mean difference before and after the mutation was significant;the ratio of precipitation in flood season to annual precipitation do not have a mutation in the whole research period(1960-2015),showing a significant increase trend(up to 0.05 significant level).The significant level of 0.05 indicates that the annual precipitation in the basin is more concentrated in the flood season,and the possibility of extreme precipitation events is increased.(2)Evolution characteristics of underlying surface conditions: the dynamic change of comprehensive land use in the area is not obvious,in which natural vegetation is slightly reduced,the total area of residential and urban-rural construction land is small,but the dynamic degree of land use change reaches 77.46%;the vegetation development in most areas of the basin shows an increasing trend,and the overall evolution of vegetation cover is in a better state;(3)Construction of Distributed Hydrological Model(WEP Model): Based on the distributed WEP hydrological model,the existing data of the study area are unified and integrated according to the input format,and a distributed hydrological model suitable for the study area is obtained.The relative error of annual runoff of Foziling station during calibration period is-9.44%,Nash efficiency coefficient and correlation coefficient are 0.71 and 0.87,respectively,and-7.15%?0.70?0.85 during verification period.The relative error of annual runoff of Huangweihe station is 2.65%?0.92 and 0.97 respectively(during calibration period),and 9.68%?0.86?0.94 during verification period,showing that the distributed hydrological model is suitable for the the upper basin of Pi river and can accurately simulate the hydrological process of the basin.(4)Identification of impacts of climate change and human activities: climate change and human activities play opposite roles in the process of runoff generation and confluence.Climate change mainly reduces the runoff of the basin by affecting precipitation,while human activities contribute to increasing the discharge of the basin,but the impacts of climate change on runoff process are much greater than that of human activities;The impact of runoff process reduced by climate change and human activities is mainly reflected in summer and autumn;The sharp decrease of runoff in the basin is mainly caused by climate change,and human activities increase runoff slightly,accounting for 2% to 3% of the annual decrease caused by climate change.However,the relative contribution in the month varies greatly,especially in winter.(5)The future prediction of meteorological and hydrological elements: the response of runoff to climate change is a complex process,in which there is a negative correlation between temperature and precipitation,temperature and runoff,and a positive correlation between precipitation and runoff.To a certain extent,the increase of temperature will lead to the decrease of precipitation and runoff;There will be an overall warming trend in the basin,in which the average surface temperature and extreme temperature of the basin will increase,and the trend is that the extreme high temperature is more severe.Under different climate scenarios,the decrease of annual precipitation in the basin is mainly reflected in the decrease of summer precipitation(most significant in July),while the precipitation in spring and autumn increases slightly.Among them,the increase of spring precipitation accounts for about 2/3 of summer precipitation reduction,and the increase of autumn precipitation accounts for about 1/3 of summer precipitation reduction.Besides,the peak month of runoff also advanced to June in July.