Study On The Response Of Runoff And Sediment To Rainfall Change And Human Activities In Nanliujiang River Basin

Nanliujiang river basin is facing serious ecological problems such as soil erosion and water pollution.Under the background of climate change and basin underlying environment change,in recent years,the study of runoff and sediment transport evolution of Nanliujiang River and its influencing factors has been widely concerned by many scholars.In this paper,taking the basin as the research object,the methods of moving average,Hurst coefficient,cumulative anomaly,Mann Kendall mutation test and wavelet analysis are used to analyze the trend,variation,mutation point and periodicity of rainfall,runoff and sediment transport in the basin.According to the mutation points of runoff and sediment transport,different periods are divided to calculate the rainfall change and human beings in each period Finally,combining with the characteristics of land use change,soil erosion change and other human activities,the paper analyzes the specific causes of runoff and sediment transport change,and provides scientific reference for watershed management,soil and water conservation,water conservancy construction,water resource utilization and management.The main achievements are as follows:1.The paper analyzes the changes of rainfall and water and sediment in the basin from 1965 to 2015.Trend analysis shows that: the annual rainfall shows a weak increasing trend,the spring and winter rainfall shows a decreasing trend,the summer and autumn rainfall shows an increasing trend,the flood season and non flood season rainfall shows an increasing trend;the annual,spring,summer and flood season runoff of Bobai station decreases weakly,the autumn,winter and non flood season runoff increases weakly;the annual,spring,summer,autumn and flood season runoff of Changle station decreases weakly In addition,runoff in winter and non flood season increased weakly;sediment transport in Bobai station decreased significantly in year,spring,summer and flood season;sediment transport in autumn,winter and non flood season increased weakly;sediment transport in Changle station decreased significantly in year,spring,summer and flood season.The analysis of variability shows that there is no variation in rainfall and runoff in each period,no variation in sediment discharge in each period at Bobai station,weak variation in annual,spring and flood season at Changle station,and no variation in the rest.The abrupt analysis shows that: the abrupt years of annual rainfall are 1987 and 1992;the abrupt years of annual runoff at Bobai station and Changle station are 1985 and 1993;the abrupt years of annual sediment discharge at Bobai station are 1980,1987 and 2002;the abrupt years of annual sediment discharge at Changle station are 1987.The periodic analysis shows that the main periods of annual rainfall are 30 a,20a,11 a,8a and 5A respectively;the main periods of annual runoff are 30 a,19A,10 a,8a,6a and 3a respectively in Bobai station;the main periods of annual runoff are 30 a,19A,10 a,8a and 3a respectively in Changle station;the main periods of annual sediment transport are 29 a,20a,8a and 3a respectively in Bobai station;the main periods of annual sediment transport in Changle station are 29 a,18a and 8 respectively a?3a?2.Based on the analysis of land use change in the basin from 1990 to 2015,the results show that the area of forest land(including Eucalyptus),urban village,industrial and mining land,transportation land increases,while the area of grassland and cultivated land,other land,water area and water conservancy facilities decreases.All land types are mainly transformed into eucalyptus,grassland is also mainly transformed into forest land,cultivated land is transformed into urban village and industrial and mining land,forest land,water area and water conservancy facilities land and garden land,garden land is transformed into urban village and industrial and mining land,cultivated land and forest land,forest land is transformed into grassland,urban village and industrial and mining land,cultivated land and garden land.In 2015,more than 70% of the area of each species in the basin was below 200 m above sea level.With the increase of slope,the distribution of all kinds of land first increased and then decreased,mainly distributed on gentle slope,slope and steep slope;the larger the slope,the larger the proportion of Eucalyptus and forest land,and the urban village,industrial and mining land,cultivated land and transportation land are mainly distributed on the slope and below.On the flat land,there are mainly urban and rural land,industrial and mining land,cultivated land,land for transportation and water area,while on the north slope,northeast slope and east slope,they are smaller.From 1990 to 2015,the conversion of forest land to Eucalyptus was the main change type of all elevations,slopes and slopes,followed by the conversion of cultivated land and garden land to Eucalyptus.3.Based on the RUSLE model,the soil erosion of Nanliujiang river basin is estimated.The results show that the soil erosion modulus of Nanliujiang river basin is fluctuating,which is closely related to rainfall erosivity and other human activities.The watershed is dominated by micro and light erosion,and its area is inversely related to rainfall erosivity.The influencing factors of soil erosion change are as follows: first,rainfall erosivity,which will directly cause splash erosion and surface runoff erosion,to a certain extent,the change of rainfall erosivity will affect the change of soil erosion.In 2015,the average erosive rainfall and days in Nanliujiang River Basin are the largest,but the average soil erosion modulus is not,which contains many Human factors.Secondly,topography,the higher the altitude and the greater the slope,the more likely to cause serious soil erosion.The third is land use / cover.Eucalyptus is more likely to cause mild and moderate erosion than woodland,while woodland is more likely to cause strong and above erosion than Eucalyptus.Except for water area,the soil erosion intensity of other land types is generally greater than eucalyptus and woodland.From 1990 to 2015,the soil erosion effect of major land types changes shows that the measures such as returning farmland to forest can effectively reduce soil erosion,and the expansion of construction land and transportation land for urban and rural residents can increase soil erosion.4.Calculate the contribution rate of rainfall change and human activities to the change of water and sediment.The results show that: according to the sudden change years of runoff and sediment transport of two hydrological stations,the runoff change of two stations is divided into three periods: t1:1965-1985,t2:1986-1993,t3:1994-2015;the sediment change of Bobai station is divided into three periods: t1:1965-1987,t2:1988-2002,t3:2003-2015 The sediment change of Changle station is divided into two periods: T1: 1965-1987,T2: 1988-2015.According to the calculation of cumulative slope change rate comparison method,the rainfall change contribution rate of Bobai station is 19.52%-43.74%,the human activity contribution rate is 53.39%-80.48%;the rainfall change contribution rate of Changle station is-9.83%-43.81%,the human activity contribution rate is 56.19%-109.83%;the rainfall change contribution rate of Bobai station is 1.01%-2 99%,the contribution rate of human activities ranged from 88.04% to 98.99%;the contribution rate of sediment change of Changle station ranged from-3.71% to 3.64%,and the contribution rate of human activities ranged from 96.36% to 103.71%.5.Analysis of human activities,the results show that before the 1990 s,the runoff,especially the large amount of sediment transport,was mainly caused by human activities,which damaged the underlying surface environment of the basin,caused serious soil erosion,combined with natural effects such as rain erosion,resulting in large amount of sediment transport in the Nanliujiang river.After that,the decrease of runoff and sediment transport is mainly affected by the increase of forest land area in the basin,the enhancement of the overall ecological vulnerability of all parts of the basin,the reduction of total soil erosion,the construction of reservoirs and dams,and sand mining.