[Show abstract][Hide abstract] ABSTRACT: Understanding solute transport process is of fundamental significance for industrial and natural processes such as mixing and separation. Since detailed information for the transverse concentration distribution is required for associated applications, compared with the sole consideration of the cross-sectional mean concentration in previous studies, the present paper analytically explores the complete spatial concentration distribution in packed tube flow by the proposed two-scale perturbation analysis. With modification on the zeroth-order concentration up to the first-order, the deduced analytical solution gives a good prediction for the longitudinal distribution of mean concentration as well as the transverse distribution, according to comparisons with new results of numerical simulation. Importantly we show in the paper that instead of being uniformly distributed in the cross-section as expected in traditional view, the transverse concentration is highly non-uniform when Taylor dispersion model is applicable. Representing the complicated flow conditions for packed tube flow, the unique dimensionless parameter as a damping factor affects the complete spatial concentration distribution in two ways: cause the contraction of the solute concentration cloud, and the flattening of the concentration contours.
Full-text · Article · Jan 2016 · International Journal of Heat and Mass Transfer
[Show description][Hide description] DESCRIPTION: Highlights
We present a hierarchical method to build a pyramid of drainage networks.
High-level drainage networks are generated from the originals iteratively.
Each level is obtained by pruning the outer reaches and merging related sub-basins.
Topographical attributes are conserved for hydrological simulations across levels.
We speed up the digital map display using each pyramid level in the proper scale range.•
[Show abstract][Hide abstract] ABSTRACT: The present paper provides a systematical analysis for concentration distribution of Taylor dispersion in laminar open channel flow, seeking fundamental understandings for the physical process of solute transport that generally applies to natural rivers. As a continuation and a direct numerical verification of the previous theoretical work (Wu, Z., Chen, G.Q., 2014. Journal of Hydrology, 519: 1974-1984.), in this paper we attempt to understand that to what extent the obtained analytical solutions are valid for the multi-dimensional concentration distribution, which is vital for the key conclusion of the so-called slow-decaying transient effect. It is shown that as a first estimation, even asymptotically, the longitudinal skewness of the concentration distribution should be incorporated to predict the vertical concentration correctly. Thus the traditional truncation of the concentration expansion is considered to be insufficient for the first estimation. The analytical solution by the two-scale perturbation analysis with modifications up to the second order is shown to be a most economical solution to give a reasonably good prediction.
Full-text · Article · Sep 2015 · Journal of Hydrology
[Show abstract][Hide abstract] ABSTRACT: Rebalancing water allocation between human consumptive uses and the environment in water catchments is a global challenge. This paper proposes a socio-hydrological water balance framework by partitioning catchment total evapotranspiration (ET) into ET for society and ET for natural ecological systems, and establishing the linkage between the changes of water balance and its social drivers and resulting environmental consequences in the Murray–Darling Basin (MDB), Australia, over the period 1900–2010. The results show that the 100-year period of water management in the MDB could be divided into four periods corresponding to major changes in basin management within the socio-hydrological water balance framework: period 1 (1900–1956) – expansion of water and land use for the societal system, period 2 (1956–1978) – maximization of water and land use for the societal system, period 3 (1978–2002) – maximization of water use for the societal system from water diversion, and period 4 (2002–present) – rebalancing of water and land use between the societal and ecological systems. Most of management changes in the MDB were passive and responsive. A precautionary approach to water allocation between the societal and ecological systems should be developed. The socio-hydrological water balance framework could serve as a theoretical foundation for water allocation to evaluate the dynamic balance between the societal and ecological systems in catchments.
Full-text · Article · Aug 2015 · Hydrology and Earth System Sciences
[Show abstract][Hide abstract] ABSTRACT: As a result of climate change/variation and its aggravation by human activities over the past several decades, the hydrological conditions in the middle Yellow River in China have dramatically changed, which has led to a sharp decrease of streamflow and the drying up of certain tributaries. This paper simulated and analysed the impact of sediment-trapping dams (STDs, a type of large-sized check dam used to prevent sediment from entering the Yellow River main stem) on hydrological processes, and the study area was located in the 3,246 km2 Huangfuchuan (HFC) River basin. Changes in the hydrological processes were analysed, and periods of natural and disturbed states were defined. Subsequently, the number and distribution of the STDs were determined based on data collected from statistical reports and identified from remote sensing images, and the topological relationships between the STDs and high-resolution river reaches were established. A hydrological model, the Digital Yellow River Integrated Model, was used to simulate the STD impact on the hydrological processes, and the maximum STD impact was evaluated through a comparison between the simulation results with and without the STDs, which revealed that the interception effect of the STDs contributed to the decrease of the streamflow by approximately 39%. This paper also analysed the relationship between the spatial distribution of the STDs and rainfall in the HFC River basin and revealed that future soil and water conservation measures should focus on areas with a higher average annual rainfall and higher number of rainstorm hours.
