Abstract: Achieving a healthy regional water balance is an important content and challenge of China′s water conservancy work, which is of great significance to ensure national water security and the sustainable and stable development of the national economy and society. This study defined the concept of a regional water balance based on a summarization of the fundamental understanding of a water balance, then its connotation was interpreted from four aspects: ① water budget balance; ② water supply-demand balance of economic society; ③ water use balance between the economy and ecology; ④ harmonious balance of human-water relationship, and three dimensions of time, space, system. This study proposed the basic principles of a regional water balance, including water balance principle, supply and demand balance principle, synergistic balance principle and harmonious balance principle. The important scientific challenges for realizing a regional water balance supported by basic principles were analyzed. The theoretical system of regional water balance was discussed in terms of two aspects of theoretical basis and basic theory. The theoretical basis includes hydrology, science of water resources, ecology, economics, sociology and other theories. The basic theory includes the spatiotemporal equilibrium theory of water resources budget, the optimal allocation theory of water resources supply and demand, the synergy theory of water use between the economy and ecology, and the human-water harmony theory. This study also discussed the prospect of applying the basic theory. The results of this study can act as a reference for future studies of the regional water balance and can provide a research framework for management of the regional water balance in the future.
Abstract: A comprehensive understanding of the hydrological processes of thermokarst lakes on a seasonal scale is key to evaluating their environmental effects. In this study, a typical thermokarst lake was selected on the Qinghai-Tibet Plateau (QTP) and the meteorological and hydrological parameters were measured and calculated (2018—2020 to analyze the hydrological characteristics and environmental effects. The following results were obtained : ① Rainfall in spring recharges thermokarst lake water quickly and elevates lake water level rapidly; a power function was exhibited between lake water storage and water levels. ② The average annual lake evaporation and lake ice sublimation were 738 mm and 198 mm, respectively. Under the influence of rising air temperature, lake evaporation and ice sublimation may be increased in the future.③ The ionic concentrations in lake water exhibited a high value in the initial and later periods of the warm season, and the self-purification of lake ice and geochemical processes resulted in different ion migration mechanisms. The hydrological elements exhibited obvious seasonal variation characteristics owing to the effect of local factors. Moreover, the hydrological processes involving thermokarst lake evolution can cause permafrost degradation, lakeshore collapse and receding, water environment deterioration, greenhouse gases release, soil salinization, and vegetation degradation. Thus, the environmental effects of thermokarst lakes should be evaluated on the entire QTP in future studies.
Abstract: Recently, the Poyang Lake basin, one of the important commodity grain bases in China, is experiencing severe low-water conditions, frequent droughts, which reduced grain production and caused serious agricultural losses. Since the Poyang Lake basin is, decreasing the grain loss caused by drought in the region Research on the systematic agricultural drought to reduce the food loss in the Poyang Lake basin plays a critical role in guaranteeing the food security of China. Based on the daily ground soil moisture monitoring data of 22 moisture stations from 2011 to 2020, the daily rainfall data of 49 rainfall gauge stations from 1956 to 2020, and the meteorological data from 2016 to 2019 in the irrigation area where the moisture station located, we used the Soil Water Deficit Index (SWDI), which considers the physiological state of vegetation, to characterize agricultural drought. The temporal and spatial distribution of moisture content, water shortage in the aeration zone and precipitation at different time scales was analyzed, and the reliability of SWDI was evaluated in agricultural drought monitoring in the Poyang Lake basin. In addition, we revealed the temporal and spatial evolution of agricultural drought in the watershed and its response to meteorological drought, and preliminarily explored the correlation between soil texture and agricultural drought intensity. The results showed that the soil water deficit index applied well in evaluating the agricultural drought in the Poyang Lake basin. The agricultural drought in the basin has aggravated in the past ten years, and the most severe disasters occurred in 2019—2020. The summer-autumn-winter drought is the dominated seasonal agricultural drought, whi ch has a significant impact on the yield of rice, rapeseed and other grains. Compared with meteorological drought, the occurrence and ending time of agricultural drought are about 2.5 weeks and 3 weeks later, lasting 10.1 weeks longer, while the frequency is lower, and the drought intensity is smaller. Sandy soil has the worst water holding capacity and is prone to severe agricultural drought. Clay and clay loam soils have the best water holding capacity, with a higher probability of light drought and moderate drought; loam, sandy loam and loamy sandy soil are between the two of them.
