Copula-based identification of the non-stationarity of the relation between runoff and sediment load

It is important to identify the non-stationarity in the relation between runoff and sediment load under the backdrop of the changing environment. This relation helps to further understand the mechanisms of runoff and sediment yield. A copula-based method was used to detect possible change points in the relation between runoff and sediment load in the Wei River Basin (WRB), China, where soil erosion is a very severe issue. The modified Mann-Kendall trend test method was applied to obtain the trends of runoff and sediment load spanning 1960–2010 at monthly and annual timescales. Finally, the causes of the identified non-stationarity of the relation between runoff and sediment load were roughly analyzed from the perspective of climate change and human activities. Results indicated that:(1) the runoff and sediment load in the Jinghe and Wei rivers were generally characterized by noticeably decreasing trends at both monthly and annual timescales;(2) both the Jinghe and Wei rivers had a common change point (2002), implying that the stationarity of the relation between runoff and sediment load in the Jinghe and Wei River was invalid; (3) human activities including increasing water consumption and growing application of soil conservation practices are dominant factors resulting in non-stationarity in the rela-tion between runoff and sediment load in the WRB. This study provides a new idea for identifying the non-stationarity of multivariate relation in the hydro-meteorological field under the background of the changing environment.     

Analyses of trends and causes for variations in runoff and sediment load of the Yellow River

Due to the impacts of globe climate change and human activities, dramatic variations in runoff and sediment load were observed for the Yellow River. Analyses of nearly 65 years' data measured at main hydrologic-stations on the Yellow River from 1950 to 2014 indicated that, except for the Tangnaihai station in the head region, sharp downward trends existed in both the annual runoff and annual sedi-ment load according to the Mann–Kendal trend test;and their abrupt changes occurred in 1986 and in 1980, respectively, according to the rank sum test. Factors affecting the changes in the runoff and sediment load were very complicated. Results indicated that the reducing precipitation and the increasing water consumption were the main causes for the runoff decline, while the impoundment of the Longyangxia Reservoir and its combined operation with the Liujiaxia Reservoir exerted a direct bearing on the abrupt change in the annual runoff. In addition to the sediment load decrease associated with the runoff reduction, the reduced storm intensity, the conducted soil erosion control, and the constructed dam buildings all played an important role in the trends and abrupt changes of sediment load decline.     

Changes in runoff and sediment load from major Chinese rivers to the Pacific Ocean over the period 1955-2010

Changes in runoff and sediment loads to the Pacific Ocean from 10 major Chinese rivers are presented in this paper To quantitatively assess trends in runoff and sediment loads, a parameter called the ＂Trend Ratio T＂ has been defined in this paper. To summarize total runoff and sediment load from these rivers, data from 17 gauging stations for the duration 1955 to 2010 has been standardized, and the missing data have been interpolated by different approaches according to specific conditions. Over the observed 56-year study period, there is a quite stable change in total runoff. Results show that the mean annual runoff flux entering the Pacific Ocean from these rivers is approximately 1,425 billion cubic meters. It is found that all northern rivers within semi-arid and transitional zones including the Songhua, Liaohe, Haihe, Yellow and Huaihe rivers present declining trends in water discharge. Annual runoff in all southern rivers within humid zones including the Yangtze, Qiantang, Minjiang, Pearl and Lancang rivers does not change much, except for the Qiantang River whose annual runoff slightly increases. The annual sediment loads of all rivers show significant declining trends; the exceptions are the Songhua and Lancang rivers whose annual sediment loads have increasing trends. However, the mean annual sediment flux carried into the Pacific Ocean decreased from 2,026 million tonnes to 499 million tonnes over the 56-year period. During this time there were 4 distinct decreasing phases. The decrease in annual sediment flux is due to the integrated effects of human activity and climate change. The reduction in sediment flux makes it easy for reservoir operation; however, the decrease in sediment flux also creates problems, such as channel erosion, river bank collapse and the retreat of the delta area.     

Joint probability analysis of water and sediment and predicting sediment load based on copula function

The Jinsha River comprises the upper reaches of the Yangtze River, which is the river section with the highest sediment content. Monitoring of sediment transport in the Jinsha River is done to the guarantee for the normal operation of the Three Gorges Reservoir. In the current study, a copula function was used to do a joint probability analysis of the water and sediment in the Jinsha River Basin (JRB), further a sediment load prediction model based on the copula function also was constructed. The results show that the average annual flow from 2001 to 2018 at the outlet of the Jinsha River (Yibin station) is about 60.43 billion m³, and the average annual sediment load is about 58.82 million t. The linear correlation coefficient between annual flow and annual sediment load is 0.28. The best marginal distribution for annual flow and sediment load is Pearson Type Three (PE3) and Generalized Normal (GNO), respectively, and the best fit for the combined distribution of the two variables is the Frank copula function. The synchronous probability of water and sediment occurrence is 0.459, and the asynchronous probability is 0.541. Based on the copula prediction model, the sediment load can be effectively simulated, and the correlation coefficient between the simulated sequence and the measured sequence reached 0.93. The current study provides important significance for the analysis of water and sediment in the JRB, which is beneficial to the management of Three Gorges Reservoir sediment discharge in the upstream and downstream.     

