**Other Abstract** | The middle reaches of the Yellow River (from Toudaoguai to Huayuankou) water and sediment and their driving factors is the focus of attention of the community. It is also an important basis for evaluation of ecological construction and ecosystem restoration in the Loess Plateau. In this paper, the middle Yellow River as the research object, based on the precipitation, streamflow and sediment discharge series data (1950-2008), using mathematical statistical test methods and wavelet method, analysis of streamflow and sediment discharge variation and its impact on precipitation / response to human activities. And to use gray prediction model to simulate changes in streamflow and sediment discharge prediction. The main conclusions are as follows:
(i) Analysis the change of streamflow and sediment discharge eigenvalues nearly 60 years in the middle reaches of the Yellow River. The annual average streamflow and sediment discharge is 165.2×108m3 and 8.2×108t respectively; and the mean coefficient of variation(Cv) is 0.5 and 0.7 respectively in the middle reaches of the Yellow River during 1950-2008. Changes in streamflow and sediment have obvious stages. In which streamflow is divided into three periods: 1950-1970 for the wet period; 1971-1985 for the normal period and 1986-2008 for the dry period. Sediment discharge is divided into two periods: 1950-1981 for the wet period and 1981-2008 for the dry period. Streamflow and sediment discharge have significant periodicity. Streamflow periodicity is 27a, 10a and 4a, and the first main periodicity is 27a. Meanwhile, the sediment discharge periodicity is 31a, 20a and 7a, and the first main periodicity is 31a.
(ii) Results show that strong decreasing trends in annual streamflow and sediment discharge have existed of the middle reaches of the Yellow River. Change point analyses reveal that more rapid declines for streamflow began in 1985, and sediment discharge began in 1981 (P=0.05). The average annual streamflow and sediment discharge shows a significant decreasing trend at the same frequency after the transition year. Decrease of streamflow after the transition year ranged from 43.9% to 69.8%. In which its decrease 51.6% for wet years, 44.0% for normal year and 60.5% for dry year. Decrease of sediment discharge after the transition year ranged from 44.9% to 96.2%. In which its decrease 46.8% for wet years, 61.4% for normal year and 96.0% for dry year.
(iii) Precipitation and Human activity are the main driving factors affecting the streamflow and sediment changes in the middle reaches of the Yellow River. Precipitation and human activities contribute rate of streamflow reduction was 30% and 70%, and the contribution rate of sediment reduction was 20% and 80%. Quantitative analysis the impacts of human activities on changes of streamflow and sediment discharge in the middle reaches of the Yellow River. Diversion is a major factor in reduction stereamflow of the middle reaches Yellow River. The average annual water consumption is about 45.0×108m3, accounting 73.8% for the total amount of water reduction impacting of human activities. Soil and water conservation is a comprehensive water and sediment reduction measures，and it is a a major factor in reduction sediment of the middle reaches Yellow River. Annual reduction in water is 6.0×108m3, and sediment is 2.7×108t, respectively, accounting 9.8% and 52.9% for the total amount of water and sediment reduction impacting of human activity. Large and medium-size reservoirs were constructed to reduce the sediment discharge is an important factor in the middle reaches of the Yellow River. The annual average is about 1.8×108t due to sediment deposition of reservoirs and riverway, accounting 35.3% for the total amount of sediment reduction impacting of human activities.
(iv) Gray prediction model was used to simulate changes in streamflow and sediment discharge prediction in middle reaches of the Yellow River. This method is good for waveform simulation, and be able to more accurately determine the peak and valley trend of hydrological data series. |

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