| The impact of climate change and subsurface changes on hydrological processes is one of the current hot issues of concern to the international community and the scientific community.Although the existing studies on the attribution of runoff and sediment changes in the middle reaches of the Yellow River can largely separate the effects and contributions of the two categories of average climate change and human activities,they lack the consideration of vegetation changes due to climate change and the effects of coupled human activities and extreme precipitation on runoff and sediment changes,resulting in some errors in the scientific analysis of the causes of the sharp decrease in water and sand in the middle reaches of the Yellow River in recent years.Therefore,the effects of climate(extreme precipitation),potential natural vegetation and human activities on runoff and sediment changes in the watershed need to be urgently studied in the identification of runoff and sediment changes in the middle reaches of the Yellow River.This study took the middle reaches of the Yellow River and its typical watersheds(Huangfu River,Sanchuan River and Yanhe River)as the object of study,and used trend analysis and other methods based on hydro-meteorological data from 1960-2017 to elucidate the characteristics of changes in average precipitation,mean temperature,runoff and sediment content in the watershed at different time scales(interannual and interdecadal),and quantitatively assessed the influence of key extreme precipitation indicators on runoff and sediment changes;Simulated the spatial pattern of potential natural vegetation in different eras in the Hekouzhen-Longmen region using the comprehensive and sequential classification system,conducted the analysis of spatial and temporal changes in potential natural vegetation and land use,and further explored the intervention intensity of human activities on potential natural vegetation.Finally,the contributions of watershed climate,potential natural vegetation and human activities(land use and other human activities)to runoff and sediment changes were quantified by combining the SWAT model and scenario combination settings.The main conclusions were as follows:(1)The trends of hydro-meteorological elements in the Huangfu River,Sanchuan River and Yanhe River watersheds from 1960-2017 were analyzed.The average precipitation in the Huangfu River and Yanhe River watersheds showed a nonsignificant increasing trend,while the Sanchuan River watershed showed a significant increasing trend,and the average precipitation in all three watersheds had a significant periodicity at the 5a time scale;The mean temperature in the three watersheds showed a significant increasing trend,with abrupt changes in 1997,1996 and 1986,respectively,all with significant cyclicity at the 29 a time scale;The runoff from the three watersheds showed a significant decreasing trend with abrupt changes in 1998,1979 and 1996,with significant periodicity at the 14 a,18a and 18 a time scales,respectively;The sediment in the three watersheds showed a significant decreasing trend,with abrupt changes in 1998,1991 and 1996,and significant cycles at the 9a,17 a and 18 a time scales,respectively.(2)The spatial and temporal variation characteristics of extreme precipitation were described.Temporally,extreme precipitation indicators showed a non-significant decreasing trend in the Huangfu River watershed in general and a non-significant increasing trend in the Sanchuan River and Yan River watersheds in general,with CDD showed a significant decreasing trend in both the Huangfu River and Sanchuan River watersheds and PRCPTOT showed a significant increasing trend in the Sanchuan River watershed;Spatially,the extreme precipitation indicators showed an overall increasing characteristic from north to south in the Huangfu River watershed,a decreasing characteristic from west to east in the Sanchuan River watershed,and an increasing characteristic from northeast to southwest in the Yanhe River watershed,and the spatial variation characteristics of each indicators were not the same.The volume and intensity of rainstorms in the three watershed s vary dramatically from year to year,and the future response to extreme rainstorms should develop relevant preventive measures.(3)The contributions of extreme precipitation to runoff and sediment changes were quantified.The correlations of R90 p,FSPTOT and ERPTOT with runoff and R95 p,R90p and R25 p with sediment in the Huangfu River watershed were high,and the contributions of extreme precipitation to runoff and sediment reduction ranged from 4.3 to 13.5% and 8.0 to16.9%,respectively;The correlations of R25 p,RX5day and R95 p with runoff and I50,RX1 day and SDII with sediment in the Sanchuan River watershed were high,and the contributions of extreme precipitation to runoff and sediment reduction from-12.9 to-20.3%and-6.3 to-13.1%,respectively;The correlations of R10 p,ERPTOT and FSPTOT with runoff were high,and the correlations of FSPTOT,R10 p and ERPTOT with sediment in the Yanhe River watershed were high,and the contributions of extreme precipitation to runoff and sediment reduction from-2.1 to-6.8% and 4.1 to 7.6%,respectively.The contribution of extreme precipitation to runoff and sediment reduction in the three watersheds were small,while the contribution of non-extreme precipitation factors(human activities)dominated the impact.(4)The spatial pattern of potential natural vegetation in the Hekouzhen-Longmen region was simulated to elucidate the intensity of human activities intervention on potential natural vegetation.Four potential natural vegetation types existed stably between 1960-2017 in the Hekouzhen-Longmen region,and their areas ⅣC18>ⅣD25>ⅢC17>ⅢD24.Spatially,grassland-type potential natural vegetation was mainly distributed in the north,while foreststeppe type potential natural vegetation was mainly distributed in the south.The land use types between 1990-2018 in the Hekouzhen-Longmen region were mainly grassland,cropland and forest land.Under the influence of human activities,45.6～51.6% of the potential natural vegetation of grassland type remains as grassland area,and the rest were transformed into cropland,forest land and construction land,67.1～80.4% of the potential natural vegetation of forest grassland type remains as forest land or grassland area,and the rest were transformed into cropland and construction land.The greatest intensity of human activities on potential natural vegetation in the Hekouzhen-Longmen region was 4.5% from 2010-2018.(5)The contributions of climate,potential natural vegetation and human activities to runoff and sediment changes were separated.The average contributions of climate change to runoff changes in the Huangfu River,Sanchuan River and Yan River watersheds were 29.1%,31.1% and 19.8%,respectively,and to sediment changes were 25.7%,15.0% and 14.1%,respectively;The average contributions of potential natural vegetation changes to runoff changes were 7.5%,9.2% and 10.8%,respectively,and to sediment changes were 10.5%,6.7%and 11.8%,respectively;Human activities had always been the dominant factor in the sharp reduction of runoff and sediment,with average contributions of 63.4%,59.8% and 72.5% for runoff reduction and 63.9%,82.0% and 74.2% for sediment reduction,respectively.Among the human activities contributions,the average contributions of land use change to runoff reduction were 47.9%,5.5% and 28.8%,and the average contributions to sediment reduction were 43.4%,7.5% and 30.7%,respectively;The average contributions of other human activities impacts to runoff reduction were 15.5%,54.3% and 43.7%,and to sediment reduction were 20.5%,74.5% and 43.5%,respectively.This study analyzed the trends of hydro-meteorological elements in three typical watersheds in the Hekouzhen-Longmen region,elaborated the spatial and temporal patterns of extreme precipitation and quantified its contribution to runoff and sediment changes,simulated the spatial pattern of potential natural vegetation in the region and elucidated the intensity of human activities on potential natural vegetation,and separated the contributions of climate,potential natural vegetation and human activities to runoff and sediment changes.The results of the study were helpful to deepen the understanding of runoff and sediment changes in response to climate change and human activities,and provided a reference basis for ecological restoration and soil and water conservation management in ecologically fragile areas. |
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