Full-text · Article · Aug 2015 · Hydrological Processes
[Show abstract][Hide abstract] ABSTRACT: Vegetation plays an important role in soil erosion control, but few studies have been performed to quantify the effects of vegetation stems on hydraulics of overland flow. Laboratory flume experiments were conducted to investigate the potential effects of vegetation stems on Reynolds number, Froude number, flow velocity and hydraulic resistance of silt-laden overland flow. Cylinders with diameter D of 2.0, 3.2 and 4.0 × 10−2 m were glued onto the flume bed to simulate the vegetation stems, and a bare slope was used as control. The flow discharge varied from 0.5 to 1.5 × 10−3 m3 s−1 and slope gradient was 9°. Results showed that Reynolds number on vegetated slope was significantly higher than that on bare slope due to the effect of vegetation stems on effective flow width. All the flows were supercritical flow, but Froude number decreased as D increased, implying a decrease in runoff ability to carry sediment. The Mean flow velocity also decreased with D, while the velocity profile became steeper, and no significant differences were found in surface flow velocities among longitudinal sections on all slopes. Darcy–Weisbach friction coefficient increased with D, implying that the energy consumption of overland flow on hydraulic resistance increased. Reynolds number was not a unique predictor of hydraulic roughness on vegetated slopes. The total resistance on vegetated slopes was partitioned into grain resistance and vegetation resistance, and vegetation resistance accounted for almost 80% of the total resistance and was the dominant roughness element. Further studies are needed to extend and apply the insights obtained under controlled conditions to actual overland flow conditions. This article is protected by copyright. All rights reserved.
Full-text · Article · Aug 2015 · Land Degradation and Development
[Show abstract][Hide abstract] ABSTRACT: Floods in mountainous river basins are generally highly destructive, usually causing enormous losses of lives and property. It is important and necessary to develop an effective flood forecast method to prevent people from suffering flood disasters. This paper proposed a general framework for a service-oriented architecture (SOA) for ensemble flood forecast based on numerical weather prediction (NWP), taking advantage of state-of-the-art technologies, e.g., high-accuracy NWP, high-capacity cloud computing, and an interactive web service. With the predicted rainfall data derived from the NWP, which are automatically downloaded, hydrological models will be driven to run on the cloud. Judging from the simulation results and flood control requirements offered by users, warning information about possible floods will be generated for potential sufferers and then sent to them as soon as possible if needed. Moreover, by using web service in a social network, users can also acquire such information on the clients and make decisions about whether to prepare for possible floods. Along with the real-time updates of the NWP, simulation results will be refreshed in a timely manner, and the latest warning information will always be available to users. From the sample demonstrations, it is concluded that the SOA is a feasible way to develop an effective ensemble flood forecast method. After being put into practice, it would be valuable for preventing or reducing the losses caused by floods in mountainous river basins.
Full-text · Article · Aug 2015 · Journal of Hydrology
[Show abstract][Hide abstract] ABSTRACT: Due to climate change and its aggravation by human activities (e.g., hydraulic structures) over the past several decades, the hydrological conditions in the middle Yellow River have markedly changed, leading to a sharp decrease in runoff and sediment discharge. This paper focused on the impacts of climate change and hydraulic structures on runoff and sediment discharge, and the study area was located in the 3,246 km2 Huangfuchuan (HFC) River basin. Changes in annual runoff and sediment discharge were initially analysed by using the Mann-Kendall trend test and Pettitt change point test methods. Subsequently, periods of natural and disturbed states were defined. The results showed that both the annual runoff and sediment discharge presented statistically significant decreasing trends. However, compared with the less remarkable decline in annual rainfall, it was inferred that hydraulic structures might be another important cause for the sharp decrease in runoff and sediment discharge in this region. Consequently, sediment-trapping dams (STDs, a type of large-sized check dam used to prevent sediment from entering the Yellow River main stem) were considered in this study. Through evaluating the impacts of the variation in rainfall patterns (i.e., amount and intensity) and the STD construction, a positive correlation between rainfall intensity and current STD construction was found. This paper revealed that future soil and water conservation measures should focus on areas with higher average annual rainfall and more rainstorm hours.
[Show abstract][Hide abstract] ABSTRACT: Study on contaminant transport in wetland flows is of fundamental importance. Recent investigation on scalar transport in laminar tube flows (Wu, Z., Chen, G.Q., 2014. J. Fluid Mech., 740: 196-213.) indicates that the vertical concentration difference in wetland flows may be remarkable for a very long time, which cannot be captured by the extensively applied one-dimensional Taylor dispersion model. To understand detailed information for the vertical distribution of contaminant in wetland flows, for the first time, the present paper deduces an analytical solution for the multidimensional concentration distribution by the method of mean concentration expansion. The solution is verified by both our analytical and numerical results. Representing the effects of vegetation in wetlands, the unique dimensionless parameter α can cause the longitudinal contraction of the contaminant cloud and the change of the shape of the concentration contours. By these complicated effects, it is shown unexpectedly that the maximum vertical concentration difference remains nearly unaffected, although its longitudinal position may change. Thus the slow-decaying transient effect (Wu, Z., Chen, G.Q., 2014. J. Hydrol., 519: 1974-1984.) is shown also apply to the process of contaminant transport in wetland flows.