Abstract: In order to effectively improve the accuracy of urban rainstorm flood simulations, the catchment hierarchical partition based on the spatial information method and the determination method of stormwater inlets were proposed in this study, owing to the complicated urban underlying surfaces and the absence of stormwater inlets. Taking the Qingshan district of Wuhan City as a study area, the SWMM model was calibrated and verified for the two typical rainfall hydrographs. Different simulated results were obtained by the validated SWMM model, with the results using different sub-catchment division methods being compared with the actual waterlogging data. The results indicate that: ① The determination method of stormwater inlets had a certain reliability and applicability in urban flood modelling if the measured data of inlet nodes were unavailable. ② The ponding depths and waterlogging nodes were simulated by the SWMM model using the catchment hierarchical partition method, the Thiessen polygon combined with the hydrology method, and the Thiessen polygon method. The simulated maximum ponding depths obtained from these three catchment partition methods accounted for 100%, 63% and 75% of the actual inundation degrees. About 80.0%, 76.4% and 77.4% of the simulated waterlogging nodes were consistent with the waterlogging risk map for a 5-year rainfall return period. The simulated results obtained from the catchment hierarchical partition method were more accurate than the results from other two partition methods. It is found that the simulated results from the catchment hierarchical partition method was relatively consistent with the actual real waterlogging situation. ③ In terms of the 5-year rainfall return period, the simulated flooding obtained from these three methods had a small influence on the urban management. However, different degrees of waterlogging would occur for the rainfall return period greater than 5 years. This study can provide a new method for the discretization of ground surfaces and the determination of inlet nodes in simulating urban rainstorm floods, and provide the reference for urban flood control and disaster reduction.
Abstract: The calibration processes of current distributed hydrological model has been a critical issue in data-scared or ungauged regions. In this study, we setup a hydrological model, in which, the variable saturated zone concept originated from the real-time interactive basin simulator is applied for runoff generation and the grid water droplet method for flow concentration. We also proposed a method for parameter estimation based on the characteristics of underlying surface. Based on field infiltration experiments and parameter sensitivity analysis, the quantitative statistical relationships were built between two sensitive parameters (surface saturated hydraulic conductivity K0z, coefficient of attenuation of saturation hydraulic conductivity with depth f) and the topographic parameters and soil types. The overland confluence parameters were determined by field overland flow observation experiments. The proposed parameter estimation method was verified in selected basins. Our results showed that: ① The proposed method for K0z estimation contributes to a better modeling performance for flood simulation in Jiangwan experimental watershed, the average Nash-Sutcliffe efficiency coefficient increased from 0.82 to 0.86, and the average absolute values of peak and flood volume errors decreased by 2.2% and 0.95%, respectively, but the average absolute value of the peak present time error increased by 4% (still controlled within 2 h). ② Using the measured flood data of 14 basins such as Jiangwan to calibrate the parameter f, we established the quantitative relationship between the calibrated parameter f and the soil type data of different depths was built in 14 basins including Jiangwan, and further tested in other six basins. The parameter f estimated by the soil type data of different depths was very close to that determined by traditional model calibration processes, the average absolute relative error is 2.8%, the average Nash-Sutcliffe efficiency coefficient of the flood simulation is 0.83, and the average absolute values of flood peak error and flood volume error were 10.07% and 6.86%, respectively, and the average absolute peak present time error was 2.61 h. Our results indicated that the proposed methods for determining sensitive runoff generation parameters are applicable in data-sparse areas and could provide a better or comparable parameter estimation and flood simulation than that determine by field measurements, model calibration, remote sensing data estimation, and other methods.
