Hydrologic Processes And Its Responses To Ecological Restoration In Different Geomorphological And Vegetation Types In The Past 60 Years

The purpose of this study was to reveal the effects of ecological environment construction such as soil and water conservation and the Grain for Green Program on hydrological processes in different vegetation type areas on the Loess Plateau.This work takes the Beiluo River Basin as an example,based on the measured hydrological data from 1960s to 2010s,has compared and analyzed the effects of ecological restoration on streamflow and sediment quantity,streamflow and sediment relationship,and the contribution of influencing factors in three hydrological regions of upstream（Located in the Forest grassland-hilly gully region）,midstream（In the Secondary forest region-Gully region of plateau/Rocky mountain area）and downstream（Situated at the Rock mountain-terrace plain area）with non-parametric test,empirical formula,elastic coefficient method based on Budyko hypothesis and fractal theory,and Copula function.The results showed that:（1）The average annual streamflow depth in the up,middle,and lower reaches was 30,23,and 21 mm,respectively.The average annual streamflow volume of middle reaches accounted for 51%of the total volume in the whole basin.All the streamflow depth in the three regions showed a significantly decreasing trend with the changing rates of-0.28,-0.16and-0.43 mm/a in past 60 years,respectively.The maximum proportion of streamflow in the upstream and middle reaches in summer,accounting for 51%and 43%,respectively.The streamflow of downstream accounted for a largest proportion of 50%appeared in spring.Compared with 1960s,the high flow（R₅）decreased by 63%,45%and 60%in the 2010s,respectively,in the three regions.While the low flow(R₉₅)increased by 68%in the up reaches,and in middle reaches it began to increase in 1980s.（2）The average annual sediment was 60,5,and-2 million tons in the up,middle,and lower reaches,respectively.The average annual sediment delivery in the up reaches accounted for over 90%of the total sediment in the basin.The sediment delivery of up reaches demonstrated a significant downward trend at a rate of 2 million t/a,and in middle and lower reaches it was only a slight reduction in past 60 years.The sediment of upper,middle and lower reaches concentrated in the main flood season from July to September.For 5%,the sediment of three hydrological regions reflected a continuous decrease trend,and no sediment days raised.（3）Compared with 1960s,human activities contributed the reduction of streamflow by66.3%,97.2%,and 64.3%in 2010s in the upper,middle and lower reaches,respectively,and it dominated the reduction of sediment delivery of 81.7%,59.2%and 92.7%in 2010s,respectively.Among the contribution of human activities in the upper reaches,the slope protection project and gully control engineering works are 39.0%and 42.7%.The estimation results of human activities contribution in the middle and lower reaches showed that the high vegetation coverage and agricultural irrigation have effects on regional streamflow and sediment.The influence of human activities on streamflow change in the upstream has gradually increased since 1970s.Climate change dominated the change of streamflow in the middle reaches.In the lower reaches,human activities dominated the reduction of streamfow.While the change of sediment was always dominated by human activities in the whole basin.（4）In order to explore variation characteristics of frequency coupling between streamflow and sediment in the period 1 of base（1960—1979）,period 2 of comprehensive harness of soil and water conservation（1980—1999）and period 3 of the Grain for Green project（2000—2019）on the Loess Plateau,the copula function were used for frequency statistics and simulation of streamflow and sediment.The results indicate that:The Gumbel-Houggard Copula function can well simulate the joint probability distribution of streamflow and sediment in the upper and middle reaches.Frank copula function has high accuracy in simulating the joint probability distribution of downstream streamflow and sediment.Sreamflow and sediment transport decreased significantly in each return period in 5～100 years.Compared with period 1,extreme events with a return period of over 20 years,the joint return period and co-occurrence return period were increased by about 100%in the period 3.The effects of ecological restoration on streamflow and sediment processes in the Loess Plateau are mainly reflected in significantly reducing streamflow,weakening soil erosion,regulating the abundance and frequency of streamflow and sediment,and reducing the occurrence probability of extreme sreamflow and sediment events.The responses of different landforms and vegetation type areas to ecological restoration showed obvious differences in streamflow and sediment processes.In the hilly and gully region with low vegetation coverage and high degree of ecological restoration,the characteristics of streamflow and sediment change dramatically and the influence of human activities gradually expands.In the natural secondary forest with high vegetation coverage and little disturbance,the change of streamflow and sediment is mainly dominated by climate change,and the variation of streamflow and sediment characteristics is weak.The transformation of streamflow and sediment in agricultural area of Weibei Plain is mainly influenced by human activities,and the evolution of streamlfow and sediment characteristics is complicated.