Abstract: The accuracy assessment of global gridded precipitation products is the first step in local application. Based on the daily precipitation observation data of 824 weather stations in China during 1979-2015, this study intends to evaluate quantitatively the accuracy of newly released Multi-Source Weighted-Ensemble Precipitation (MSWEP) products in China by using a variety of indicators such as relative bias and correlation coefficient. Moreover, a corrected Mann-Kendal trend monitoring method was used to analyze the reliability of MSWEP products in trend analysis of precipitation. The main conclusions show that:① Precipitation estimated from MSWEP is higher than that of the gauged precipitation in China. However, there is an underestimate in the North China region. ② MSWEP daily rainfall data have overestimation and underestimation of slight-precipitation and heavy-precipitation events, respectively. ③ There is a good relation between monthly/annual gauge precipitation and MSWEP. ④ There is an obvious spatial difference between the trends of annual precipitation based on MSWEP and site observations in China. However, in spring, autumn and winter, the spatial distribution of the trend is in good agreement.
Abstract: In order to study the influences of rainfall characteristics on the effectiveness of total runoff capture in different regions of China, the study draws on the daily precipitation data of 186 meteorological stations over nearly three decades for spatial analysis and statistical calculation, obtaining the volume capture ratio of annual rainfall. The corresponding design rainfall depth and annual controlled rainfall on average are determined. Then, the relationship between annual ratio of total runoff capture in each city and annual average of controlled rainfall is used to divide China into nine types. The results show that the design rainfall depth in China has considerable geographical variation, with the standard deviation of Guangdong, Sichuan, Guangxi, Hebei, and Henan approximating 1.5 to 3 times the national average, and that it increases with the increase in volume capture ratio of annual rainfall. Meanwhile, the marked spatial variation of rainfall within the province has led to significant differences in effectiveness of runoff capture at the source. In southern China, runoff control mostly features "high volume and low ratio."In contrast, areas of "low volume and low ratio" are found in Gansu, Ningxia and other places; covering 9.44% of the area, they have precipitation characteristics unfavorable for source control facilities. The "high-volume and high-rate" areas most suitable for source control are located in Southwestern China, accounting for 3.80% of the area.
Abstract: Evaluating the impact of urbanization on changes in river systems is crucial for urban planning and protecting river systems. We selected the Wuchengxiyu and Yangchengdianmao hydrological zones in the lower Taihu Lake basin as regions of study. We compared river system data and land use types from the 1980s and 2010s to identify changes in river system patterns and measure how these changes were related to land urbanization. To measure this relationship, we used global and local autocorrelation analyses and a Geographically Weighted Regression model. The results indicate that drainage density decreased at a rate of 11.3% over the last 30 years in area of study. The 1st-and 2nd-order tributary density decreased at the rate of 18.1% and 11.3%, respectively. The most drastic decreases were observed mainly around large cities. The intensive degradation of the river system, caused by deposition, landfill and annexed by the lakes were the main reasons for the decrease in drainage density. The degradation of tributaries had an exponential relationship with the velocity of land urbanization:the rate of decrease in tributary length accelerated when the land urbanization reached 40%. In addition, the intensity of river system variation and recession increased in regions with greater river density. Thus, more attention should payed to the protection of river systems during rapid urbanization.
Abstract: The main reason for the decrease of sediment transport in the lower reaches of the Jinsha River is reservoir trapping, which has a great influence on the condition of the inflow and sediment of the Three Gorges Reservoir. To develop a complete understanding of the situation of reservoir trapping in the Jinsha River basin, based on the reservoir construction and the hydrology and sediment data collected in the basin since 1956, with the method of empirical formulas and by using typical investigations and the principle of sediment transport balance, the sediment storage capacity in the middle and lower reaches of the Jinsha River in the recent 60 years and the contribution weight of reservoir trapping to sediment reduction in the Jinsha River basin were studied quantitatively. The results indicated that the contribution weight of reservoir trapping to the sediment reduction at Pingshan station from 1956 to 1990, 1991 to 2005, and 2006 to 2015 was 0.3%, 48%, and 83%, respectively. It is apparent that the effect of reservoir trapping increased gradually. From 1956 to 2015, the annual sediment trapping in the reservoirs of the Jinsha River basin was 0.310×108 tons, and 89% of this sediment was silted in the large reservoir. Especially, from 2011, with the completion of the reservoirs in the middle and lower reaches of the Jinsha River and the Yalong River, the annual reservoir trapping in the Jinsha River basin increased to 1.92×108 tons, the contribution weight to the annual sediment reduction at Pingshan station was more than 90%.
