| In recent years,with the intensification of climate change and human activities,runoff and sediment regime have exhibited significant changes in the Yellow River Basin,runoff and sediment yield has dropped sharply,bringing a series of new problems to managemant of water and water resources utilization of the Yellow River.Therefore,it is of great theoretical and practical significance to discuss the variation characteristics of runoff and sediment in the Yellow River and to find out the driving factors.The Jing River is an important branch of the middle reaches of the Yellow River.Therefore,this study uses the Jing River Basin as the study area,to analyze the spatial and temporal variation characteristics of runoff,sediment and precipitation in the Jing River Basin over the years,and quantitatively evaluate the contribution of precipitation and human activities to reduction of runoff and sediment,and moreover to reveale the impact of vegetation,terrace and check dams on runoff and sediment,by using Mann-Kendall trend test method,ordered cluster analysis method,cumulative anomaly method and double mass curve.The main conclusions are as follows:(1)Spatial and temporal variation characteristics in runoff and sediment in the Jing River Basin.The average annual runoff and sediment load in the Jing River Basin from 1934 to 2016 were 1.608billion m~3 and 221 million ton.Runoff and sediment load were significantly reduced in Jing River Basin(p=0.01),and the changing year of runoff was in 1996 and sediment was in 1997,runoff and sediment decreased by 38.5%and 62.3%respectively after the changing year.Through the cumulative anomaly analysis,it is found that the variation of runoff and sediment has obvious periodic characteristics.The change process of runoff is in the three stages of wet-normal-dry,which is stair-like reduction;the change of sediment load is the four stages of wet–dry-wet–dry.The first wet period and the second dry period lasted the longest.In the past 60 years,the runoff and sediment of most hydrological stations in this basin has shown a significant decrease(p=0.05).The changing years of the hydrological stations in the Malian River Basin have been proved in 2002~2004,and the mutation points the hydrological station in the middle and lower reaches of Jing are mostly in 80s to90s.runoff and sediment at Jing Chuan station,located in the upstream of the Jing,was found to mutate earliest,they are 1970 and 1973.(2)Temporal and spatial changes in the intensity of runoff and sediment in the Jing River Basin.The runoff modulus of the Jing River Basin decreased from 50,200 m~3/km~2·a to 24,000 m~3/(km~2·a)from 1960s to 2000s,a decrease of 52%,and the runoff modulus decreased stepwise in each decade.After 2000,the area of runoff modulus above 70,000 m~3/(km~2·a)disappeared,and the area of 50,000to 70,000 m~3/(km~2·a)decreased significantly,and the area of less than 35,000 m~3/(km~2·a)increased.The specific sediment yield of the Jing River decreased from the 1980s,increased in the 1990s,then decreased sharply after 2000,from 6262 t/(km~2·a)in the 1960s to 2138 t/(km~2·a)after 2000,a decrease of 61%.The regine of he specific sediment yield above 8000 t/(km~2·a)disappeared,the region of 5000 to 8000 t/(km~2·a)decreased,and the area below 2000 t/(km~2·a)increased.(3)Temporal and spatial variation characteristics of precipitation.The average annual precipitation is 515 mm in the Jing River Basin from 1966 to 2014,and the concentration of precipitation in the flood season was high.The precipitation in the 1980s and 1990s was generally dry,and the precipitation has gradually turned wet from 2000s.After 2010,rainstorms frequently occurred.From 1966 to 2014,during the interannual and annual distribution of precipitation,the precipitation of light rain and moderate rain showed a decreasing trend,however,the precipitation of heavy rain,heavy rain and heavy rain showed a increased trend.The rainfall has obvious spatial differentiation characteristics,decreasing from southeast to northwest.The precipitation in the loess tableland is relatively abundant.The precipitation in the loess hilly area in the upper reaches of the Malian River is less,the annual precipitation is below 500mm,rainstorms occurs frequently mainly in the middle and lower reaches of the Malian River,Puhe and the upper reaches of the Ruihe.(4)Precipitation and human activities are the main factors leading to changes in runoff and sediment.The main driving force for the significant reduction of runoff and sediment is human activity in recent years in the Jing River.The decrease in runoff before 1996 was mainly due to the precipitation,while the main cause of the decrease in runoff was human activities from 1997 to 2014,with a contribution rate of 97%for human activities and a contribution rate of 3%for precipitation.The dominant factor in the reduction of sediment transport in the basin has been human activities.The contribution rate of human activities from 1979 to 2014 was 61.4%,and the contribution rate of precipitation was 38.6%.After 2000,human activities played an absolute leading role in reducing sand,with a contribution rate of 92%and precipitation impact of 8%.The impact of human activities on water and sediment has increased significantly since 2000.Since 2010,due to frequent rainstorms,precipitation has played a role in increasing water and sand,and human activities have contributed more than 100%to runoff and sediment reduction.(5)Responses of runoff and sediment to the major human activities,including vegetation,terraces,and check dams in basin scale.The vegetation situation in the basin has improved significantly in recent years.The vegetation coverage of forest and grass has increased from 41.8%in 1978 to 68.6%in 2016.There is a nonlinear negative correlation between vegetation and runoff,vegetation and sediment.Runoff and sediment reduction of vegetation has a significantly critical phenomenon.When the vegetation coverage is less than 40%,the effect of runoff reduction and sediment reduction is not obvious;when the vegetation coverage is between 40%and 60%,the maximum runoff reduction and sediment reduction benefit is obtained for each 1%increase of vegetation coverage,and the maximum marginal benefit is obtained;When the vegetation coverage exceeds 60%,the effect of runoff reduction and sediment reduction tends to be stable.As of 2012,the terraced area in the basin is 7758.7km~2.The relationship between the proportion of terraces and the runoff coefficient and sediment yield coefficient is found.At the basin scale,the terraces have a nonlinear negative correlation with runoff and sediment.When the area of the terraces is small,the effect of reducing runoff and reducing sediment is not obvious.After the proportion of terraces is more than 5~10%,with the increase of terraces,the runoff and sediment yield of the basin is weakened.The sedimental reducing capacity of terraces is stronger than that of runoff-reducing capacity,and the terraces are more significant for runoff reduction and sediment reduction in the loess area.As of 2016,there were 633 large and medium-sized check dams in Jing River Basin,and92%of the silt dams were built from 1990 to 2009.As of 2011,the average siltation rate of check dams in the basin was 25%.Till 2011,a total of 1.216 billion tons of sediment was intercepted.The check dams are the main anthropogenic driving factor for the reduction of sediment in the Malian River Basin. |
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