Variations Of Erosive And Extreme Rainfall And Their Effects On Runoff And Sediment Load In Hekouzhen-Tongguan Reach Of The Yellow River

The Yellow River has a low water discharge and an extremely high sediment concentration and sediment load, harnessing silted is the key issues in managing the Yellow River. The middle reach of the Yellow River(MYR) is located between Hekouzhen and Tongguan where the river flows across the Loess Plateau. The MYR contributes approximately 90% of sediment to the Yellow River. Since 1980 s, there was a significant decreasing in sediment load of the Yellow River, especially in recent 10 years. Reasons of this change caused the extensive concern. In this study, we analyzed daily and monthly precipitation data from 26 meteorological stations for MYR and with runoff and sediment load data from 2 hydrological stations in the study area during the period of 1958-2013, with the methods of tendency analysis, spatial interpolation, Mann-Kendall method, Empirical Orthogonal Function(EOF) decomposition, Ensemble Empirical Mode Decomposition(EEMD), traditional statistics, Hurst exponent, Double mass curve and accumulated variance analysis, which can be used to detect spatial, multiscale characteristics of periodicity and trend for precipitation in MYR. Based on variations of sediment load in the MYR, we constructed statistical models of Rainfall-Runoff and Rainfall-Sediment through the hydrologic method to quantitiatively evaluatethe impacts of precipitation change and human activities on runoff and sediment load. The main conclusions are drawn as follows:(1) There were highly significantly decreases in both sediment discharge and natural runoff in MYR during 1958-2013. Mann-Kendall test showed that natural runoff significantly decreased with MK statistical value Z=-6.18(P < 0.01); sediment load significantly decreased with MK statistical value Z=-6.45(P < 0.01). Compared to reference period(1958-1979), the runoff and sediment load decreased by 40.9% and 61.4% in 1980-2013, respectively; In particular, the runoff and sediment load decreased by 56.6% and 82.2% in 2000-2013,respectively.(2) There was no trend in different rainfall index, flood-season rainfall and main flood-season rainfall. Though annual flood-season rainfall and main flood-season rainfall exhibited decrease trends, they were not significant(P > 0.1) in the study period.Mann-Kendall test showed that there were no trends in both the rainy days of erosive rainfall(RD12), heavy rainfall(RD25) and rainstorm(RD50), and the precipitation of erosive rainfall(R12), heavy rainfall(R25) and rainstorm(R50). Analysis on variation of different rainfall index showed that compared to reference period(1958-1979), though different rainfall index decreased in most area in 1980-2013 and 2000- 2013, it showed weak decrease. Moreover, in the different soil erosion regions, compared to reference period(1958-1979), though different rainfall index of rainy days decreased in most area in 1980-2013 and 2000-2013, rainy days decreased within one day, rainfall of different rainfall indice decreased within 15 mm. When comparing the variation of sediment load in the MYR with the variation of different rainfall indice, the weak decrease of rainfall and rainy days of erosive rainfall, heavy rainfall and rainstorm since 1980 cannot lead to the serious and sharp decrease of sediment discharge in Hekouzhen-Tongguan region of the Yellow River basin.(3) Though the precipitation was distributed highly and concentratedly in a year, there were no significant trends(P > 0.1) in different annual precipitation concentration index during the study period in the MYR. Annual average precipitation concentration index(PCI)and annual average concentration index(CI) exhibited no significant trends, with a linear increase from 1958 to 2013. The majority of MYR stations for CI had increasing MK trends,although the trends were not significant at P < 0.05. For PCI, fifteen stations exhibited positive trends and eleven stations had negative trends. However, trends were not significant at any station, except for the Xi’an station(with a significantly positive trend at the P < 0.05 significance level). The precipitation CI exhibited complex spatial patterns on a seasonal scale throughout the entire MYR. During spring and winter months, precipitation was distributed more regularly, while higher CI values were found in summer months, particularly in the eastern MYR. At seasonal scales, CI exhibited significantly increasing trends in winter at most stations, except for the Xi’an station. The majority of MYR stations for PCI had no trends.(4) The turning points of the annual rainfall, flood-season rainfall( ??) and main flood-season rainfall( ?mf) were all detected between 1960 and 1970. By employing the EEMD technique, the original annual ?? and ?mf time series from 26 stations were decomposed into four independent Intrinsic Mode Functions(IMFs) and one residue,respectively. The periodicity properties of ?? at each station show few periodic variabilities.The periodicity of IMF1 at each station ranged from a minimum of 2.3 to a maximum of 3.1,the periodicity of IMF2 at each station ranged from a minimum of 4.3 to a maximum of 7.0,the periodicity of IMF3 at each station ranged from a minimum of 8 to a maximum of 14, and the periodicity of IMF4 at each station ranged from a minimum of 18.5 to a maximum of 47.8.However, only the periodicity of IMF1 at Hengshan station, the periodicity of IMF2 at Xifeng station, the periodicity of IMF3 at Suide station and Xixian station and the periodicity of IMF4 at Yuncheng, Baoji, Lishi, and Yan’an station passed the statistical significance. In addition, according to the results of Hurst index, the trends of future precipitation in most stations will be persistent.(5) Based on hydrologic method, we constructed statistical models on Rainfall-Runoff and Rainfall-Sediment in the MYR. Given the turning point of the runoff was 1984, the streamflow series could be divided into three stage: reference period(period I: 1958-1984),Impacted period(period II: 1985-2013), Impacted period(period III: 2000-2013). According to Rainfall-Runoff statistical model, precipitation change contributed 18.7% to runoff reduction, while human activities contributed 81.3% to the runoff reduction in period II;precipitation change contributed 6.5% to runoff decline, while human activities contributed93.5% to the runoff reduction in period III. Based on the turning point in 1979 for sediment load, the sediment load series were also divided into three periods: reference period(period I:1958-1979) and Impacted period(period II: 1980-2013), Impacted period(period III:2000-2013). According to Rainfall-Sediment statistical model, precipitation change contributed 8.7% to sediment reduction, while human activities contributed 91.3% to the sediment reduction in period II; precipitation change contributed 9.0% to sediment decline,while human activities contributed 91.0% to the sediment reduction in period III.