Long-term Vegetation Pattern Change,Cause Of Formation And Its Influence On Hydrological Process In The Upper Yellow River

Vegetation has functions such as balancing energy distribution,changing soil structure,and regulating land water circulation.As an important ecological barrier in China,the Yellow River Basin plays an irreplaceable role in maintaining our country's food,energy and ecological security.In this study,the upper Yellow River above the Lanzhou section(hereinafter collectively referred to as the"the upper Yellow River")were selected as the study area.The pattern and cause of vegetation,and its influence on hydrological processes in this area from 1982 to 2015 were systematically studied.First,based on the Normalized Differential Vegetation Index(NDVI),the characteristics of temporal and spatial changes of vegetation in the upper Yellow River are explored through the unary linear regression method.Then,this study briefly analyzed the characteristics of climate change in the upper Yellow River,and revealed the reasons for the changes of NDVI in the upper Yellow River through correlation analysis and multiple linear regression-residual analysis.In addition,considering that the process of vegetation's influence on regional runoff and sediment is not isolated,and the time series of water and sediment are nonlinear,unstable,and multi-scale features.This study uses Multivariate Empirical Mode Decomposition(MEMD)to reveal the time multi-scale relationship between water and sediment and its influencing factors in the upper Yellow River.The main results are as follows:(1)There are significant differences in the temporal and spatial distribution patterns and changing trends of vegetation in the upper Yellow River.The NDVI in the upper Yellow River shows an upward trend as a whole,and there is degradation in some areas,but the variation is relatively small(mostly in-1×10^-3 yr^-1 to 0 yr^-1).The annual NDVI shows a downward trend in the area accounted for 20%,concentrated in the medium vegetation coverage area(0.3?NDVI<0.5).The areas where NDVI showed a downward trend during the growing season are mainly concentrated in areas with medium and high vegetation coverage(NDVI?0.7).The pixels with a relatively large increase rate of NDVI(slope?4×10^-3 yr^-1)are mainly distributed in northeast Taohe Basin.(2)The climate in the upper Yellow River is warmer and more humid than before,but there is a big difference between the local climate and the overall climate.The climate of west"Tangshang Region"(the section from the source of the Yellow River to Tangnaihai)is becoming colder and more humid than before.And the climate in some parts of its eastern is developing in the opposite direction(warmer and drier than before).The combined effect of climate change and human activities is the leading factor in the change of NDVI in the upper Yellow River.The pixels of annual NDVI increase are mainly distributed at the“edges”of the upper reaches of the Yellow River,which is only caused by climate change.Among them,in the area where the precipitation is less than 500 mm(the middle part of the upper Yellow River),the vegetation degradation may be caused by the fact that the water obtained by the vegetation cannot meet its own water consumption due to the rising temperature.The impact of human activities on NDVI(mainly an increase)is more pronounced during the growing season.(3)In recent years,the runoff and sediment in the upper Yellow River have been reduced.The water and sediment fluctuate greatly from year to year in Tangshang Region,and in the Tanglan Region(Tangnaihai to Lanzhou section),the water and sediment is relatively stable.The Huangshui Basin is similar to the Tanglan Region,but the stability of water and sediment in Taohe Basin is the worst of all(CV_runoff is 26.51%,CV_sediment is 75.29%).In a short time(3 years),the improvement of vegetation can effectively restrain the increase of sediment in the Tanglan Region(mainly the inner tributary of Taohe Basin),and cause the decrease of runoff in Tangshang Region.The increase in precipitation may cause a certain amount of soil loss to the upper Yellow River in a short time,and temporarily increase the amount of sediment.However,an increase in precipitation can effectively cause an increase in runoff and is the main factor for the increase in runoff.In the long term(more than 30 years),the improvement of the ecological environment,the increase of precipitation,and the increase of temperature can effectively reduce the amount of sediment.The increasing vegetation coverage has the effect of restraining the increase in runoff in the long run,which is only manifested in Tangshang Regions and Huangshui Basins.Precipitation only has a long-term effect on the runoff of the Taohe Basin.Rising temperature and actual evapotranspiration are increasing the"loss"of runoff,which leading to a decrease in runoff over a long time.In summary,the vegetation in the upper Yellow River has great spatial heterogeneity over the past 34 years.The temporal and spatial changes of vegetation are closely related to climate change and human activities.Under the co-led by climate change and human activities,the vegetation has improved as a whole,but it has degraded in some areas.Both runoff and sediment have declined in recent years.The relationship among runoff-sediment,vegetation and climate is too complicated.In the short term,the increase in precipitation will cause a certain amount of soil loss in the upper Yellow River,and lead to an increase in runoff.In the long term,the improvement of vegetation can play a better role in soil and water conservation with a warmer and more humid climate.According to the characteristics of the spatial and temporal distribution patterns,changes of vegetation and its impact on regional runoff and sediment in the upper Yellow River,it can provide certain basic data support for monitoring regional ecological environment,allocating water resources,mitigating soil erosion and other ecological environmental issues;meanwhile,it can provide certain theoretical support for the upper Yellow River to formulate strategies to deal with climate change,improve relevant ecological policies,and realize the protection and sustainable development of the ecological environment.