| The Yellow River was the famous heavily silt-carrying river in the world. Sediment deposition and river rose up were the primary cause of the disaster in the lower reaches of the Yellow River. Since the 1980 s, runoff and sediment load decreased, water and sediment relations changed significantly, caused more attention. This study based on hydrological station and meteorological station nearly 60 years’ observation data, employed statistics and hydrological method to analyze the mainstream and tributaries’ runoff and sediment load spatial and temporal evolution characteristic of the middle reaches of the Yellow River basin, reveal the relationship of runoff and sediment load changes between mainstream and tributaries, and its response to climate change and human activities. Study on the runoff and sediment changing law and its driving factors is the basis of scientific control of runoff and sediment regulation of the Yellow River. It would be of great significant in maintaining healthy and stability of the Yellow River. The results are as follows:(1) we used the annual runoff index(WI) and specific sediment yield(SSY) to analysis the spatiotemporal changes in runoff and sediment for mainstream sections and 57 sub-catchments in the MRYR, including 3 periods(1957-1979 yr,1980-1999 yr,2000-1012yr). The results showed that the WI and SSY exhibited significant(P<0.05) decreasing trend in most sub-catchments. The spatial differentiation characteristics of WI was obvious, the most significant decreasing section of WI was between Toudaoguai~Fugu and Tongguan~Huanyuankou(P<0.01). Meanwhile the distribution of WI also reflected the precipitation characteristics of the MRYR basin, which decreases from the south to the northwest. The most significant decreasing region of SSY was in the north part of the MRYRB. During three periods, the significant reduction section of SSY was in Toudaoguai~Tongguan, its average annual SSY in 2000-2012 yr was much lower than that of 1957-1979 yr, and represented more than 80% reduction. Based on the runoff and sediment budget analysis indicated that the tributaries inflow in the Longmen~Tongguan section contributed the most runoff. The inflow of sediment load was mainly from the Toudaoguai~Tongguan section. Among four mainstream sections, two mainstream sections(Longmen~Tongguan, Tongguan~Huanyuankou) belonged to deposition regions. During study periods, the tributaries in the Longmen~Tongguan section also contributed the most sediment load. And the tributaries significant reduction section was in Toudaoguai~Wubao.(2) Used the Mann-Kendall test and linear regression for trend test, and sequential cluster method for abrupt changing point analysis for annual observations at 21 hydrological gauging stations(4 mainstream and 17 tributaries stations) to investigate the spatial and temporal variations of sediment load in the He-Long region in the middle reaches of the Yellow River basin from 1962 to 2009. The results showed that sediment load from both mainstream and tributaries show significant decreasing trends, evident abrupt changing points were detected around late 1970 s for most stations. The comparison among three mainstream sections implied that the sediment was mainly from Wubao~Longmen section, and the highest sediment modulus section was Fugu~Wubao. And contribution of tributaries’ in each mainstream section was mainly from Huangfuchuan River, Kuye River, Wuding River, Qingjian River and Yanhe River. Sediment budget analysis indicated that the 17 tributaries contributes 70.6% sediment load to the whole He-Long region. Both in mainstream and tributaries’ sediment load reduction was mainly caused by human activities, especially soil and water conservation projects.(3) The linear trend test showed that both annual runoff and sediment load displayed a gradually downward trend(P<0.05) in 1954-2010 yr in two large-scale sub-catchments(in the north was Huanfuchuan River and south was Yanhe River) in the He-Long Region. The accumulated anomaly method was used to detect the stage changes in annual runoff and sediment load, both river had three stages, in Huangfuchuan, the high flow period was in 1954-1979, the normal flow period was in 1980-1996,while in Yanhe, these two stages were in 1954-1971 and 1972-1996, respectively. The low flow periods of two rivers were all in 1996-2010. After the “Grain for Green” project, both catchments vegetation increased significantly. And the NDVI value of Huangfuchuan increased 2.5 times, especially in Yanhe River increased siginificantly( increasing rate was 0.01/a).(4) A linear regression function was applied to examine the relations of changes in discharge and suspended sediment concentration at event scale. The results showed that at the annual scale, annual runoff depth(h) and suspended sediment yield(SSY) exhibited good relations with higher correlation coefficients(R2) above 0.8(p<0.05). And at flood event scale, discharge and suspended sediment concentration also exhibited good liner relations. The relation had limited relations with the size of catchment area. The magnitude and frequency of flood events had obvious decreased trend during periods. Hysteresis loops analysis was employed to analysis single flooding events process in two rivers. Diversiform of C–Q loops type could be found, At Huangfu station, negative(anticlockwise) hysteretic loops appeared the most frequently, while at Ganguyi station the complex(multi-peak) hysteresis are the most. The results confirmed the complex and heterogeneous nature of sediment response in the middle reaches of the Yellow River basin.(5) Did a further study on the relevant factors, which influenced runoff and sediment in the MRYR.The results showed that:(1) The MRYR basin existed a relatively dry and warm period during recent 60 years. Average annual precipitation exhibited downward trend with an average decreasing rate of –1.28 mm/yr, average annual temperature increased by 0.3°C/yr, climate warming increases potential evapotranspiration and reduces runoff water to some degree.(2) The intensive human activities were the dominant factors for the significant decline in annual runoff and sediment load in the MRYR basin, especially extensive soil and water conservation measures leaded to rapidly decreasing of runoff and sediment load in the Toudaoguai~Tongguan section. The NDVI values in the MRYR basin displayed an obvious increase between 2000-2012 yr, The engineering measures structured, especially check-dams measures caused consequently decrease both the magnitude and the variability of runoff, and its storage capacity made flood frequency and magnitude decreased in the catchment.(3) In recent years, both mainstream and tributaries’ large, medium and small reservoirs combined operation leaded to the significant reduction of runoff and sediment load in the MRYR basin. The reservoir accumulation of water accounted seventy percent of the outflow runoff of the Middle Reaches, and trapped sediment load accounted forty percent of the outflow sediment of the Middle Reaches.(4) Water withdrawal for industry and agriculture was one of the factors leaded to sharply reduction of runoff inflow in the MRYR. Annual average water withdrawal and water consumption were rising in 1998-2012 yr, and which accounted 69% of runoff inflow in the MRYR.(6) Double mass curve method were used to distinguish precipitation and human activities for reduction of runoff and sediment load in the MRYR, it showed that after the changing year, human activities and precipitation contribution rate of runoff reduction were 80% and 20%,. And for sediment was 92% and 8%, respectively. Further confirmed that in the middle reaches of the Yellow River the effects of human activities on runoff and sediment load reduction was higher than rainfall effects, especially for the influence of sediment was more apparent. |
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