Abstract: A theoretical analysis framework considering the tradeoff between water supply benefit and water diversion cost is established to deal with real-time operation issues in inter-basin water diversion projects. First, the optimal decisions for the recipient reservoir are discussed. Then, how the forecast information uncertainty may influence the reservoir operation is investigated, and a criterion on forecast information utilization is established based on the acceptable risks of water shortage and spill. Finally, the Biliuhe reservoir in Liaoning Province is taken as an example to verify the analytical framework. The results show that when the water availability amount of the recipient reservoir is small, water diversion could increase both the immediate benefit of water use at the current time step and the potential benefit of water storage at the end of the decision period. At optimality, the marginal benefit of storage equals the marginal benefit of current release, as well as the marginal cost of water diversion. With the increase of the water availability (before water diversion), water is released based on the current water demand, and the increase of water diversion improves only the benefit of water storage. In this case, at optimality, the marginal cost of water diversion is equal to the marginal benefit of water storage. Moreover, when the expected water storage is small, the reservoir has a relatively high probability of water shortage. The utilization of the forecast information depends on the acceptable risk of water shortage. Otherwise, the reservoir has a higher probability of flow spill, and the utilization of the forecast information depends on the acceptable risk of spill.
Abstract: In order to investigate the effects of soil bulk density, initial soil moisture content, pressure head, and clay particles content and infiltration time on the characteristics of point source infiltration under film hole irrigation. The experiment was carried out by orthogonal design method including 9 tests and 3 verification tests. Based on laboratory test under film hole irrigation, the paper analyzed the effects of multifactor interaction on accumulative infiltration per unit film hole area, infiltration rate and average incremental of volumetric moisture content under point source infiltration under film hole irrigation. Moreover, the one empirical model of the relationship between the accumulative infiltration per unit film hole area and the influence factors and another empirical model of the relationship between the average soil volumetric moisture content increment in wetted body and the influence factors had been built respectively. Furthermore, the root mean square errors of these empirical models were 0.032 cm and 0.006 cm3/cm3, respectively, the correlation coefficients were greater than 0.97, the determination coefficients were over 0.95(P<0.01), and the relative error between the simulated values and the measured values of these empirical models were within ±13%. The results of this study indicated that pressure head had no significant effect on accumulative infiltration per unit film hole area (P>0.05). However, the initial soil moisture content had a significant effect on accumulative infiltration per unit film hole area (P>0.05), and the soil bulk density, clay particles content and infiltration time had extremely significantly effect on accumulative infiltration per unit film hole area (P<0.01). Infiltration time was the main factor affecting the accumulative infiltration per unit film hole area, followed by clay particles content, soil bulk density and initial soil moisture content. The average soil volumetric moisture content increment in wetted body was not affected by the infiltration time. Soil bulk density, initial soil moisture content, pressure head, and clay particles content have extremely significantly effects on average soil volumetric moisture content increment in wetted body (P<0.01). Initial soil moisture content was the main factor affecting the average soil volumetric moisture content increment in wetted body, followed by soil bulk density, pressure head and clay particles content. The study can provide a theoretical basis for further optimizing the combination of technical factors for film hole irrigation.
Abstract: In order to estimate the risk of dike-break flood in the flood protected zone under multi-flood sources and complex boundary conditions, a one-dimensional multi-breach model and a two-dimensional flood routing model were established based on unsteady flow control equation. A dam-break model was used to couple rivers and flood protected zones, and local mesh refinement method and approximate structure generalization method were used to deal with water division and blocking resulting from the complex boundaries such as roads in the flood protected zones. Four flood scenarios of the flood protected zone from Hannan to Baimiao from three flood sources including the Yangtze River, Hanjiang River, and Dongjinghe River were simulated. The flooding area, economic loss and flood-affected population were compared and analyzed by the loss rate method based on inundation depths. The results showed that the model established in this study was reasonable and stable. The complex boundary had a significant impact on the flood routing, and the risk differed significantly depending on the flood sources. The worst situation was observed in a 300-year flood occurred in 1954 in the Yangtze River at the dike-break position of Xiangxin, resulting the submergence of 3 790 km2 area, a flood-affected population of 1.968 million, and an economic loss of 80.2 billion yuan. This study can provide strong technical supports for flood risk management and decision-making of refuge and migration.
