Abstract: Vegetation is one of the critical environmental factors driving the hydrologic cycle. The impact of vegetation change on runoff is a hot issue. As a pronounced greening region, the Huang-Huai-Hai River basin (HHHRB) was selected as the studying area. Based on long-term (1982-2016) hydro-meteorological and Normalized Difference Vegetation Index (NDVI) datasets, the spatial and temporal change in NDVI was detected using Mann-Kendall's test methodology. The relationship between NDVI and the parameter ω of Budyko-Fu's model was analyzed using an empirical formula and an elasticity method. Taking the parameter ω as a link, the impact of NDVI change on runoff in HHHRB was investigated with the chain rule for derivatives of complex functions. Several new findings were investigated: ① There was a statistically significant increasing trend in NDVI during the last 35 years over HHHRB. ② An increase in NDVI would increase the model parameter ω, thereby leading to a decrease in the runoff. ③ There might be an average reduction of 8.3% in runoff as a 10% increase in NDVI in the HHHRB. ④ The runoff was more sensitive to NDVI change under drier climate and sparser vegetation conditions. The results could improve the understanding of the mechanism of the water cycle in a changing environment and might provide scientific and technological support for water resources planning and vegetation management.
Abstract: Climate change and human activities has severely changed streamflow and eco-flow in the Yellow River basin. It is of great significance for the long-term management and planning of water resources to analyze whether future climate change will alleviate the impacts of human activities on the river. Here, daily runoff was simulated using hydrological model which was driven by the dataset of 13 climate models from Coupled Model Intercomparison Project Phase 6 (CMIP6) after correction. Then, both annual and seasonal variation of eco-flow at Huayuankou station from 2026 to 2100 were estimated based on the method of flow duration curve under scenarios SSP1-2.6, SSP2-4.5 and SSP5-8.5. The results suggest that bias was reduced obviously after correction, human activities severely affected eco-flow at the Huayuankou station during 1986-2010, the increasing trends of annual mean temperature and annual precipitation will be significant from 2026 to 2100 with the smallest rate under SSP1-2.6 but the largest rate under SSP5-8.5, and climate change will alleviate the negative impacts of human activities on eco-flow in the Yellow River Basin to some extent, with the most under scenario SSP5-8.5, and the most in winter but the least in summer and autumn.
Abstract: In order to objectively identify the regional rainstorm and flood processes and quantitatively assess the intensity of regional rainstorm processes, based on the intensity-duration curve for extreme rainfall, the maximum precipitation intensity within 5 days is regarded as an index to define a flood-waterlogging event and the intensity of regional flood-waterlogging processes. Using the daily precipitation dataset at 502 meteorological stations in the middle and lower reaches of the Yangtze River from 1961 to 2019, the frequency, intensity and variation trend of the regional flood-waterlogging processes in the middle and lower reaches of the Yangtze River are analyzed. The results show that the frequency of the regional waterlogging processes in this region exhibits an increasing trend in recent 60 years. It is obvious that the regional flood-waterlogging processes occur more frequently in the last 20 years, and the proportion of the regional flood-waterlogging processes lasting for 5-9 days to all the waterlogging processes is two-thirds or above. The regional flood-waterlogging days display the spatial distribution of "more in the southern area and less in the northern area" and "decreasing in the northwestern area and increasing in the southeastern area". Moreover, the spatio-temporal variations of the annual precipitation and flood-waterlogging lead to a stronger difference of drought and flood between different areas in this region. The eastern area with more precipitation gets wetter, but the northern and western area with less precipitation gets drier.
