| The Loess Plateau,as one of the regions with the most serious soil erosion in the world,has low forest coverage and a shortage of water resources.For a long time,the water and sediment process in the Yellow River Basin has been affected by both climate change and human activities,showing a significant reduction trend.How to quantify the impact of climate change and human activities on water and sediment changes in the river basin has become a hotspot and problem in current hydrological research.To this end,this paper takes the Qingshui River Basin in the middle of the Yellow River as an object,using Mann-Kendall nonparametric test,Mann-Kendall mutation point test and cumulative anomaly method to analyze the precipitation,evapotranspiration,Dynamic changes of runoff and sediment;using land use area analysis,landscape pattern index analysis,multiple linear regression and other methods to analyze the spatio-temporal changes of land use and landscape pattern in the Qingshui River Basin since 1987,and screen the key landscapes that affect the water and sediment in the watershed Index;quantified the contribution rate of climate and human activities to the changes of water and sediment in the river basin through the elastic coefficient method and the double cumulative curve method.The main research conclusions are as follows:(1)The average annual precipitation in the basin from 1960 to 2015 was 514.31 mm,and the general trend was a volatility decline.There was no significant trend change in 56 years.Rainfall is unevenly distributed in each month of the year,mainly concentrated in the flood season(June-September),accounting for 71.0% of the average annual precipitation.The average annual potential evapotranspiration of the watershed from 1960 to 2015 was 1346.51 mm,and the general trend was a decline in fluctuations.There was no significant trend change in 56 years.The potential evapotranspiration is unevenly distributed in each month of the year,and it is a single-peak curve.The peak value is concentrated from May to August,accounting for 51.2% of the annual evapotranspiration.(2)From 1987 to 2015,the forest area continued to increase,the grassland continued to decrease,the farmland fluctuations decreased,the water area remained unchanged,and urban and construction land increased rapidly.The depth of runoff decreases linearly with the increase of forest area,and the amount of sediment transport changes exponentially.(3)From 1987 to 2015,the degree of fragmentation and heterogeneity of the watershed landscape increased,and there was a certain correlation between the same type of landscape index.The higher the degree of fragmentation of the watershed landscape patches,the stronger the heterogeneity,resulting in the enhanced effect of the surface landscape on water and sand blocking,and the smaller the amount of water and sand produced.Through multiple linear regression analysis,it is concluded that the water and sediment in the river basin are closely related to the changes in ED,PARFAC,and SHEI.The PARFAC indicator has the most significant impact.In general,the connectivity of the overall landscape degree of the watershed promotes the occurrence of runoff and soil erosion,and the simple boundary makes the landscape less restrictive to water and sediment,which is more likely to cause soil erosion.(4)Between 1960 and 2015,the average runoff depth was 29.08 mm,and the average sediment transport was 2.165 million tons,both of which showed a very significant downward trend(P<0.01).The runoff and sediment transport were abruptly changed in 1980 and 1995.The climate and water and sediment changes in different periods are different.With changes over time,the average annual precipitation shows a decreasing trend and the degree of variability decreases.Potential evapotranspiration showed a fluctuating trend,and the degree of variation decreased.The runoff depth gradually decreases,but the stability becomes smaller.The amount of sediment transport is generally decreasing,and the degree of dispersion is increasing.In different periods,there is a certain difference in the relationship between precipitation,runoff depth and sediment transport.The decreasing sensitivity of change over time indicates that changes in human activities have weakened the impact of precipitation on runoff and sediment transport.At different time scales,the monthly scale is more relevant.(5)The amount of precipitation,potential evapotranspiration and the absolute value of the elastic coefficient of the underlying surface increase significantly.And with time,the runoff is more sensitive to different factors.According to the analysis and calculation of the comprehensive elastic coefficient method,the runoff change caused by climate change was 26.23% from 1981 to 1995,and the runoff change caused by human activities was 73.77%.From 1996 to 2015,the change in runoff caused by climate change was 5.49%,and the change in runoff caused by human activities was94.51%.In summary,human activities are the main factors affecting runoff changes in the Qingshui River Basin,followed by changes in precipitation,and the smallest impacts of potential evapotranspiration.According to the analysis and calculation by the double-cumulative curve method,the change in sediment transport caused by climate change from 1981 to 1995 was 2.20%,the change in transport sediment caused by human activities was 97.80%,and the change in transport volume caused by climate change from 1996 to 2015 was 6.11%.The change in sediment transport caused by human activities is 93.89%.It shows that human activities play a leading role in the reduction of sediment transport in the basin. |
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