| It is of great significance to study the influence of climate change and human activities on the hydrological process of river Watershed for rational utilization and protection of water and soil resources.In this thesis,the Hantaichuan Watershed,a first-level branch on the upper right bank of the Yellow River,was studied.Mann-kendall trend analysis method,cumulative anomaly method and wavelet analysis were used to analyze the variation characteristics of precipitation,runoff and sediment transport in the Watershed from 1990 to 2020.Based on Google Earth Engine(GEE)cloud platform,land use classification results in 1990,2000,2010 and 2020 were obtained by random forest method.Using Arc GIS10.5,Fragestats4.2 and other software to obtain land use,landscape pattern and vegetation cover change indicators,the spatio-temporal changes of land use,landscape pattern and vegetation cover change process and characteristics were analyzed in this period.Based on the double accumulation curve method,correlation analysis method and regression analysis method,the response of water and sediment change to precipitation and land use change was explored,and the contribution rate of precipitation and land use change to water and sediment change in the Watershed was calculated.The main research conclusions are as follows:(1)Significant changes in land use and vegetation cover occurred in Hantaichuan Watershed.From 1990 to 2020,the main land use types in the Hantaichuan Watershed were woodland,grassland and cultivated land,and the dominant landscape changed from grassland to woodland.The change trend of landscape horizontal pattern index in the Watershed was different,the patch shape was irregular,the landscape fragmentation degree was increased,the landscape heterogeneity was strengthened,the edge effect was enhanced,the proportion of small patches was increased,the aggregation degree was decreased,the spatial distribution was more dispersed,and the human disturbance was stronger.NDVI in the Hantaichuan Watershed increased year by year from 1990 to 2020,with an average rising rate of 0.63/10a and an average annual NDVI of 0.27.The improved area of NDVI was much larger than the subtracted area,and the vegetation recovery was more obvious.(2)Variation characteristics of water and sediment in the Hantaichuan Watershed:the annual average precipitation in the Hantaichuan Watershed was 449.74mm from 1990to 2020,decreasing at a rate of-0.81mm/10a.The precipitation in the Watershed is unevenly distributed throughout the year,mainly in flood season precipitation,a unimodal distribution,precipitation mainly concentrated in June to September,accounting for more than 75%of the annual precipitation.From 1990 to 2020,the annual average temperature in Hantaichuan Watershed was 7.46℃,and the annual average temperature increased significantly at a rate of 0.58℃/10a(P<0.05).The average temperature in July was the highest level of the whole year.From 1990 to 2020,the average annual runoff of Hantaichuan Watershed was 833.47×10~4m~3,and the average annual sediment transport was 93.70×10~4t,both of which decreased significantly(P<0.05).The distribution of runoff in the Watershed is uneven during the year,which comes from the flood season precipitation.The annual runoff has an obvious periodicous change of"abundant and dry"on the interannual scale.(3)The response of water and sediment change to precipitation and land use/cover change in the Hantaichuan Watershed:Annual precipitation,flood season precipitation and summer precipitation were significantly correlated with runoff and sediment transport in the Watershed from 1990 to 2020(P<0.05),while LPI index was negatively correlated with runoff change(P<0.05).COHESION index of water area,DIVISION index of arable land and unused land were negatively correlated with sediment transport(P<0.05).NDVI distribution in high(0.6-0.8)and high(0.8-1.0)grades was significantly positively correlated with sediment transport(P<0.05).From 1990 to 1997,the change of runoff and sediment transport in the Hantaichuan Watershed was mainly influenced by precipitation,and from 1998 to 2020,the contribution rate of precipitation to the reduction of runoff and sediment discharge was 22.79%and 6.63%,respectively,due to the combined influence of precipitation and human activities.The contribution rate of human activities to the reduction of runoff and sediment discharge was 77.21%and 64.56%,respectively.Human activities play a leading role in the variation of runoff and sediment discharge. |
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