Abstract: As a primary tool in climate change research at the watershed scale, the role of horizontal resolution in regional climate simulations needs to be evaluated. This paper investigates the role over the Huang-Huai-Hai (HHH) River basin using a regional climate model, RegCM4, coupled with the National Center for Atmospheric Research (NCAR) Community Land Model, version 4.5 (CLM4.5) including the modules of CN and DV (RCM_CLM4.5). The model is driven by ERA-Interim reanalysis data, and the time period is 1990-2010. Model validation shows that RCM_CLM4.5 at 50 km and 25 km resolutions can reproduce the present climate well, including the spatial distribution of winter and summer mean temperature and precipitation, the annual cycles of temperature and precipitation, and the climate extremes. However, some biases can also be found. For example, compared with the observation, cold and warm biases are found in the winter and summer mean temperature, respectively; wet biases are found in the precipitation. Comparison between the two simulations, better performance of the magnitude and spatial distribution of mean temperature is found in the simulation at 25 km resolution; while for precipitation, better performance is observed in the one at 50 km resolution; little difference is found for the climate extremes. In general, results from this study indicate a very low sensitivity of present climate in this region to model resolution and can provide a reference for the application of this model in future climate change research over the HHH River basin.
Abstract: Water resources is one of the important factors impacting food production. It's a hot issue to investigate the response of grain yield to climate change driving water resources change. Taking winter wheat and summer maize in Fenhe River basin as the study object, six machine learning algorithms were used to build the yield prediction models, including Linear Regression, Back Propagation Neural Networks, Support Vector Machine Regression, Random Forest, Radial Basis Function, and Extreme Learning Machine. Based on the response of water resources to climate change by the climate-elasticity coefficient, the comprehensive response of grain yield was analyzed on a catchment scale, which was related to climate change driving water resources change. The results indicated that ① Machine learning algorithms performed well on the simulation yield of the winter wheat and summer maize in Fenhe River basin. ② There was a 19.4% increase in water resources as a 10% increase in precipitation, otherwise, a 1℃ increase in temperature might lead to a 4.3% decrease in water resources. ③ If precipitation decreased by 10%-30%, the yields of winter wheat and summer maize would decrease by 6.4%-19.3% and 4.0%-15.0%, respectively. ④ When the temperature increased by 0.5-3.0℃, there might be 1.8%-17.1% and 1.2%-7.9% increases in the yields of winter wheat and summer maize, respectively. ⑤ The yield of winter wheat was more sensitive to climate change than that of summer maize in the Fenhe River basin. The results were useful for future adaptive strategies of water resources management and agricultural production.
Abstract: This research aimed to determine the temporal and spatial variations of the hydrometeorological extremes, and their impacts on vegetation across the Yangtze River basin (YRB). First, a meteorological reanalysis dataset with high spatial and temporal resolution from 1982 to 2015 was used to identify and analyze the annual variation trends of the extreme drought (Dry-PRE or Dry-VPD) and extreme wetness (Wet-PRE or Wet-VPD), based on precipitation and vapor pressure deficit (VPD), respectively. Subsequently, the responses and sensitivities of vegetation growth (indicated by normalized difference vegetation index, NDVI) to the hydrometeorological extremes were explored. The results showed that the frequencies of Dry-VPD, Wet-PRE and Dry-PRE generally increased while that of Wet-VPD decreased during the study period. Particularly, Dry-VPD increased significantly in the upstream and downstream of the YRB after 1998. The vegetation across the YRB was more sensitive to extreme drought (mainly with a negative correlation) than extreme wetness. Moreover, the vegetation growth in the upstream was sensitive to Dry-VPD, while that in the midstream and downstream was sensitive to Dry-PRE. To summarize, this study found that extreme drought was on the rise across YRB. Different from the general notion that precipitation limits vegetation growth, our study indicates that vegetation in the upper reach of the YRB is more negatively impacted by atmospheric drought.