Full-text · Article · Jun 2015 · Journal of Hydrology
[Show abstract][Hide abstract] ABSTRACT: With the increasing resolution of digital elevation models (DEMs), computational efficiency problems have been encountered when extracting the drainage network of a large river basin at billion-pixel scales. The efficiency of the most time-consuming depression-filling pretreatment has been improved by using the O(NlogN) complexity least-cost path search method, but the complete extraction steps following this method have not been proposed and tested. In this paper, an improved O(NlogN) algorithm was proposed by introducing a size-balanced binary search tree (BST) to improve the efficiency of the depression-filling pretreatment further. The following extraction steps, including the flow direction determination and the upslope area accumulation, were also redesigned to benefit from this improvement. Therefore, an efficient and comprehensive method was developed. The method was tested to extract drainage networks of 31 river basins with areas greater than 500,000 km2 from the 30-m-resolution ASTER GDEM and two sub-basins with areas of approximately 1000 km2 from the 1-m-resolution airborne LiDAR DEM. Complete drainage networks with both vector features and topographic parameters were obtained with time consumptions in O(NlogN) complexity. The results indicate that the developed method can be used to extract entire drainage networks from DEMs with billions of pixels with high efficiency.
[Show abstract][Hide abstract] ABSTRACT: Mesoscopic structures form in dense granular materials due to the self-organisation of the constituent particles. These structures have internal structural degrees of freedom in addition to the translational degree of freedom. The resultant granular elasticity, which exhibits intrinsic variations and inevitable relaxation, is a key quantity that accounts for macroscopic solid- or fluid-like properties and the transitions between them. In this work, we propose a potential energy landscape (PEL) with local stable basins and low elastic energy barriers to analyse the nature of granular elasticity. A function for the elastic energy density is proposed for stable states and is further calibrated with ultrasonic measurements. Fluctuations in the elastic energy due to the evolution of internal structures are proposed to describe a so-called configuration temperature T(c) as a counterpart of the classical kinetic granular temperature T(k) that is attributed to the translational degrees of freedom. The two granular temperatures are chosen as the state variables, and a fundamental equation is established to develop non-equilibrium thermodynamics for granular materials. Due to the relatively low elastic energy barrier in the PEL, granular elasticity relaxes more under common mechanical loadings, and a simple model based on mean-field theory is developed to account for this behaviour.
Full-text · Article · May 2015 · Scientific Reports
[Show abstract][Hide abstract] ABSTRACT: A two-dimensional (2D) hydrodynamic model is developed for the Jing-Dongting river–lake system, with the computation domain (3,900 km 2) covered by a channel-refined grid of 300,000 cells. The model provides good descriptions of the river–lake characteristics (strongly coupled, annular branches, varying flow regimes). In a simulation of a 1-year unsteady flow process, the mean absolute error in simulated water levels is 0.11–0.15 m, and the mean absolute relative error in simulated cross-section discharges is 4.3–8.5%, compared with field data. The accuracy of the 2D model is obviously better than those of existing one-dimensional (1D) and 1D–2D nested models. The prediction–correction parallelization (PCP) method is then tested by simulating the river–lake system using 64 subdomains. The complexities of real rivers are revealed to have almost no negative effects on the quasi-coupled solutions and accuracy of the PCP method. In tests, the mean absolute error in water level is found to be approximately 0.3 cm, and the mean absolute relative error in cross-section discharges is 0.23%, comparing the results of sequential and PCP simulations. In solving linear systems, sequential runs are 40.5 to 76.8 times slower than parallel runs using 64 cores. It takes 6.6 h to complete a simulation of a 1-year unsteady flow process in the river–lake system. The 2D model is then applied to the study of regulations of real river networks.