Abstract: To finely simulate the rainfall runoff process in small and medium-sized watersheds, rational estimation of the spatial parameters of the distributed hydrological model is of great importance. Utilizing the new version of the Global Digital Soil Mapping System (SoilGrids), this study developed a scheme to obtain spatial parameters for the Grid-Xin′anjiang Model (GXM). Sixteen selected flood events in the Chenhe watershed of Shaanxi Province from 2003 to 2012 were simulated using the GXM model, which were compared with those of the Xin′anjiang model (XAJ). The sensitivity of free water storage capacity and its spatial distribution characteristics were quantitatively analyzed in various flood phases. The results indicate that the GXM simulation reduced peak time error by approximately 0.31 h, with high accuracy of peak discharge and runoff depth, and that dynamic spatial distribution of hydrological elements, such as soil moisture, can be reasonably simulated. Free water storage capacity significantly impacted the deterministic coefficients of flood rise, peak phases, and relative runoff volume error during flood rise, but insignificant influence on flood recession. Free water storage capacity in valley and ridge areas is greater than that in middle segments of slopes.
Abstract: Due to the phase transition between ice and water, soil water and heat transport show complexity during freeze-thaw periods. To investigate the distribution and transfer characteristics of water vapor can provide key information for better understanding the mechanisms of the hydrological cycle in the vadose zone of the sandy land. By establishing the in-situ observation site in the Mu Us Sandy Land, as well as building a coupled water, vapor, ice, and heat model through the modified software program, an analysis was conducted for the vapor transfer process during the freeze-thaw cycles. Results showed that the simulated water content and temperature are highly consistent with the measured data, suggesting a good accuracy and applicability of the proposed model. By comparing the simulation results of the selected non-freezing, initial freezing, downward freezing, and thawing periods, it can be found out that the distribution of soil water content, ice content, and vapor density in the profile changed, and the variation in vapor density was closely related to soil temperature. When soil was frozen, the thermal vapor flux driven by temperature gradient accounted for more than 90% of the total soil water fluxes, and vapor transfer played a critical role in affecting the soil moisture distribution of the profile.
Abstract: The Yangtze River is China′s "golden waterway". Through systematic waterway improvement projects and dredging maintenance, the channel depth of the Jingjiang reach downstream of the Three Gorges Dam has increased from 2.9 m in 2002 to 3.5—3.8 m in 2020. However, the water depth is still lower than that of the upstream Three Gorges Dam reservoir area (4.5 m) and the downstream reach (4.5—6.0 m). The water depth of the Jingjiang reach does not connect with the upstream and the downstream, which restricts the comprehensive benefit of the channel of the Yangtze River. In order to adapt to the water depth of upstream and downstream, it is necessary to improve the channel scale of the Jingjiang reach, and it is urgent to clarify the relationship between channel water depth resources, navigation obstruction characteristics and river evolution. Based on the data of discharge, sediment transport and topography during the period of 1960—2020, this study analyzed the relationship between the beach trough evolution and the elevation of channel water depth resources in the Jingjiang reach of the Middle reaches of the Yangtze River. The results indicate that after the operation of the Three Gorges Project, the scouring of the Jingjiang reach mainly occurred in the dry river channel, accounting for 90.97% of the total scour. The central bar area and point bar area was reduced by 18.3%, of which the area of the the central bar and point bars ware reduced by 9.4% and 24.9% respectively. Under the combined action of riverbed scouring and channel improvement projects, channel scale verification was carried out with 4.5 m×200 m (water depth ×width). The total length of navigation obstruction in Jingjiang reach accounted for 5.3% of the whole reach. The navigation obstruction characteristics of 4.5 m water depth were as follows: the decrease of dry water level in sandy pebble riverbed reach was higher than the undercut depth of river channel, resulting in insufficient water depth of channel. The scouring of convex bank and siltation of deep channel in concave bank, in sandy riverbed reach cause the instability of beach trough shape and channel boundary. Low flow route instability and insufficient water depth were caused by the shrinkage of the shoal and uneven scouring between the branching channels during the dry season.