Abstract: Deep run-of-the-river reservoirs, like Xiluodu Reservoir, are characterized by large storage volumes and high flow rates. The thermodynamics and temperature distributions within these reservoirs are different from natural rivers and lakes. Research on Xiluodu Reservoir was conducted based on a series of long-term and high frequency thermistor chain data from November 2016 to September 2017. Annual heating and cooling periods were differentiated and the diurnal variations in surface water temperature as well as metalimnetic internal wave phenomena were analyzed. The surface heat flux was calculated using both observed data and a constructed numerical model to investigate its influence on the change in water temperature. Results revealed that the formation of the surface mixing layer is dominated by the total surface heat flux. Surface heat flux is an important factor when considering the increase of epilimnetic water temperature during the heating period. During the cooling period, the heat loss at the surface incites vertical mixing while inflow intrudes into the stratified volume, which together govern the cooling processes in the epilimnion. Results also revealed the formation of a metalimnetic internal wave with a period of 24 hours. This suggests that internal wave formation within Xiluodu Reservoir is related to dam hydropeaking. When comparing the surface heat flux calculated from measured meteorological data and that calculated by the numerical model with daily-averaged boundary conditions, the values from the numerical model tend to be smaller.
Abstract: Riverbank erosion occurs frequently on fluvial rivers, which is one of the key issues for river regulations. The measurement of lateral erosion rate is critical for the investigation of bank erosion mechanism and revetment. A measurement system for bank erosion rate is developed based on the technology of image processing. A new method to measure the lateral erosion rate of riverbanks is proposed by tracking the location of tracing meshes on riverbanks. As a case study, the lateral erosion rate of riverbanks in an experimental channel is measured by the proposed method, and the measured data agreed with the calculated results by a semi-analytical method. Both measured and calculated results indicated that the lateral erosion rate depended on the flow velocity as well as the anti-scour ability of riverbanks. The proposed method is able to provide a technology for the further study on bank erosion mechanisms.
Abstract: The convective outflow boundary condition is used in the multistep characteristic-based operator-splitting(MCBOS) finite element method. A numerical model for linear shear flow past square cylinders in tandem arrangement is established. The model is proven feasible through the simulation of the linear shear flow past isolation square cylinder. Effect of the shear parameter and spacing ratio has been studied. The numerical results show that the shear flow can suppress vortex shedding and decrease the Sr number. The stagnation point of the upstream square cylinder shifts toward the lateral separation point. When shear parameter is equal or greater than 0.3, the location of the stagnation point remains unchanged. The results also show that the wake field of the gap of two tandem square cylinders changes from the steady state to the unsteady state without vortex shedding and then to the unsteady state with vortex shedding as the spacing ratio increases. The reason for this in deeply connects with the continuous negative value. The dramatic increase in the pressure difference between stoss and lee side leads to increased the mean drag coefficient abruptly.
Abstract: Flow division is a classic issue encountered in river dynamics. Bifurcations in subtidal estuarine environments are influenced by river discharge as well as tidal motion. Residual circulation usually occurs in estuaries owing to the effect of tidal dynamics. Flow divisions that occur at bifurcations are generally more complicated compared to those occurring in regions without tides. This study aims at quantifying tidal influence on subtidal discharge distribution in the Yangtze Estuary through application of the discharge asymmetry index. A 2-D numerical model was developed based on the bathymetry of the estuary, as measured in 2002. To decompose the effect of tidal motion, river discharge, and river-tide interactions, the model was subjected to three independent operating conditions-river discharge and tides, river discharge only, and tides only. In addition, the effect of Deepwater Navigation Channels on flow division has been qualitatively explained. Through use of the proposed model, the discharge asymmetry index at the South and North Channel bifurcations was reduced by 32.95% and 35.71%, respectively, during the dry and wet seasons. As observed, the general effect of tides is to make flow divisions more even. In the South Channel, river-tide interactions were strengthened by construction of the Deepwater Navigation Channel. Consequently, the subtidal water level was enhanced, and the dominant trend of flow allocation to the South channel was weakened.
