Abstract: Sediment load entering the Yellow River Estuary has been greatly reduced and the river bed has been eroded because of the operation of the Xiaolangdi Reservoir,with the occurrence of remarkable channel adjustments. In order to describe comprehensively the bankfull channel geometry and flood discharge capacity in the Yellow River Estuary,a reach-averaged method was used in this study to calculate the reach-scale bankfull channel dimensions and bankfull discharge in the Yellow River Estuary annually from 1990 to 2016. Effects of channel evolution on the reach-scale bankfull channel dimensions and bankfull discharge in the study reach were investigated. In addition,empirical relationships were established between the reach-scale bankfull characteristic parameters and the previous four-year average fluvial erosion intensity during flood seasons in the Yellow River Estuary. These results show that both channel widening and channel incision occurred in the reach after 2003,and the variation in cross-sectional profiles in the Yellow River Estuary was mainly characterized by channel narrowing. Furthermore,bankfull discharge decreased with channel aggradation,and increased with channel degradation. Additionally,the correlation between each bankfull characteristic parameter and fluvial erosion intensity in the Yellow River Estuary was greater than 0.8,indicating the remarkable response of bankfull parameters to the altered flow and sediment regime at Lijin.
Abstract: This study implements ensemble runoff forecasts using the Xin'anjiang model driven by ECMWF,UKMO,NCEP and other seven control forecast products of TIGGE data center in Chitan reservoir watershed in Jinxi,Fujian Province,China. By means of ensemble selection,pre-processing via multi-model integration,and post-processing based on the BMA model,the influences of data-processing scheme and initial set quality on the accuracy and uncertainty of the meteo-hydrological runoff predictions were investigated. The results show that different treatment schemes could effectively improve the accuracy and stability of runoff forecasts. Combination of pre-processing and post-processing led to better performances than other schemes due to its error reduction in two aspects,i.e. lowering the input errors and controlling the output errors. Although the initial set quality exerted a perceptible impact on the ensemble runoff forecasts,the effect was not significant since either pre-processing or post-processing procedure effectively controlled the forecasting errors. Overall,we conclude that the pre-processing and post-processing processes are indispensable to improve the accuracy and reliability of meteo-hydrological runoff forecasts,which should be paid attention to.
Abstract: Watershed hydrological modelling provides a methodology to evaluate the effects of wetlands on stream flow and watershed hydrology. In this study,the PHYSITEL/HYDROTEL modelling platform was used to assess the added-value of specific isolated wetland and riparian wetland modules. To achieve this goal,two sets of simulations were performed (with and without wetland modules) to assess the capacity of the modelling platform with respect to two river segments of the Nenjiang River Basin,Northeast China. The results showed that integration of specific wetland modules can improve the performance of HYDROTEL to simulate both stream flows and effects of wetlands watershed hydrology. During the calibration and validation periods,Nash-Sutcliffe and Kling-Gupta efficiency improved 3.08% and 4.64% and percent bias root and mean squared error decreased 12.72% and 55.93% as a result of the integration of wetland modules,respectively. This explicit assessment framework could be applied to study the effect of wetland restauration and recovery projects on watershed hydrology.
Abstract: At present,high-accuracy water level data are urgently required for small experimental catchments. However,conventional water level gauges (WLGs) with an accuracy of one centimeter do not meet the need of high-accuracy water levels. To determine an applicable high-accuracy WLG for small experimental catchments,we primarily selected five types of relatively high-accuracy WLGs,and systematically compared these gauges under the following four conditions:a stationary water level,step-by-step increasing and decreasing water levels,a simulated food peak,and a natural rainfall-runoff process in situ. The results demonstrated that:① Based on the comprehensive absolute error,the accuracies of probing-needle,pressure-type model 2(i.e. capacitance-type),and magnetostriction WLGs were within 0.4 mm;② The accuracy of the float-type WLG was around 0.5 mm;③ The accuracy of the pressure-type model 1(i.e. resistance-type) WLG was higher than 1 mm;and ④ the magnetostriction WLG can accurately determine water level processes under the condition of low water head;therefore,this gauge is suitable for the measurement of high-accuracy water levels in small experimental catchments. These findings provide a significant reference for high-accuracy monitoring runoff,rainfall,evaporation,and infiltration processes.
Abstract: This study further clarified the influence of the westerlies and the South Asia monsoon on both the water vapor transport and precipitation in the Three-River Headwaters Region. ERA-20C reanalysis datasets were used to perform an empirical orthogonal function (EOF) decomposition of the water vapor transport flux of the Three-River Headwaters Region during the rainy seasons of 1948-2010. The EOF mode and index were acquired that can indicate whether the water vapor transport in the investigated region is controlled by the westerlies or the South Asia monsoon. The spatial patterns of water vapor transport and precipitation in the region were found under control of the westerlies or the South Asia monsoon. Under control of the westerlies,the water vapor transport direction inside the region was NW-SE. The westerlies and the South Asia monsoon converged at the south boundary of the region;furthermore,precipitation increased significantly in the east and south of the region (i.e.,the majority of the Lancang River headwater and east of the Yellow River headwater). Under control of the South Asia monsoon,the water vapor transport direction was S-N,the two water vapor transport paths converged in the north,and precipitation increased significantly in the north (i.e.,the majority of the Yangtze River headwater,and the north of the Yellow River headwater). The westerlies and the South Asia monsoon both exerted a vital influence on precipitation in the Three-River Headwaters Region. Water vapor transport paths controlled by either the westerlies or the South Asia monsoon led to positive precipitation anomalies in different parts of the region.
