The Flow Regime Variation Of The Mainstream Of The Yellow River In Recent 60 Years

In recent decades,the great variation in the runoff and sediment pattern of the Yellow River have had a noticeable impact on the water safety and disaster prevention in the basin.Understanding the flow regime of the Yellow River is related to the future strategy of managing the Yellow River.Therefore,this paper analyzed the variation characteristics of runoff and sediment in the Yellow River basin in the past 60 years,explored the temporal and spatial changes of ru noff and sediment characteristics,and distinguished the degree of influence of the main effect factors on runoff and sediment,providing reference for river management strategies according to local conditions.Based on the daily average runoff data of 27 hydrological gauge stations and the daily average sediment load data of 20 stations,the paper proposed a segmentation for the Yellow River basin based on the variabilities of the discharge and sediment load by using a combination of various statistical methods such as cluster analysis.This segmentation provided an effective and reasonable framework for subsequent analysis.The segmentation divided the basin into five segments:(1)Upstream Tangnaihai;(2)Tangnaihai – Shizuishan;(3)Shizuishan – Longmen;(4)Longmen – Huayuankou;(5)Downstream Huayuankou.A parametric global method was used to detect abrupt change points of discharge and sediment load series,and a variety of statistical analysis methods such as the Mann-Kendall trend test and wavelet analysis were used to explore the multi-scale variation characteristics of discharge and sediment load in the mainstream of the Yellow River.According to the change point detection,the historical period was divided into three periods: before 1987(A),198 7 – 1999(B),and after 1999(C).Annual runoff and sediment load in all the regions except upstream Tangnaihai displayed significant decreasing trends.The closer to the downstream,the larger the annual runoff decr ease and the larger the coefficient of variation of the runoff.Toudaoguai – Tongguan was once the area with the largest sediment yield per unit area in the basin,but after 1999,the sediment yield per unit area in some sections of its downstream exceeded that in this area.The characteristics of the maximum event flood in the mainstream of the Yellow River were analyzed.The maximum event flood duration in segment 2 decreased significantly.The flood volume and sediment load during the maximum event flood in segment 2 – 5 showed significant downward trends.The ratio of flood volume to annual runoff decreased greatly from period A to period B,and increased at 20 stations in period C.The ratio of flood sediment load to annual sediment load displayed significant decreasing trends in the upper reaches and significant increasing trends in the lowest reaches.The maximum suspended sediment concentration(SSC)in the flood season in the upstream appeared in the medium and small discharge level s,while the maximum SSC below Tongguan was in the maximum levels.The sediment transport ability with large discharge obviously weakened after 1987.The operation of cascade reservoirs in the upstream smoothed the annual runoff process in segment 2.The effect of reservoir sediment trapping had little effect on the change of sediment load,but the reservoir runoff regulation had great influence on the siltation of the upstream river channel.From period A to B,the contribution rate of water consumption variation to runoff variation in the upper,middle and lower reaches was 21.7%,13.0% and 70.1% respectively.From period B to C,The increment of water consumption in the upper and middle reaches accounted for 126.4% and 59.0% of runoff reduction in the reaches respectively.The annual water consumption in the downstream decreased,contributing 72.7% to the increase of runoff.The effect of sediment diversion on sediment load variation was generally small.From period A to B,the contribution rate of sediment diversion variation to the sediment load variation in the upper,middle and lower reaches was 12.9%,-6.0% and 24.0% respectively.From period from B to C,the contribution rate of sediment diversion variation to the sediment load variation in the upper,middle and lower reaches ws-33.4%,-0.1% and 21.2% respectively.