Abstract: The heat transfer process between ice and water draws special attention due to wide applications on the development and decay of sea ice and glaciers. The study on water-ice energy transportation can enhance the understanding of the mechanism on the process of ice ridge consolidation and glacier melting. In this paper, the thermodynamics processes and solidification characteristics of ice is investigated experimentally with submerging tests and numerically with the Finite Element Method (FEM). The measurements and numerical simulations are performed to determine the ice growth and temperature variation with various initial thicknesses and temperatures. From the measured and simulated results, we can see that the ice temperature is non-linear distributed in the short beginning and linear distributed in the later process. The mean temperature increases sharply in the early stage and gently afterward, depending on the internal temperature gradient. For the colder and larger samples, it proceeds longer time to reach the thermal equilibrium. Moreover, the ice also grows quickly in the beginning and slowly in the later stage. The dimensional analysis denotes that, rather than initial condition, the true driven factor for temperature development is the Fourier number, which is the ratio between conduction energy and inertial energy. The relative ice growth is dominated by the difference between the Stefan number and the Biot number, which represent latent energy and convection energy, respectively.
Abstract: The presence of ice jam increases the possibility of upstream water level, which can easily trigger ice jam flooding. Bridge pier types can significantly influence backwater heights. By using a series of experiments in the laboratory, the impact of bridge-pier types were investigated. It shows that, when Fr ≤ 0.145, the backwater height from ice jam without pier is smaller than that with piers. When Fr>0.145, the backwater height from ice jam without pier is larger than that with piers. The sequence of backwater height due to ice jam is the tip-shaped pier, rectangular pier, Round end-shaped pier, cylindrical pier from large to small. For rivers that have ice jam problems, it is better to choose cylindrical pier or round end-shaped pier.
Abstract: Wave force is the main load on the offshore structure, accurate predictions the wave loads on structures are significant in practice engineering design. Most of the research associated with wave interaction with structures has been focused on unidirectional waves. But in reality, sea waves are multidirectional. Based on a physical model experiment, the characteristics of wave force loads on a large-scale cylinder under multidirectional random waves action are studied systematically. The result shows that the wave directionality has an obvious impact on wave loads, especially on the transverse forces. For the wider directional distribution, the transverse force cannot be ignored anymore and is about 40% of the normal force. This fact needs to be taken into account in order to make accurate predictions of the wave loading on cylinders in practice.
Abstract: Water resources is one of the most essential natural resources throughout the world. Development of the early warning methods of water resources carrying capacity, by revealing the interaction mechanism between the water resources, the social economy and the ecological environment, can aid in the effective evaluation and warning of the sudden and abnormal state of the water resources carrying capacity, and is the key part of the early-warning on the carrying capacity of water resources. For this reason, the main methods of early warning of water resources carrying capacity has been summarized into four aspects:diagnosis, prediction, evaluation and regulation, and the main points for future research on water resources bearing capacity early warning have been put forward based on its concepts and mechanisms:① to further refine the concept of water resources carrying capacity warning system; ② to establish the indicator system of water resources carrying capacity; ③ to lucubrate the holistic study of water resources carrying capacity; ④ to realize the dynamic warning of water carrying capacity; ⑤ to enhance the study of the adjustment measure of water carrying capacity.
Abstract: The hyporheic zone, which is an important interface between the surface of the water and groundwater, is an active ecotone in a river ecosystem. The study on hyporheic zones and hyporheic exchange mechanism impose significant effects in assessing the development and utilization of water resources, and in maintaining and restoring the health of a river ecosystem. The present study systematically introduces the analytical model of a one-dimensional vertical steady-state and transient heat transport equation in the temperature tracer method, and combines and summarizes the research progress of temperature time-series data collection methods, signal analysis methods, and numerical methods, based on temperature tracing. Finally, the advantages and main problems of the temperature tracer method were compared and discussed, indicating that the uncertainty in temperature measurements, multi-dimensional groundwater flow, and impact of non-ideal conditions on these analytical models should still be emphasized in future studies on temperature tracer methods.