Abstract: Extra long-term data series for precipitation is essential to the analysis of climate change and projection of regional water security in the future. While the observation data are often limited to only a few decades, historical documents could be used to reconstruct and estimate the historical precipitation to extend the data series. Based on the 'Atlas of drought/flood in China for the last 500-year period' and the observed precipitation data from 1959, this study reconstructed the long sequence of precipitation for the monsoon region of eastern China from 1470 to 2019. This study also investigated the variation pattern of precipitation for this 500-year period, explored the impacts of human-induced climate change and natural climate variability on precipitation, and projected the change trend of precipitation for the four scenarios from CMIP6. The results show that: ① The interannual distribution of precipitation in the region wasuneven with obvious wet and dry variations, 1470-1691 was in the dry period, 1692-1924 was a wet period, and turned into another dry period since 1925 with a quasi-181-year cycle; ② The impact of human-induced climate change has become apparent since 1991, wit a significant change in the tendency rates for precipitation in the Hai River, the Lower Yellow River and Yangtze River basins and the accelerated increasing trend of precipitation in the region; ③ Under the future climate change scenarios, the multi-year average precipitation would have a significant increase comparing to the baseline period of 1961-1990, greater seasonal variations, and higher risk of regional drought and flood. However, further study is needed on the liability for the change trend and risk analyses due to the uncertainty of future climate scenarios.
Abstract: The Jiuduansha shoal (JDS, in Yangtze River Estuary, China) is a national nature reserve. Understanding the relationship between fractional vegetation coverage and tidal creeks is significant to the JDS protection. In this study, the distribution of tidal creeks and fractional vegetation coverage in JDS were extracted and analyzed based on long-time series Landsat and HJ-1A multispectral images during 1996-2020. The results indicate that the total length of tidal creeks increased by 11.71 km/a during 1996-2020. The vegetation coverage increased by 4.79 km2/a. In 2020, the highest order of tidal creeks was fourth. The distributions of tidal creeks were closely related to high vegetation coverage areas. Among the different orders of tidal creeks, those at the first order significantly affect the overmarsh path length (LOP). Among the different vegetation coverage parameters, the impact of the vegetation coverage area on the LOP was most remarkable. The relationship between the LOP and vegetation coverage area satisfied an exponential function. As the vegetation coverage area increased, the LOP gradually decreased. The drainage efficiency of the creek network increased.
Abstract: Natural runoff in the Yellow River basin has suffered a sustained decline under the influence of climate change and human activities. Earlier studies mainly investigated the attribution of climate change and human activities, while focusing on the observed runoff decline and rarely on natural runoff, which is not conducive to the efficient utilization and scientific management of water resources in the Yellow River basin. In this study, natural runoff during different periods in the Yellow River basin were evaluated by means of a dualistic water-cycle model, while the contribution of the main factors on the declining natural runoff was elucidated by a multi-factor attribution method. Results revealed that the 1956-2016 annual natural runoff at the Huayuankou station was reduced by 11.46 billion m3 compared to 1956-1979. The contributions of climate change, land-use change, and social water use were estimated to be 24.4%, 25.0%, and 50.6%, respectively. Analysis of the contributions in each Yellow River zone indicated that climate change and human activities were the dominant factors upstream and downstream of Lanzhou, respectively. In order to mitigate the declining trend of natural runoff and promote the ecological protection and high-quality development in the Yellow River basin, stronger measures, such as deep water saving, rigid water control, moderate water increase, efficient water management, and water protection legislation, should be implemented.
Abstract: For rivers flooded by rain, the annual sediment load is mainly derived from one or several rainstorms in the flood season and is of research significance. Based on the theory of runoff erosion power, the M-K method and the daily measured runoff and sediment data for the Xiaoheba, Luoduxi, Wusheng and Beibei stations in the Jialingjiang River basin, this paper analyzed the sediment transport characteristics during flood events in different periods and regions. The results of this paper are as follows: the runoff and sediment discharge during the flood season mainly came from the main stream of the Jialingjiang River. In general, the runoff discharge of flood events in different regions showed no significant decreasing trend, while the sediment transport modulus (sediment load) showed a significant decreasing trend except at Xiaoheba station in 2013-2020. With the gradual increase in runoff erosion power, the variation ranges of the sediment transport modulus (sediment load) in different periods in the Jialingjiang River basin were correspondingly narrowed. The degrees of influence of human activities on the sediment transport processes were weakened in the order of the Wusheng station in mainstream Jialingjiang River, Luoduxi station in Qujiang River and Xiaoheba station in Fujiang River. Affected by the Wenchuan earthquake and rainstorms, the average sediment loads in flood event of the Xiaoheba and Beibei stations in 2013-2020 were higher than those in 1998-2012, while the rate of increase rate at the Beibei station was less than that of the Xiaoheba station.
