To reduce the sediment load of China's Yellow River, soil conservation measures have been progressively implemented across the Loess Plateau region since the 1950s. The effectiveness of these soil conservation measures (which were also coincident with reduced rainfall and streamflow) in controlling sediment movement remains to be ascertained. Here the association between sediment movement, hydrological variability, and the implementation of soil conservation measures is examined for the Coarse Sandy Hilly Catchments region of the Yellow River basin. The hypothesis that the soil conservation measures have reduced suspended sediment yields beyond that associated with rainfall reductions alone, principally by reducing sediment concentration, is examined. Annual sediment yield decreased significantly over time in all subcatchments, and the timing of the change (between 1971 and 1985) was consistent with the timing of change in streamflow. Annual mean sediment concentration in 7 of the 11 catchments exhibited a statistically significant decreasing trend over time, indicating that soil conservation practices reduced the mobilization of sediment in most areas, typically accounting for ∼75% of the observed reductions in annual sediment yield. Lesser reductions in area‐specific sediment yield at larger catchment areas after the soil conservation measures were emplaced suggests that larger rivers may be reeroding stored sediment. As these sediment stores are likely to be relatively large given the high historic yields, relatively high area‐specific sediment yields may persist at larger catchment areas even with improvements to sediment management in smaller tributaries.
Abstract. To control severe soil erosion on the Loess Plateau, China, a great number of soil conservation measures have been implemented since 1950s and subsequently, the "Grain for Green" project was implemented in 1999. The measures and the project resulted in a large scale land use/cover change (LUCC). Understanding the impacts of the measures and the project on streamflow, sediment load and their dynamic relation is essential because the three elements are closely related to the sustainable catchment management strategy on the Loess Plateau. The data for seven selected catchments in the middle reaches of the Yellow River were used and standardized with the precipitation and the controlling area for analysis. The nonparametric Mann-Kendall test and the Pettitt test were employed to detect trends and change points of the annual streamflow and annual sediment load. Simple linear regressions for the monthly streamflow and sediment load from May to October were made to express their relationship. Based on the change point identification and the time when the project began to be implemented on the Loess Plateau, the complete time for the data records was divided into three periods to compare the change degrees of streamflow, sediment load and their relation for the catchments.Results show that there are three types of responses in streamflow, sediment load, and their dynamic relations for the seven catchments. The effects of the LUCC on streamflow, sediment load, and their relationships are greatest in the three transition zone catchments followed by the two rocky mountain catchments. The effects are much weaker in the two loess hilly-gully catchments. In general, the change degrees for sediment load are much greater than those for streamflow, which results from the decreased streamflow and weakening trend of their dynamic relation period by period in catchments.
Abstract:Spatiotemporal trends in precipitation may influence vegetation restoration, and extreme precipitation events profoundly affect soil erosion processes on the Loess Plateau. Daily data collected at 89 meteorological stations in the area between 1957 and 2009 were used to analyze the spatiotemporal trends of precipitation on the Loess Plateau and the return periods of different types of precipitation events classified in the study. Nonparametric methods were employed for temporal analysis, and the Kriging interpolation method was employed for spatial analysis. The results indicate a small decrease in precipitation over the Loess Plateau in last 53 years (although a Mann-Kendall test did not show this decrease to be significant), a southward shift in precipitation isohyets, a slightly delayed rainy season, and prolonged return periods, especially for rainstorm and heavy rainstorm events. Regional responses to global climate change have varied greatly. A slightly increasing trend in precipitation in annual and sub-annual series, with no obvious shift of isohyets, and an evident decreasing trend in extreme precipitation events were detected in the northwest. In the southeast, correspondingly, a more seriously decreasing trend occurred, with clear shifts of isohyets and a slightly decreasing trend in extreme precipitation events. The result suggests that a negative trend in annual precipitation may have led to decreased soil erosion but an increase in sediment yield during several extreme events. These changes in the precipitation over the Loess Plateau should be noted, and countermeasures should be taken to reduce their adverse impacts on the sustainable development of the region.
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