Abstract: Tidal flats are fragile and delicate. Studies show that anthropogenic pressures and sea level rise due to climate changes have an effect and play a vital role in the morphodynamics of tidal flats worldwide. It is very important to understand the regularities and morphodynamic variations of tidal flats whilst investigating the external condition responses to tidal flats. Seasonal variations' regularity on tidal networks and dissects of Doulong River intertidal flats located in Jiangsu Province,China were analysed,observed and researched. With the applications of Unmanned Aerial Vehicle (UAV) coupled with Structure-from Motion (SFM) algorithm,the Digital Elevation Modes (DEM) and orthoimage of the Doulong River intertidal flats were obtained. Results from the research clearly show the variations in tidal flats elevations are remarkable. With an increase as high as ±0.5 m within the period studied. The vertical and horizontal error of the tidal flats are less than 9 cm and 2 cm respectively. The tidal creek meanders are active with no regular changes in season throughout the year studied. The ratio of width to depth ranges from 10 cm to 25 cm during the development of a tidal creek. UAV technology provides a platform to monitor the variation trend of tidal flats elevation and the short-term evolution of small-scale tidal creeks. Thus,providing strong technical support for monitoring the short-term morphodynamic evolution of estuary and coast,which can be hard to sense by traditional remote sensing.
Abstract: Droughts have such essential features as long duration,large scope of influence,and spatio-temporal continuity,and a single variable or double variables were mostly considered in previous studies. By identifying arid patches and judging the continuity between two adjacent droughts based on the given threshold,a spatio-temporally continuous 3D identification method has been proposed for drought events,measuring a drought event in terms of 5 characteristic variables of duration,area,severity,intensity,and central location;the duration-area-severity frequency analysis method based on Copula function has been proposed. Taking Southwest China as an example,SPI drought indices are used to identify the drought events that lasted 3 months or longer in recent 52 years,which totaled 78. Among them,the most serious event from Aug. 2009 to Jun. 2010 has a recurrence period of 94 years. The comparison between the probability distribution function and the Copula function indicates that the 3 characteristic variables of duration,area and severity shall be simultaneously considered upon frequency analysis of drought events.
Abstract: Some well-known notions in univariate statistical analysis becomes misleading in the multivariate context,and have not been correctly understood by many researchers. One example is the relationship between the expected number of events with return periods greater than or equal to T year over a N year period with the ratio N/T. Additionally,in some empirical studies,the relationship between the joint return period of a hydrological multivariate event and the corresponding return periods of its marginal distributions were found and applied. However,there lacks a detailed derivation and interpretation of this relationship. In this paper,based on the GH copula,the relationships between the bivariate return period of an event and return periods of its corresponding marginal distributions was derived theoretically,as well as the relationship between the number of occurrences of bivariate events and their primary return periods at different degrees of correlation between the two marginal variables. The theoretically derived relationships in this study were tested with drought events in Kunming identified on SPI and SRI series with 56 years of monthly precipitation and runoff data. The results suggest that the correctness of drought multivariate return period analysis could not be supported by the closeness of the primary return period of an event and the average inter-arrival time of drought events more severe than the studied event. Future researchers also need to avoid using the maximum marginal return period to approximate the primary return period of an ‘and’ event when the two marginal variables only have a weak correlation.
Abstract: Prediction of typhoon-induced debris flow can help to reduce casualties and losses. This study focused on 47 debris flow catchments of the Chenyulan River Watershed in Nantou County,Taiwan,China. Seven variables-mean channel slope,effective drainage area,shape coefficient,channel length,lithology,area ratio of collapses and landslides,and mean rainfall intensity-were selected from the conditioning factors of rainfall-induced debris flow,including geomorphology,loose debris materials and rainfall conditions. According to the importance of these variables,the area ratio of collapses and landslides as well as the mean rainfall intensity were used to construct the predictive model. Random sampling selection was used to divide the dataset into the training and validation datasets (70% and 30% of the dataset,respectively). The former was used to develop the predictive model and the latter to validate the model. Four indicators,namely precision,accuracy,false negative rate,and false position rate,were employed as indexes of quantitative assessment and used to determine the optimal model. The results show that the prediction model of typhoon-induced debris flow based on Fisher discriminant analysis has good prediction performance,and it compensated for the deficiency of rainfall intensity-duration thresholds model that analyze critical rainfall of debris flow based only on rainfall data. A comparison of the predictions of the I-D model and our model indicated that ours had superior prediction performance. The results provide strong technical support for the prediction of rainfall-induced debris flow.
