Abstract: The changes in runoff and sediment in the Yellow River is a hot issue in the field of hydrology and river management. The Kuye River located in the Loess Plateau was selected as a typical basin. Based on the observed hydro-meteorological data during 1960-2014, there was a statistically significant increasing trend in annual mean temperature, an insignificant decreasing trend in annual precipitation, and significant decreasing trends in annual runoff and sediment. 1980 and 1999 were two breakpoints of the relationships among runoff, sediment and climatic factors. The ensemble modelling framework of runoff and sediment was constructed by the VIC model and eight sediment models based on machine learning algorithm. The Nash-Sutcliffe efficiency coefficient and correlation coefficient for monthly runoff and sediment simulation were higher than 0.6 and 0.7, respectively, that denoted acceptable performance. Based on the comparison between the reconstructed natural runoff and sediment versus observed values, the attribution of runoff and sediment changes were quantitatively analyzed. The results indicated that the contributions of climate change and human activities to runoff reduction in 1980-2014 were 24%-39% and 61%-76% respectively, and the contributions to sediment reduction were 15%-36% and 64-85%, respectively, compared with that in the natural period from 1960 to 1979. Human activities are the main reason for the decreases in runoff and sediment in the Kuye River basin. The related results could not only improve an innovative methodology to understand the attribution of runoff and sediment changes, but also play an important supporting for the utilization of water resources and conservation of runoff and sediment in the Yellow River basin.
Abstract: Flash droughts are extreme events characterized by a short duration, strong intensity, and rapid onset. Owing to their unusually rapid occurrence, the use of current drought monitoring tools to capture them is still challenging. In this study, using ERA-Interim soil moisture reanalysis products from the European Center for Medium-Range Weather Forecast, an approach that considers the rate of decrease in soil moisture content during the onset stage of flash droughts was proposed for the identification of the flash droughts that occurred during the 1979-2018 period. Further, the anomalies associated with meteorological variables during the drought onset stage of these events were analyzed, and the feasibility of using precedent meteorological driving forces to realize flash drought simulation based on a multivariate linear regression model was investigated. The results obtained showed that the rate of soil moisture intensification during the onset state of flash droughts showed significant spatial discrepancy. Specifically, during this stage, soil moisture content declined much faster in the south of the Yangtze River than in Northwestern China. Additionally, the mean, peak, and variation ranges of the anomalies of the different meteorological variables exhibited more significant spatial discrepancies during this stage. Particularly, the peak values showed the most significant variation, and were half the standard deviation higher than the slowly-evolving drought. Furthermore, by integrating the anomalies associated with multiple meteorological variables during the early and onset stages of the flash drought events, the regression model provided satisfying simulations of the soil moisture intensification rate, suggesting that it is possible to simulate flash droughts from a meteorological perspective.
Abstract: Affected by global climate change, the status of meteorological and hydrological drought in the Lancang-Mekong River basin (LMRB) has witnessed great changes. Therefore, predicting the temporal and spatial variations of future drought as well as its propagation characteristics has become the basis for tackling climate changes and conducting cooperation on the water resources of the Mekong River. By applying the SWAT model and the method of atmosphere-land coupling, both the history (1960-2005) and the future (2022-2050 and 2051-2080) hydrological processes of the LMRB were simulated. Moreover, the future temporal and spatial variation trends of meteorological and hydrological drought were also predicted and analyzed by using the Standardized Precipitation Index and Standardized Runoff Index. The research results indicated that: ① In the future, the amount of precipitation will be increased. Thus, the meteorological drought will be alleviated to some extent, but at the same time, the inhomogeneous distribution of intra-annual precipitation and increased evaporation from the river basin together will cause a severer hydrological drought and an intense propagation process. ② Hydrological drought will have significant spatial heterogeneity. Specifically, the hydrological drought will be the most intense in Jinghong and Chiang Saen stations, followed by Luang Prabang, Mukdahan, and Pakse stations, and be the weakest in Vientiane station. ③ Over time, the frequency of hydrological drought in the river basin will be slightly decreased, but the proportion of heavy drought and severe drought will be increased, and the extreme drought will be enhanced both in amount and strength. Furthermore, the features of spatial variation will be more significant and noticeable.