Abstract: Bend flow structure plays an important role in the evolution of natural river morphology. In order to further deepen the understanding of circulation structures and turbulence characteristics in the bends. Taking the continuous bend in Huanghuacheng reach of the Three Gorges Reservoir area as the research object, Acoustic Doppler Current Profiler(ADCP) was used to carry out prototype three-dimensional instantaneous velocity measurements. Scale distribution characteristics of bend circulation structures and turbulence structures were further analyzed. Results show the presence of macro-scale channel circulation structures in the Huanghuacheng reach. Due to the influence of local backwater and riverbed topography, measured vertical lines were characterized by full-depth circulation structure or double-circulation structure. Very large-scale turbulence structures still exist in the continuous curved rivers of the reservoir area, but their scale and distribution are greatly affected by the curved circulation structures.
Abstract: Understanding hydrodynamic process is key to study and improve the environment problems in reservoirs. Based on high-frequency field monitoring of flow and water level in the Xiangxi Bay (XXB) of the Three Gorges Reservoir(TGR) during release period (May 2019) and filling period (September 2019), the characteristics of flow oscillation in XXB are analyzed. The results are as follows. ① The barotropic flow oscillation with fundamental Helmholtz mode is a universal hydrodynamic process in the TGR. ② This oscillation has a period of nearly 2 h, causes strong periodic flows with an amplitude up to ±0.05—0.1 m/s and periodic water level fluctuation with an amplitude up to 0.2 m at the upstream of Xiangxi Bay. ③ The tributary oscillation is driven by the gravity wave at the mouth of tributary, which is induced by the diurnal discharge operation of TGR. This study further reveals the response mechanism of the hydrodynamics in a tributary bay to the discharge operation in TGR. It also provides a new perspective for the future basic and applied research in the reservoir operation.
Abstract: The macro-tidal estuary with a mountain stream is a typical estuary on China′s southeast coast, characterized by intense dynamic forces, active sediment movements, and a robust response to human activities. It demands further investigation of its evolution mechanism under both natural conditions and anthropogenic stresses. This paper takes the Jiangxinyu reach of the Oujiang River estuary as a case example to explore the river′s evolution under the influence of human activities, including effects of longitudinal dam construction, sand mining, and shoreline adjustment since the 1970s. Measured data analysis and a two-dimensional flow and sediment mathematical model were used. The results confirm that anthropogenic stresses have altered the natural periodic alternation of the river′s main and side branches. Moreover, the undercutting of the riverbed caused by sand mining from the beginning of this century has increased tidal range by 0.5 m and tidal prism by 47%, while weakening the discharge asymmetry and sediment transport asymmetry indices by 0.08—0.30 and 0.28—0.54, respectively. The undercutting of the riverbed has also weakened the ebb and flood asymmetry. These conditions have subsequently helped slow down the shrinkage of the side branch. The evolution mechanism is dominated by both natural and human activities. Overall, the research findings may be a reference for river evolution and protection under the influence of human activities.
Abstract: Neck cutoff as an extreme geomorphic process, occurred in flood season of July 2018 in the lower Black River in the Yellow River Source region. Neck cutoff strongly adjusting flow structure is of great importance to understand the hydrodynamic mechanism of meander cutoff. Acoustic Doppler Current Profiler (ADCP) was used to conduct field measurements on a total of 45 cross-sections in May 2019 and August 2020 in order to compare and analyze three-dimensional flow structure in different cross sections. Results demonstrated that due to the influence of channel shape, boundary conditions, hydrological conditions and other factors, the flow structure in the upstream straight reach is less affected by the neck cutoff, while the flow structure of the bend section is obviously adjusted after neck cutoff. The specific performance is that the neck cutoff has little effect on the spatial distribution of velocity and circulation intensity in the straight reach. It will change the transverse distribution of the maximum velocity at the top of the bend and the section distribution of circulation structure, and affect the scale and distribution position of the separation area. The results also deepen the understanding of the impact of flow structure adjustment after neck cutoff on deposition of oxbow lake, scouring and depositing of riverbed in separation area and scouring of new channel.