Abstract: This study proposes a design flood estimation method for cascade reservoirs during operation period to maximize the comprehensive benefits both for flood control and hydropower generation.The joint probability distributions of floods occurring at all sub-basins are established using the multivariate t-Copula function. The most likely regional composition is derived using the Monte Carlo simulation and the Genetic Algorithm (GA) methods. The design floods as well as the flood limited water levels of the Wudongde-Baihetan-Xiluodu-Xiangjiaba cascade reservoirs duringoperation period are analyzed. It is shown that:① The results of the most likely flood regional composition are reasonable and can offer a meaningful reference for the analysis of design flood estimation for cascade reservoirs.② The design floods of downstream reservoirs are significantly influenced by the operation of upstream reservoirs. The 1000-year design flood peak,3 d,7 d and 30 d design flood volumes at Xiangjiaba reservoir significantly decrease by 15 400 m3/s (35%),3.56 billon m3(33%),7.02 billon m3(30%) and 8.50 billion m3 (11%),respectively.③ The obtained flood control water levels of downstream reservoirs are slightly higher than flood limited water levels but without reducing the original flood prevention standards. The flood control water levels (flood limited water levels) of Baihetan,Xiluodu and Xiangjiaba reservoirs during operation period are 788.82 m(785 m)、570.67 m(560 m) and 371.36 m(370 m),which can increase the hydropower generation by 2.1%,6.1% and 1.4%,respectively,and in total 1.71 billion kW·h annually.
Abstract: The transport patterns of pollutants in natural bodies of water are closely correlated with deposition and resuspension processes of sediment,to which the pollutants attach,and yet the influences of sediment on the transport of pollutants have not been fully studied. We here used transport age to quantify the influences and applied these findings to the Xiyang Sea area of the radial sand ridges in the South Yellow Sea. The governing equations of transport age considering sediment's impacts were derived from sediment-pollutant interaction model. Numerical simulation resolving these equations was used to analyze the transport pattern discrepancy in the presence and absence of sediment and the influence of the partition coefficient of pollutants (KD). The results show a significant trend of decreased pollutant concentration and corresponding increases in transport age,which indicates that sediment plays a role in weakening the dilution and diffusion processes of pollutants. The partition coefficient of pollutants,which is representative of adsorption capability of sediment,has a positive effect on the transport age. This effect tends to decline to zero when a value of 103 L/g is achieved for the partition coefficient of pollutants,when the sediment adsorption capacity tends to be saturated.
Abstract: Land use/landscape pattern has become an important factor affecting water environment quality in China. Nonetheless,in-depth study of the complex relationship between land use/landscape pattern and water environment quality is still lacked. We applied a structural equation model (SEM) which accounted for the bidirectional regulation of water environmental carrying capacity and environmental pressures in the upstream of Huangshui River basin. Subsequently,we calculated the contribution rates of various variables of land use/landscape pattern on water environment quality in accordance to their relative weights. The results indicated that the contribution rate of point source pollution on the water quality is the greatest (37.3%). From landscape pattern perspective,the contribution rates of the spatial distributions of impervious surface and green space,the connectivity and aggregation of the wetland on the water quality are 9.14%,7.91%,7.44% and 6.96%,respectively. To significantly improve the water quality in the study area,more efforts are needed to control point source pollution,enhance the connectivity and aggregation of the wetland,increase green space and decrease impervious surface in the upstream of Huangshui River basin.
Abstract: As an important factor affecting water quality,the migration and transformation of biogenic elements in rivers have attracted much attention. The process of mass exchange at the water-sediment interface is affected by many environmental and hydrodynamic factors,and it is extremely complex,which not only affects the flux of river biogenic elements but also relates to the water environment. In order to explore the migration and transformation process and mechanism of biogenic elements at the water-sediment interface,this paper summarizes the micro-interface mechanism of sediment particles and biogenic elements,the migration and transformation of biogenic elements at the water-sediment interface and the influence of flow conditions on sediment adsorption/desorption of biogenic elements. The water quality model of the river and the water environment regulation model are described under the condition of sluice and dam pumps. Finally,the influences of sediment transport on the transport of biogenic elements and water environmental regulation is pointed out in plain rivers. Furthermore,future research directions are also outlined.
Abstract: For the Lower Yellow River,which has a major role of transferring sediment to the sea,the boundary condition has an important influence on the sediment transport efficiency. From the literature survey,this paper reviewed studies of the effects of boundary condition on sediment transport. An increasing number of studies have shown that the boundary condition and its evolutionary processes affect the spatial-temporal variation of the sediment transport efficiency. For different calculation methods of sediment transport,such as the hydrology method and hydrodynamics method,the influencing ways of boundary condition have both the difference and the relation. The mathematical comparison results show that the derivatives of sediment concentration of the two above mentioned methods are first order approximate. The key criterion used to check the effectiveness of simulation for sediment model is to determine whether the coupling effect between the sediment movement and bed deformation can be reflected. Seeing as how the sediment transport and bed deformation processes are very closely interlinked,it is proposed that the regulation of sediment transport efficiency can be studied from the aspect of the riverbed evolution process,potentially providing new insights and support for further studies.