Abstract: Accurate river runoff simulation is of great importance for basin hydrological simulation, water resources plan and sustainable management, hydrological disaster prevention and control. With the rapid development of the earth observation satellite remote sensing techniques, microwave remote sensing monitoring can provide a new method for river runoff simulation. To date, for river runoff simulation, further exploration is needed based on passive microwave remote sensing brightness temperature observations. This paper applied the M/C signal method to river runoff simulation in typical river basins in China based on a high-resolution passive microwave remote sensing brightness temperature dataset. The usability of this method was discussed. To analyze the influence factors of the simulation results, 7 geomorphologic and hydrometeorological factors were chosen, including river width, mean observed discharge, control area, vegetation percentage, elevation, land use/cover and climate type. Results of the monthly runoff simulation show that among the 61 typical sites in 7 river basins in China, R2 of 59.0% of them exceeded 0.5 and more than 41.0% reached an ENS larger than 0.5. Performance for the sites in basins of the rivers in Southwest China is obviously better, especially for the sites on the Tibetan Plateau; elevation, mean observed discharge, climate type and width of the river cross section are found out to be the dominated factors; and the M/C method is more suitable for sites on small rivers of high elevation under mountain plateau climate. Generally, this paper can inspire research ideas for river runoff simulation, thus providing reliable guidance for extending the hydrological application of microwave remote sensing in China.
Abstract: Definition of interconnected water systems determines its level of science and engineering technology. How to summarize existing research and engineering practice scientifically, the research on the definition and classification of interconnected water systems is of great value in theoretical and applications. In response to this problem, this article discusses the meaning and important role of defining the term based on formal logic methods and norms combined with an analysis of the main problems associated with the current term definitions used in the water conservancy industry. Then, we systematically demonstrated the necessity and scientificity of applying an interconnected water system instead of a river and lake water system connection and analysed the various problems that exist in association with the current definition of an interconnected water system. On this basis, the following definition of an interconnected water system is given: "science and technology that study the formation of connected water system channels, the evolution of water systems, and the impact of water body transfers"; interconnected water system engineering is herein defined as "a project that constitutes water transfer channels among water systems through water engineering techniques, rivers and lakes". Then, the connotations and classifications of these two definitions are discussed. These definitions are more scientific, concise and reasonable than the currently used definitions, and more importantly, each definition can scientifically handle the relationship between the adaptability of the concept definition and the completeness of the classification.
Abstract: To investigate the effect of irregular shape and distribution of porous media on groundwater flow and solute transport, a double-layer herringbone structure was artificially constructed and numerical simulations of groundwater flow and transport were performed. Such a structure simulates the existing inclined cross-bedding geological features and forms macroscopic anisotropic hydraulic conductivity field. The results show that the macroscopic anisotropic structure creates helical flow, leading to the stretching and folding of the streamlines and the irregular deformation of the solute plume, and thus, solute dilution is enhanced significantly. The vertical relative position between the macroscopic anisotropic structure and the solute plume has a remarkable effect on solute transport.
Abstract: Conventional drainage network models that use the link-node approach often fail to simulate the detailed internal flow processes in drainage pipes. To address the problem, a one-dimensional drainage network hydrodynamic model based on a Godunov-type finite volume scheme and the Preissmann slot theory was proposed in the present work. The Harten-Lax-van Leer (HLL) approximate Riemann solver was employed in the model to calculate the numerical flux at the cell interfaces, the impacts of the water levels at the nodes of the pipeline on the connected pipes were identified, and the water levels were calculated and updated by the strict mass conservation equation and the theory of characteristics. Verification on three classical computation examples proved that the proposed model achieved accurate and stable performance in simulating pressured transient flows, transient mixed flows, and other complex flow regimes. The model was applied to simulation of rainwater drainage at Tianfuheyuan Community in Xixian New District, Shaanxi, China, and the simulation results agreed with the measured data. The proposed model can be used to identify the hydrodynamic factors at the central node of all computation cells and junctions of the pipeline, and is hence expected to provide a basis for detailed simulation of urban pluvial flooding.
