A Preliminary Study On The Response Of Streamflow And Sediment Load To Landform-vegetation Combination In Beiluohe Watershed

In this article,5watersheds with different landform and vegetation coverage on theLoess Plateau were selected to study the inter-annual fluctuation, inner-annual distributionand annual evolution trend of streamflow and sediment load with Concentration Period&Degree analysis, M-K test, Pettitt test and Rank-sum test.The flow duration curve was alsoadopted to investigate how streamflow and sediment load responded to the soil and waterconservation implemented from the1970s, and especially the Grain for Green Project since1999. And the influence of climate change and human activities on the change of streamflowand sediment load were separated for providing scientific basis for scientific regulation ofstreamflow and sediment load, comprehensive management of water and soil conservation，and the construction of eco-environment. The results showed that：(1)Annual streamflow and sediment load averaged over the study period were highest inthe hilly gully region, moderate in plateau gully region, and lowest in the forest. Thedifference of sediment load between watersheds forested and poor-forested was much moresignificant than that between hilly gully region and plateau gully region. The hydrographs ofstreamflow inner-annual distribution in the forested region were more gentle than that ofpoor-forested，and the concentration period was nearly1month behind. Sediment load washigh concentrated irrespective of the difference in landform and vegetation cover.(2)Over the past50years, neither annual nor seasonal streamflow showed significantdecrease in forest region. Though insignificant, streamflow in winter showed increasing trendin the poor-forested region, and streamflow of other seasons and of the year showedsignificant declining trend. Annual sediment showed significant decreasing trend irrespectiveof landform and vegetation difference, except Zhang Cunyi which was greatly influenced byextreme events.In contrast to forest watersheds, change points of streamflow were inconsistence withsediment load, and change points were accordant in the poor-forested watersheds. Base on themutation year and the condition of soil and water conservation, the whole study period wasdivided into3phases: phase I before1980, phase II from1980to1999and phase III since 2000.(3)Both the correlation and coefficient of linear streamflow-sediment relationship inpoor-forested region were far greater than forest region. And they decreased as soil and waterconservation measure implemented especially since Grain for Green policy, irrespective oflandform and vegetation difference,(4)Climate change contributed57%~84.1%to streamflow change, and human activities64.1%~105.4%to sediment change in phase II, while in phase III, the contribution of humanactivities increased to73.7%~81.8%and88.1%~105.3%respectively. The influence ofhuman on the change of streamflow and sediment tended to intensify irrespective of landformand vegetation difference.