Abstract: Water security is an integrated concept of multiple dimensions, including water resource utilization, water eco-environmental protection, water disaster prevention among others. In this paper, an evaluation indicator system of water security are constructed, focusing on five dimensions: water quantity, water quality, sustainability, affordability, and flood security. It is based on a summary and discussion of the definition, connotation, and evaluation of water security dimensions. We provide increased understanding of applicability and usefulness of water security evaluation systems, thereby avoiding content duplication or overlaps between different dimensions and promoting the accuracy of problem diagnosis. In doing so, we used a multi-expert decision-making method to quantify indicators weight and to standardize data. Compared with evaluating the water quantity security using indicators of water resources endowment alone, this evaluation system implies concepts of water demand and sustainable security that included in resource security, which is capable of analyzing actual reasons for water shortage. Affordability, which is seldom considered in previous studies, is supplemented to ensure system integrity of water security assessment. Finally, a case study of water security evaluation of China is carried out with cluster analysis. The main water security problems in each region of China were diagnosed well, which are in line with the current situation of China. This test indicates that the evaluation system is both reliable and practically applicable.
Abstract: In order to improve the calculation accuracy of flow exchange between groundwater and surface water, a method that integrates the hydraulic connection and hydraulic head difference between groundwater and surface water, water temperature, Radon-222 and stable hydrogen and oxygen isotopes (HHTRO) was proposed and successfully applied to quantify the flow exchanges between groundwater and surface water in the Suzhou section of the Xinbian River. The study results showed the recharge from surface water to groundwater varied from 8.69 to 366.82 m3/(d·m) per unit river length and the discharge from groundwater to surface water varied from 0.72 to 120.90 m3/(d·m). The discharge from groundwater to surface water was predominantly on the left bank with a net discharge of around 45.26 m3/(d·m) and the recharge from stream to groundwater was mostly on the right bank with a net recharge of approximately 214.33 m3/(d·m). Overall, the flow exchange between surface water and groundwater in this river section was dominated by the recharge from surface water to groundwater with a replenishing ratio of 55.14% for groundwater. This method would not only improve the accuracy for estimating the flow exchange between groundwater and surface water provide necessary theoretical support for water resources development in river basins.
Abstract: Traditional methods for discriminating the genetic type of groundwater regime have some problems, such as being substantially affected by human subjective factors and the ease of producing multiple solutions. To improve the discrimination reliability of the genetic type of groundwater regime, this study proposes a comprehensive discrimination method combining a qualitative analysis of hydrogeological conditions and quantitative analysis of measured data. Firstly, according to the meteorological, hydrological, and hydrogeological conditions of the study area, the effects of various groundwater recharge and discharge items on groundwater regime are quantified. Then, a correlation analysis and logical judgment are carried out based on the dynamic data. The recharge and discharge items that contribute the most to the groundwater regime are identified, and the genetic types of the groundwater regime are named and determined accordingly. Based on this method, this study uses the same 57 groundwater observation points in the Songhua River basin of the Sanjiang Plain as in 2011 and 2015. These data were combined with the groundwater level depth, groundwater level, river water level, precipitation, distance from the observation well to the river, and lithology of the unsaturated zone. The genetic types of the groundwater regime in the study area were divided into six types: hydrological type, precipitation infiltration type, precipitation infiltration-evaporation type, precipitation infiltration-runoff type, precipitation infiltration-exploitation type and exploitation type. Comparing the changes in the genetic type of groundwater regime of the study area from 2011 to 2015 revealed thatthe proportion of natural genetic types, such as precipitation infiltration type and precipitation infiltration-runoff type, decreased, whereas the proportion of the genetic type affected by human activities increased. This indicated that the impact of human exploitation activities on the groundwater regime is increasing. Compared with the traditional qualitative analysis method based on the characteristics of the groundwater regime curve, the comprehensive discrimination method proposed in this study can identify the driving factors of groundwater regime, while fully considering the hydrogeological conditions and combined with quantitative methods, which improves the scientific reliability and work efficiency of the discrimination of the genetic type.