Abstract: The significant variation of seasonal streamflow in mountainous regions prevents the existing irrigation channel of the water volume facilities from meeting the flow measurement accuracy requirements in an extensive flow range. In this paper, a combined water measuring facility based on crump weir and sluicing flume is proposed, and the mechanism of its flow measurement is explored in an extensive flow range. Based on the hydraulic performance test of the generalized flume in the range of 5-79 L/s, the changes of the water surface line, Froude number (Fr), vertical longitudinal average velocity, and the characteristic length and width of a thin water layer were analyzed under different flow rates. Furthermore, the flow measurement equations were formulated in different flow threshold ranges. The results show that: ① As the flow rate increases, the flow pattern of the combined facility changes from being an in-groove flow to a weir flow, and the relative water depth of the threshold value corresponding to the flow rate is 0.885. The flow measurement formulas of the in-groove flow and the weir flow are obtained through a fit. The relative error is less than 3% compared with the measured flow rate. ② The in-groove and the weir flow have different characteristics in the combined facility; when the in-groove flow occurs, the average longitudinal velocity, the characteristic length and width of the thin water layer, and the comprehensive flow coefficient of each measuring point increases with the flow rate. When the weir flow occurs, the average longitudinal velocity of each measuring point in the front of the flume decreases as the flow rate increases. In contrast, the average longitudinal velocity of each measuring point in the back increases with the flow rate. The average section velocity near the middle of the contraction torsion surface in the flume is the same. ③ The combined facility flow coefficient m0 decreases as the discharge increases. ④ The characteristic length and width of the thin water layer downstream decrease as the flow rate increases, and the maximum value is reached at the flow threshold. This study effectively solves the lack of flow measuring facilities in open channels with a significant flow fluctuation. It can provide a reference in the application of flow measuring facilities for seasonal streams in mountainous regions.
Abstract: A series of experimental tests to study the effect of the height and period of ocean wave on composite coral reef topography was performed. Our study found that the breaker height and breaker distance to a certain reference location increases as either the height or the period of the deep water wave increases, and were relatively less affected by reef flat water level. Wave set-up over reef flat was found to decrease with the rise of water level while transmission wave height over reef flat was found to increase; both of them were greatly affected by the reef flat water level. The dimensionless analyses have shown that the ratio of breaker height to deep water wave height was found to be associated with deep water wave steepness and the ratio of transmission wave height to deep water wave height with the ratio of reef flat still water depth to deep water wave height. In addition, for the dimensionless breaker position (the ratio of horizontal distance between the breaker point and the reef edge to the reef-edge shallow-water wave length) and the dimensionless reef flat wave set-up (the ratio of wave set-up to deep water wave height), a parametric formulae can be established through the ratio of reef-edge water depth to deep water wave height.
Abstract: The bed shear stress under the action of waves is one of the important essential parameters to estimate the incipient motion and transport of offshore sediment, and its bottom friction effect will also have an impact on the offshore hydrodynamic environment. Due to the difficulty in field observation, the understanding of the wave boundary layer and the bed shear stress under the action of waves is mainly based on indoor experimental observation. This paper reviews the relevant theoretical models and experimental measurement research at home and abroad, sorts the advantages and disadvantages of various research methods and measurement techniques, as well as the application conditions, collates a lot of experimental data from 1970s to now, generalizes and analyzes the existing research results, including the impact of wave nonlinearity, wave breaking and other factors on the bed shear stress, summarizes the limitations of the existing research, and puts forward the future research focuses. Super-large water flume is an important facility to break through the bottleneck in the theoretical study of the layer of wave boundary in the future. The development of high-adaptability underwater two-dimensional shear stress sensor is the key to making a breakthrough in the study of bed shear stress under complex dynamic conditions.
Abstract: Braided river as a primary alluvial river pattern is of great importance on fluvial geomorphology, aquatic habitat, river exploitation and ecological protection. According to the literature review on braided rivers, the basic definition and morphological characteristics are described, and several classic methods and parameters of morphological characterization are introduced. In addition, field methods of obtaining high-precision DEM are summarized as well as the application of flume experiment and numerical model which advantages and disadvantages are pointed out, respectively. This paper describes the general composition of different morphological units of braided river, and lists the evolution mechanism of bar formation, anabranching adjustment and lateral migration, bank erosion and floodplain deposition. The primary controlling factors of morphodynamics and evolution of braided river, such as stream power, bed sediment particle size, sediment supply, riparian vegetation and valley width, are compared and analyzed. Meanwhile, a promising direction for alluvial river dynamics is to unravel formation causes, morphological characteristics, and fluvial processes of braided rivers with different spatial scales on the Qinghai-Tibet Plateau.