Abstract: Drought levels are multi-level and transitional. A method quantitatively evaluating changes in drought levels and overall changes in drought are needed. By constructing a probability matrix of drought level change and introducing the multipliers of rate and degree of change, this study derived their formulas and developed a quantitative method for evaluation of drought level change. Standardized precipitation-evapotranspiration index and standardized streamflow index were employed to analyze drought levels in the Dongjiang River basin as an example. Changes in drought levels from meteorological drought to hydrological drought and those in hydrological drought levels under the influence of reservoirs regulation were evaluated at monthly, seasonal and annual scales. Results showed that: ① The degree of overall change in drought was small, but the change in drought level was distinct. The rates of overall change in drought levels at monthly, seasonal and annual scales were 45.9%, 46.8%, and 34.9%, respectively. ② The change in drought level decreased by approximately 1.4 level in October, but increased by 1.3 level in Spring. ③ The impact of reservoir regulation on the hydrological drought was significant at monthly and seasonal scales. The rate of overall change in drought level was 33% and the drought level decreased approximately by one level in spring. The proposed method can be applied not only for drought evaluation, but also for the assessment of other disasters.
Abstract: The 1987 Water Allocation Scheme has been in effect in the Yellow River basin for more than 30 years. This scheme needs to be adjusted dynamically to suit changing flow, sediment, and water use patterns. In order to allocate water resources scientifically in a changing environment, this paper develops a dynamic water allocation idea: "keeping the stock and allocating the increment", taking the Yellow River basin as the study area. This method involves first seeing to the water needs of the river habitat and the coastal waters. Then hyperconcentrated floods are created according to the dynamic sediment condition to cut down the amount of water needed to transport sediment. The water saved instream is regarded as the increment of off-stream water allocation. The increment is allocated to provinces along the Yellow River with consideration of both equilibrium and efficiency. Results show that the instream water allocation decreases 0.507-2.424 billion m3 though the highly efficient sediment transport method when the amount of sediment coming into the downstream area is 0.400-0.500 billion t. The increment of off-stream water allocation is mainly allocated to provinces in the upstream area. The dynamic water allocation method gives first priority to the environmant and natural habitats and ensures that the adjusted water allocation to each province is no less that the current amount. This paper provides a scientific method for sandy rivers to allocate water dynamically and provides a reference to the adjustment of the 1987 Water Allocation Scheme.
Abstract: The Lower Jinsha River is an important sediment-producing and sediment-transporting area in the Yangtze River basin. Since 2013, the third level Xiluodu Reservoir (XLDR) has directly intercepted the sediment of the Jinsha River, and its sediment discharge effect will affect its operation and the inflow of sediment into the Xiangjiaba Reservoir (XJBR) and the Three Gorges Reservoir (TGR). Based on the observation data of hydrologic sediment and river section, the sediment discharge effect of XLDR and its influencing factors were studied in this paper. The results showed that: ① From 2014 to 2019, the total sediment discharge of XLDR in was 14.9 million tons, and the sediment discharge ratio was only 3.1%, which was obviously smaller than the design value and the downstream XJBR and TGR. The sediment discharge of XLDR was mainly concentrated in the main flood season from July to September, accounting for about 71% of the total.② There were three main reasons for the small sand discharge ratio of XLDR. The first reason is the poor coordination between flood peak and sediment peak. The sediment peak often enters the reservoir area before the flood peak, which leads to greater probability of sediment deposition in the reservoir area. Secondly, the operation mode of XLDR determines that the reservoir area is in the state of large water depth and small velocity almost throughout the year, especially the long flood detention period, which weakens the dynamic condition of sediment transport. Third, in the initial state of the impoundment of XLDR, there are two natural hidden ridges in the reservoir area, which have obvious blocking effect on the sediment transport at the bottom of the reservoir area, and so that the density current sediment transport unable to reach the front of the dam.
Abstract: This paper presents a simulation method of the dust wash-off process using sediment dynamics theory to clarify the physical mechanism of the Sartor-Boyd exponential model for the dust wash-off process.Related theories including the non-equilibrium transport model for non-uniform sediment and the delayed response model for sediment transport rate during fluvial processes were summarized, and compared with the Sartor-Boyd exponential model. The comparison revealed that the Sartor-Boyd exponential model is formally equivalent to the delayed response model of sediment transport rate during the clear-water scouring process. Given the delayed effect of rainwater conflux, and the non-equilibrium wash-off characteristics of group-sized dust, a non-equilibrium wash-off model for non-uniform dust was proposed and applied to simulate the dust wash-off processes with different rainfall-runoffs in the Niushan industrial district of Dongguan City, Guangdong Province. Comparison of the application and effects indicated that the estimated dust concentration values using the proposed model were more consistent with the measured values than those calculated using the Sartor-Boyd exponential model; furthermore, the determination coefficient and Nash-Sutcliffe coefficient of efficiency increased from 0.15 to 0.73 and from 0.59 to 0.70, respectively. The results of this research provide a physical basis for the simulation of the dust wash-off process.