Abstract: The concept of conditional value-at-risk (CVaR) is introduced into the field of flood damage assessment, and a joint design method for multi-reservoir system flood storage based on CVaR is then proposed. The purpose of the method is to research on the mutual feedback mode for coordinated flood control among reservoirs by analyzing the degree of influence of each reservoir′s storage capacity on the flood control risk of the reservoir group system as a whole. The five-reservoir system of the Hanjiang River is selected as a case study, and the flood damage assessment index CVaR values corresponding to the present flood storage combination scheme (FSCS) are regarded as upper limit values. The results show that: ① The sensitivity rankings of the five reservoirs, in descending order, are: Sanliping, Yahekou, Danjiangkou, Ankang, and Pankou reservoirs. ② The flood damage assessment index CVaR values corresponding to different FSCSs may not be the same when the total flood storage of the multi-reservoir system is fixed as a constant, and there is a viable FSCS for the multi-reservoir system. ③ If the same CVaR constraint conditions are selected, the Danjiangkou Reservoir has greater flexibility to adjust its flood control capacity in the five-reservoir system than in the two-reservoir system, because the former considers the flood control coordination effect of more reservoirs (such as Pankou, Sanliping, Yahekou reservoirs).
Abstract: In the context of wave-current, the characteristics of vortex structure and wave propagation around the upstream inclined pile differ significantly than around the vertical pile, which affects the local scour process of the pile foundation. To better understand the characteristic of local scour around upstream inclined piles under the joint action of wave and current, local scour tests were carried out in the wave-current flume for the upstream inclined piles with different inclination angles. Besides, a preliminary investigation was conducted into the ephemeral characteristics of local scour depth, influencing factors, and the characteristics of bed morphology. As suggested by the research results, the local scour depth of upstream inclined piles is smaller compared to vertical piles under the context of clear water scour at all times. The equilibrium scour depth decreases progressively, and the relative time scale (T0*)shows an incremental increase when the pile inclination rises. Besides, the width of scour hole is smaller compared to vertical piles, showing a teardrop-like shape, while the dune after the tail exhibits a longer single-peaked band structure with a more significant irregularity. Despite a close correlation between the depth of local scour and Froude number, the wave deformation caused by upstream inclination makes it more difficult to determine the specific function of scour depth, radio of velocities and Keulegan-Carpenter number.
Abstract: Climate change potentially increases the severity, frequency, and duration of cyanobacterial blooms in freshwater lakes, thereby threatening the ecosystem service functions of these lakes and negatively impacting human health. This paper provides a comprehensive review of the external environmental factors and internal eco-physiological characteristics that may trigger these blooms. In particular, detailed discussions are provided to explain the processes leading to cyanobacterial blooms caused due to rising temperature, CO2 concentration, and extreme climate change. A preliminary research framework to determine the impact of climate change on the blooms, namely, the "mechanism analysis — model construction — simulated prediction — risk assessment" research framework, is established. Strategies for the control and prevention of bacterial blooms are developed. Several key future research themes, including the construction of big data monitoring platforms, mechanism models for development and risk prediction, and formulations of nutrient control standards and prevention policies, are proposed to tackle the current problem. This review provides scientific evidence for the prevention and control of cyanobacterial blooms in lakes.
Abstract: Hydrological models are integrated approximations of complex hydrological phenomena and processes in nature, and have been extensively applied for many practical purposes, such as flood and drought disaster prevention, water resources development utilization and management. In the current study, difficulties lying in the applications of large-scale hydrological models were discussed, research progresses on the uncertainty of model parameters were summarized, and a framework for parameter uncertainty analysis named, 'Sensitivity analysis—Optimization—Regionalization (SOR)' was introduced with special emphasis on its basic concepts, importance and applications. To improve the accuracy of large-scale hydrology simulation and prediction, a more comprehensive SOR was suggested for the application process of hydrological modelling, so were the developments of advanced distributed hydrological model and more accurate hydrometeorological observation systems to reduce the extra forcing-driven and model structure-driven uncertainty.