Effects of Caragana Korshinskii Kom. Cover on runoff,sediment yield and nitrogen loss

Soil erosion is serious in the Loess Plateau of China. Deposition of the eroded sediment in lakes or rivers may lead to eutrophication, because the sediment carries a lot of nutrients. Field experiments were conducted to study soil erosion and loss of nitrogen (N) from a 15o hillslope with 30% (low) or 80% (high) coverage of the shrub Caragana korshinskii Kom. A bare soil plot was used for the comparison. The results showed that Caragana korshinskii cover significantly reduced runoff and soil erosion. In comparison to the bare soil plot, the vegetation covered plots had about 20% less runoff and 65% less sediment. In general, the concentration of N in both runoff water and the eroded sediment decreased with time and approached a steady value. However, the species of nitrate nitrogen (NO3) was exceptional which increased with time slightly. The soil erosion caused an N loss of about 250 mg/m2 for the bare soil plot, the low coverage of Caragana korshinskii reduced the N loss by 20% and the high coverage of Caragana korshinskii reduced the N loss by 40%. Moreover, the amount of total N in eroded sediment was 2 to 3 times higher than the value in runoff water. In the total N loss, the organic N was about 75-80%. Nevertheless, inorganic N in runoff water was 5 to 10 times higher than the value in eroded sediment. The species of NO3 was obviously higher than the species of ammonium nitrogen (NH4). NO3 was the main species of inorganic N loss and was about two thirds of total. The organic N was the main species of N in the eroded sediment.     

Simulated multi-scale watershed runoff and sediment production based on GeoWEPP model

The runoff and sediment yield data from the Qiaozidonggou, Qiaozixigou, and Lu＇ergou watersheds, in the Loess Plateau of China are used to calibrate and validate the runoff and sediment yield simulated by GeoWEPP model of the WEPP Model at watershed scale. The indices of relative error, R, correlation coefficient, Re, and Nash-Suttcliffe efficiency coefficient Ens are used to evaluate the model fit. The eco-hydrological responses in the Luoyugou and Lu＇ergou watersheds are also forecast based on the WEPP Model. Meanwhile, the relation between vegetation pattern changes and sediment yield in the watershed is discussed, and the responses of runoff and sediment yield in the watersheds concerning forest growth stages are studied. The results show that the relative errors of simulated values of runoff and sediment yield are below 30%, the correlation coefficients axe above 0.90, and the Nash-Suttcliffe efficiency coefficients axe above 0.80. The simulation results present satisfactory performance, thus, the model could be used to simulate the runoff and sediment yield in these small watersheds. It is also observed that soil erosion tended to become severe as precipitation increased in the watershed, while soil erosion has a decreasing trend as forest cover increases and vegetation pattern is optimized. When the watershed is fully covered by forest, erosion and sediment yield are minimized. When the forest cover is about 30% and evenly distributed in the watershed, the erosion intensity is lower than if the forest cover is collectively distributed in the watershed. Erosion varies with different forest growth stages in the watershed; it is more serious at the young and near planting stage and is the smallest at the mature forest stage.     

大跨开合屋盖风荷载与风致响应试验与数值计算研究

为研究屋盖开孔对大跨结构风效应的影响,在边界层风洞中开展了大跨开合屋盖刚性模型风洞测压试验,分析了结构表面风荷载分布特征,基于时域分析法研究了屋盖开合对风致响应的影响。结果表明:大跨结构模态密集,第一阶模态能量最大,高阶成分的贡献不容忽视;屋盖开洞能有效的减小屋面风荷载,降低位移响应与加速度响应,减小风振系数。研究结果可为大跨开合屋盖风荷载设计提供参考。     

Relationship between the equilibrium morphology of river islands and flow-sediment dynamics based on the theory of minimum energy dissipation

River islands are vital geomorphic units in alluvial rivers, and the variation of their morphology and position plays a significant role in regulating flow-sediment transport and channel stability. Based on the theories of minimum energy dissipation theory of fluid movement and river morphodynamics, this study uses the river islands in anabranching channels to analyze the relationship between the shape coefficient of river island and the flow-sediment dynamics under stable equilibrium conditions...