Abstract: In order to simulate the rainfall-runoff process of the basin with high efficiency and high accuracy, the dynamic wave method is applied to solve the two-dimensional Saint-Venant equation, with the hydrological process coupled. A high-performance watershed rainfall-runoff process numerical model including processes such as rainfall runoff, confluence, infiltration, and flood evolution has been established. The advantage of this model is the use of unstructured grids, which can better deal with irregular boundaries and accurately fit the surface of complex terrain, so that the model can accurately calculate and simulate the rainfall-runoff process of the basin; besides, the GPU technology is introduced to accelerate calculations in this model, making it possible to calculate storm floods in large-scale watersheds. Finally, the model was applied to the V-type classic case and two actual watershed flood cases; the obtained results are in good agreement with the actual measurement, and the calculation time is shorter, showing that the model can quickly and accurately simulate the river basin rainfall-runoff process. The research results are helpful for realizing reasonable and efficient prediction of flood disasters in actual river basins, so as to provide strong support for emergency rescue work.
Abstract: The mechanism of water quantity increase entering the Taihu Lake from the Huxi sub-basin was explored. For the purpose, the coupled hydrology and hydrodynamic models of the Taihu Lake basin were established to simulate water stage variation of representative stations, as well as water quantity of important control lines. A knowledge graph was applied to study the flood drainage path. The simulation time series were divided into two periods; (i) 1990-2000:before integrated water resources management projects were in operation, and (ii) 2013-2018:after the Jiangnan canal upgrading, Meilianghu pump station, and city flood control projects were in operation. Three factors were identified to be responsible for the water quantity increase in the Taihu Lake from the Huxi sub-basin: increased precipitation, increase in the amount of water diverted from the Huxi and Wuchengxiyu sub-basins along the Yangtze River, and the Meiliang pump station and the Wuxi City flood control projects in operation. The results demonstrated that the annual average Taihu water stage increased by 0.05 m, and representative water stage of the river network increased by 0.02-0.16 m. The Wuxi City water stage increased significantly after the Meiliang pump station and Wuxi City flood control projects came into operation. Individually, the contribution of the three factors was almost equal, while their interactive effects were both synergistic and inhibitory, resulting in more intense flood contradiction for the Taihu Lake basin and its sub-basins.
Abstract: The changing environment around the Aral Sea has a great impact on the ecological stability of the arid area in Central Asia, to this end, the study on the wetland evolution of the Syr Darya-Aral Sea delta is of great significance to gain further insight in the changing valley and find out the solution for ecological recovery. Based on the multi-source remote sensing data, this paper extracted the wetland information from images by an object-oriented hierarchical extraction method, and analyzed temporal and spatial variations in the delta wetland of the lower of the Syr Darya River from1962 to 2019. Also, this paper took such elements as the changes of the Aral Sea and the farmlands, river discharges, the water conservancy projects and others into consideration and discussed the drivers of the near 60 a wetland changes. This study shows that as the constant shrink of the Aral Sea, the landmass of delta wetland of the lower of the Syr Darya River decreased first, then it grew bigger. During this period, the rate of change presents-38.29 km2/a, 193.35 km2/a and 14.36 km2/a in 3 phases: 1962-1980, 1981-1991 and 1992-2019 respectively. The wetland change has a distinctive spatial variation. Since 1980, two wetlands laying near the northern and the south estuaries of the Syr Darya River have largely expanded towards the Aral Sea. The area between the wetlands and the farmlands has extended to two sides of the river since 1991.Given the fact of the reduced runoff entering the delta after 1962, the irrigation network project and the ecological water allocation play the main role in growing the delta wetland. On the other hand, these projects indirectly cut down the flow entering the Aral Sea, which leads to the rapid shrink of the Aral Sea. The delta wetland recovery poses a striking contrast with the degradation of the Aral Sea. In this case, the ecological environment around the Aral Sea will undergo an inferior recovery.
Abstract: Human activities have dramatically changed the subsurface flow and caused problems such as salinization and desertification that degrade groundwater-dependent the ecosystem. Taking the shallow groundwater layer as the link, a conceptual model of phreatic water evaporation in arid plains is established. The model analyses the groundwater depth conditions of desertification and salinization in arid areas, and calculates threshold water table required for preventing these problems. The Luocheng Irrigation District at the junction of the middle and lower reaches of Heihe River is taken as an example to study the relation between groundwater depths and ecosystems patterns. The contributions of this study include: ① Describing the soil water movement principle caused by desertification outside the salinized oasis in the arid area, based on which, the method for calculating the threshold groundwater depth for preventing salinization is provided. By calculation, the critical groundwater depth leading to soil salinization is 1.3~1.5 m and the calculated critical groundwater level is 8~13 m; ② Discussing the appropriate values of soil equivalent pore diameter and liquid surface tension used for estimating the maximum rising height of capillary water; ③ Proposing a scheme to regulate the groundwater level regulation via controlling irrigation in the area to protect groundwater-dependent ecosystems while satisfying water demand for irrigation and other purposes.