Abstract: The traditional water system connectivity scheme is mainly dominated by the water system structure, ignoring the influence of connectivity on water system function. In the layout of the urban water system, the connecting structure and function of the water system are integrated to formulate the connecting scheme of the water system. Based on the centrality evaluation method, a method is proposed to quantitatively portray the importance of river segments. The functional relationship between the connectivity function and its characterization parameters is established with the help of the affiliation function, and the effect of the water system connectivity function is quantified accordingly. The two-stage optimization model of the water system connectivity scheme with structure-function coupling is established with the river network connectivity function achievement rate. The objective is to determine the effectiveness of the water system connectivity, and based on this, the optimal joint gate scheduling scheme is selected. Taking the Qingyi River as an example, the study shows that the overall connectivity of the water system of Qingyi River in the original river network is good, but the connectivity near the Hucheng River and Yinma River is poor. Improving the Hucheng River-Qingni River and the Yinma River-Xiaohong River connections could greatly enhance the connectivity of the water system near the Hucheng River and Yinma River.
Abstract: The hydrodynamic of river channels in plain cities are generally of poor conditions. Accelerating water renewal and strengthening orderly flow is one of the most important ways to improve the water environment quality of river network. In this paper, a typical plain city in the lower reaches of the Yangtze River, Qidong, was studied as an example. Making full use of the natural tidal dynamic conditions, a hydrodynamic model of the river network was established. Numerical simulation experiment was carried out to quantify the improvement of hydrodynamic characteristics of the river network through water system pattern and overall arrangement optimization and water conservancy project operation. The results show that, the optimized scheme increases the ecological velocity compliance rate of the river network from 52.3% to 94.2% for the whole period under the worst tidal condition; the maximum instantaneous ecological velocity compliance rate is raised from 42.6% to 85.0%;the regional ecological velocity continuity increases to 91.8%. Meanwhile, the urban ecological water consumption reduces 67.5%. Under the optimized scheme, the diversion water volume from external regions can be reduced, and the renewal rate of urban river network significantly increases which shrinks the accumulation time of pollutants. This study provides a novel perspective and method for improving the water environment of tidal river channels in plain cities.
Abstract: Low-slope bedrock meandering rivers widely exist in areas controlled by geological structures (the bed slope is less than 5‰). Flood has a great influence on riverbed sedimentation and erosion in bedrock meandering rivers, however, we have a poor understanding of dynamic structure of flood for bedrock meandering rivers. By geometrically generalizing bedrock meandering reaches, considering the Froude similarity and side wall roughness, a generalized model of a bedrock meandering channel is established in order to analyze the water surface profile distribution, time average flow fields and turbulence structure characteristics for bends under flood. The results show that under the flood flow, the minimum water surface transverse gradient, separation of flow on the convex bank, development of double circulation on the concave bank, velocity-dip-phenomenon and maximum secondary circulation intensity are caused at the upstream of the bend apex, and the maximum water surface transverse gradient and circulation splitting are caused at the downstream of the bend apex; the bed shear stress is distributed in the flow separation area at the convex bank as well as the center of the upstream of the bend apex, and the transverse momentum transport is concentrated at the upstream of the bend apex. The experiment results show an explanation in view of hydrodynamics in respect to bed erosion and sedimentation in bedrock meandering rivers.
Abstract: The East River is one of the confluence rivers in the Pearl River Delta, which belongs to less sediment river. Pool-riffle sequences are densely distributed in the middle and upper reaches of the channel, but the evolution characteristics of that under the condition of no sediment supply are not clear. Therefore, it is of certain scientific significance to improve the understanding of evolution rule. This study selected the sequences of the East River to measure the bed surface and bedload, and obtained the feedback between flow and bed morphology without sediment supply under different flow and slope in the laboratory. Experimental results show that the alternation of pools and riffles results in the discontinuous variation of average-velocity longitudinally, and the peak value of turbulent kinetic energy is easy to occur in the middle riffle under the shape of flood discharge. In the absence of sediment supply, the formation-destruction processes of periodic armoring layer occur alternately, in which static sorting is the dominant factor and leads to discontinuous gradation longitudinally. The increase of flow discharge and slope effectively promoted the sequence development, but at the same time shortened the length of pools and riffles. With or without upstream sediment supply, the variation of bedload transport in the sequence is enlarged, i.e., the bedload transport rate in the sequence increases with sediment supply; on the contrary, the bedload transport rate decreases without sediment supply.