Abstract: In order to explore the effect of gravel groups on the naturalization of the biological habitats in river, the Jinshaxi River in Jiangxi Province was used as an example to investigate the effect of gravel groups on the naturalization of biological habitats. The silver carp was selected as the target species. The effect of the gravel group layout and naturalized transformation of the curving river channel were determined through literature review, numerical simulation, and two-dimensional hydrodynamic and habitat simulation of the river reach. The results showed that the weighted usable area of the silver carp in the curving river channel decreased significantly. The Froude number of the flow increased near the main stream line of the original river, and the shear velocity value changed from the transverse gradient distribution of the original channel to a uniform distribution. The arrangement of gravels in the curving river channel significantly influenced the biological habitat. The high-density arrangement at the concave bank of the river channel increased the percent usable area of the habitat from 9.20% to 15.06%, and the Froude number of the water flow near the lower reaches of the group suddenly decreased from 0.13-0.68 to 0-0.15. Furthermore, the area downstream of the gravel group formed a low-speed flow area, which provided a good shelter for the silver carp in the channel with a large flow.
Abstract: A series of benchmarks were setup on the south Chuandong tidal flat (Yancheng, Jiangsu). Monthly observation of the surface elevation has been performed for 7 years (October 2012 to November 2018) using the Rod Surface Elevation Table(RSET). The edge of the Spartina alterniflora was obtained from the remote sensing images. The long-term evolution of the mudflat and the influence of growth of Spartina alterniflora were evaluated. Results show that: the subsurface processes (e.g., soil expansion and compression) is the key factor affecting the evolution of mudflat at the highest position of the tidal flat. The growth of salt marsh increases the amount of sediment deposition. With the propagation of the edge of salt marsh, the annual sediment deposition at the central salt marsh decreases gradually. In the vicinity of the frontier of the salt marsh, the annual siltation volume increases. Even though, the bare flat evolution trend is hardly affected by the advancing of salt marsh. From the landside to seaside, the cross-shore profile can be divided into 4 zones, which are erosion-deposition balance, rapid deposition, erosion-deposition balance and rapid erosion zones, respectively. The slope of the intertidal flat increases evidently. The tidal flat resources decrease constantly in the past 7 years.
Abstract: Obtaining high-precision, high-resolution precipitation data is of great significance for hydrological analysis, water resources management, and flood and drought monitoring. Although remote-sensing precipitation products can effectively reproduce the spatial and temporal distribution of precipitation, few original remote-sensing precipitation products can meet the requirements in either precision or resolution in the hydrological field. It is therefore necessary to carry out post-processing research on existing remote-sensing precipitation products. The main methods used to obtain precipitation data are introduced, including rain-gauge observations, weather radar estimates, and satellite products. The advantages of each method and their problems are discussed. Next, the research advances made in post-processing methods of remote-sensing precipitation products are summarized, including spatial downscaling, bias correction, and multi-product fusion. Then, the indices used for evaluation of post-processing precipitation products are reviewed. Finally, the following research directions that must be pursued more avidly in the future are discussed: development and improvement of precipitation-estimation techniques, construction of a more reasonable framework for multi-source precipitation data fusion, strengthening of the comparative study of downscaling methods and ideas for their improvement, and uncertainty analysis.
Abstract: The hyporheic zone is the key transition region between rivers and aquifers, which is highly dynamic and sensitive. Frequent water exchange can trigger the bio-geochemical reactions within the hyporheic zone easily, and consequently make a difference to river ecological security. On the purpose of improving the authenticity and validity of the hyporheic flow hydrodynamic monitoring, the present study introduces various monitoring methods including seepage, water pressure, solute & temperature tracing and their applicability, and summarizes the monitoring points arrangement in the riverbed and adjacent riparian zone and the data acquisition frequency of different monitoring methods. The hyporheic exchange theory and calculation methods for reach, riverbed and riparian zone are summarized systematically, and the factors causing measurement uncertainties of the hyporheic exchange are discussed in the paper. In view of the deficiencies of the existing monitoring methods and the development direction of future research, it is proposed to strengthen the application of multi-method joint and multi-technology integration in future and carry out the lateral hyporheic flow monitoring and uncertainty analysis. By this means, the reliable data for studies on the coupling mechanism between hydrological and bio-geochemical processes in the hyporheic zone might be provided.