Abstract: To improve the current simulations of hydrothermal exchange in riparian zones that do not take into account heterogeneous heat transfer of soil, this paper constructs a hydrothermal coupling model of riparian zones that includes heterogeneous heat transfer of soil by introducing the soil effective thermal conductivity model on the basis of saturated-unsaturated seepage and porous medium heat transfer theories. This study uses COMSOL software to implement the new method and simulate the hydrothermal coupling model of a riparian zone for different effective thermal conductivity models; the new model is verified by comparing and analyzing prototype observation data of temperature and water level in a riparian zone. The results show that the new model can better reflect the hydrothermal exchange process in a riparian zone than the traditional method that does not consider the heterogeneous heat transfer of the soil. In addition, the simulation of the hydrothermal coupling model based on Johansen's effective thermal conductivity model performs the best, and the simulation results are consistent with previous experimental results. This study can provide technical support for in-depth study of hydrothermal exchange and pollutant migration processes in riparian zones.
Abstract: To reveal the evolutional characteristics of a stream-groundwater system, this paper takes the seasonal losing stream-groundwater system with stratified sediments as the research object. The criterion for the hydraulic connected states and the mechanism of the seepage in the system are analyzed. The variation types and mechanism of hydraulic connected states are clarified. A theoretical framework of stream-groundwater system hydraulic connected states is developed, and six stream-groundwater system models with different stratified sediment types are constructed. The evolution and variation of hydraulic connected states of the six stream-groundwater models are simulated by numerical simulations, and the results show that the wetting front curve and saturation front curve are of great importance to describe the evolutional process of the hydraulic connected states and can be regarded as characteristic curves for analysis. The difference of permeability and capillary force caused by different types of sediments is the fundamental driving force for the variation of the evolutional process. The variation types of wetting front curve include steep, gentle slope or alternate between steep and gentle slopes, and the variation types of saturation front curve include backward, forward, or alternate between backward and forward. The results of this study provide theoretical support for further research on water quantity assessment, mass transport and energy transfer in stream-groundwater systems.
Abstract: In karst groundwater, a common phenomenon in the solute transport process is solute transient storage. The mechanism of transient solute storage between karst conduit and fissures are revealed using a constructed physical model of conduit-fissures. Quantitative tracer tests were conducted under concentrated recharge conditions, and the solute transport processes were simulated using the Dual Region Advection Dispersion model. With enhanced concentrated recharge hydrodynamic conditions, the quantity of transient water storage in fissures tends to increase linearly; and the breakthrough curves transform from single peak to double-peaks. A negative correlation between the average flow velocity in conduit and fissures was confirmed. Breakthrough curve shapes are defined by the difference between solute retention times in conduit and fissures. The overall findings are that solute transient storage induces a tailing effect and the phenomenon of double-peaks in breakthrough curves, significantly controlling solute transport processes in karst groundwater.
Abstract: Because of global climate change and human activities, the occurrence frequency of extreme precipitation events is increasing steadily, which causes devastating flood disasters. The stability of a human body can be compromised when exposed to floodwaters, which definitely threatens the life of a human being. Therefore, the investigation into mechanisms and criteria of human instability in floodwaters can provide scientific bases for urban flood risk assessments and hazard reduction measures, and can also benefit the ability of emergency response. This paper summarizes existing studies on instability mechanisms and criteria for people in floodwaters. Based on the summarization, the application scope, advantages and disadvantages of these existing criteria for human instability are evaluated in detail. Even though the calibrated formulas based on mechanical analysis and measured data involve some simplifications, they can reflect the critical mechanical condition of human instability in floodwaters, and therefore show better universality in practice. Given the limitations and shortcomings of the existing studies as well as major threats in current urban flood disasters, this study also presents key future research directions, including the instability mechanisms and criteria of pedestrian evacuation in actual flood conditions and from flooded stairs; using big data technologies to obtain and analyze online resources of flood instability videos and images, and reasonably estimate the critical instability state to construct a dataset of body instability in floodwaters; conduct in-depth studies on the influence of body posture adjustment on flood hazard degree. Besides, it is necessary to promote the application of human instability criteria in the field of urban flooding disaster prevention and reduction, in order to minimize the disadvantageous impacts of urban flood disasters.