[Show abstract][Hide abstract] ABSTRACT: Parallel solutions of linear systems arising from velocity–pressure coupling in implicit two-dimensional (2D) hydrodynamic models are usually difficult and inefficient. Using domain decomposition, a prediction–correction parallelization method is proposed to solve such systems in parallel. It is proposed as a special method for parallelizing simulations of free-surface flows in alluvial rivers. Rather than solving a large-scale global linear system over the whole domain, the method solves sub linear systems for subdomains in two steps, prediction and correction. For free-surface flows in alluvial rivers, the gravity wave propagation over subdomains is divided into internal and external parts, moving within a subdomain and across its boundaries, respectively. The external part is assumed to be well solved at the prediction step; the whole wave propagation is then solved at the correction step using updated boundary values and initial estimates. A theoretical analysis is conducted to derive the computational errors at the prediction and correction steps of this method, resulting in the condition for its application. The method is tested on five meshes whose numbers of elements are 12,800–819,200. The grid scale, which is equal to or smaller than a common scale of real applications, provides grid-independent results. The method performs well for problems of various computational granularities. In solving linear systems with the different meshes, sequential runs were 41–96 times slower than parallel runs using 64 subdomains and 64 working cores.
[Show abstract][Hide abstract] ABSTRACT: Water use efficiency (WUE) is a crucial parameter to describe the interrelationship between gross primary production (GPP) and evapotranspiration (ET). Incorporating the nonlinear effect of vapor pressure deficit (VPD), underlying WUE (uWUE=GPP·VPD0.5/ET) is better than inherent WUE (IWUE=GPP·VPD/ET) at the half-hourly time scale. However, appropriateness of uWUE has not yet been evaluated at the daily time scale. To determine whether uWUE is better than IWUE, daily data for 7 vegetation types from 34 AmeriFlux sites were used to validate uWUE at the daily time scale. First, daily mean VPD was shown to be a good substitute for the effective VPD that was required to preserve daily GPP totals. Secondly, an optimal exponent, k*, corresponding to the best linear relationship between GPP·VPDk* and ET, was about 0.53 both at half-hourly and daily time scales. Thirdly, correlation coefficient between GPP·VPDk and ET showed that uWUE (k=0.5 & r=0.85) was a better approximation of the optimal WUE (k=k* & r=0.86) than IWUE (k=1 & r=0.81) at the daily scale. Finally, when yearly uWUE was used to predict daily GPP from daily ET and mean VPD, uWUE worked considerably better than IWUE. Comparing observed and predicted daily GPP, the average correlation coefficient and Nash-Sutcliffe coefficient of efficiency were 0.81 and 0.59, respectively, using yearly uWUE, and only 0.59 and −0.83 using yearly IWUE. As a nearly optimal WUE, uWUE consistently outperformed IWUE, and could be used to evaluate the effects of global warming and elevated atmosphere CO2 on carbon assimilation and evapotranspiration.
[Show abstract][Hide abstract] ABSTRACT: The Budyko hypothesis states that the ratio of the actual evapotranspiration over precipitation (E/P) is fundamentally related to the ratio of the potential evapotranspiration over precipitation (E0/P). A number of Budyko functions have been proposed to describe such a relationship between E0/P and E/P. There is, however, no simple method to generate Budyko functions that meet the water and energy constraints. This study showed analytically that for any Budyko function, the sum of elasticity of evapotranspiration with respect to potential evapotranspiration and that with respect to precipitation equals to unity. This complementary relationship for sensitivity of evapotranspiration has important implications for evaluating hydrologic impact of change in climate and/or catchment characteristics. More importantly, this study found a function that is monotonically increasing with simple limiting properties. This function can be used to generate numerous valid Budyko functions, and can also be used to test the validity of the existing Budyko functions.
Full-text · Article · Feb 2015 · Geophysical Research Letters
[Show abstract][Hide abstract] ABSTRACT: Soil erosion is the root cause of environmental and ecological degradation in the Loess Plateau of the Yellow River. Watershed sediment dynamics was fully analyzed here, and a physically based, distributed, and continuous erosion model at the watershed scale, named the Digital Yellow River Integrated Model (DYRIM), was developed. The framework, the key supporting techniques, and the formulation for natural processes were described. The physical processes of sediment yield and transport in the Loess Plateau are divided into three subprocesses, including the water yield and soil erosion on hillslopes, gravitational erosion in gullies, and hyperconcentrated flow routing in channels. For each subprocess, a physically based simulation model was developed and embedded into the whole model system. The model system was applied to simulate the sediment yield and transport in several typical years in different watersheds of the Yellow River, and the simulation results indicated that this model system is capable of simulating the physical processes of sediment yield and transport in a large-scale watershed.
[Show abstract][Hide abstract] ABSTRACT: This paper develops an algorithm for computing spatially distributed monthly potential evaporation (PE) over a mountainous region, the Lhasa River basin in China. To develop the algorithm, first, correlation analysis of different meteorological variables was conducted. It is observed that PE is significantly correlated with vapor pressure and temperature differences between the land surface and the atmosphere. Second, Dalton model, which was developed based on mass transfer mechanism, was modified by including the influences of the related meteorological variables. Third, the influences of elevation on monthly temperature, vapor pressure and wind velocity were analyzed, and the functions for extending these meteorological variables to any given altitude were developed. Fourth, the inverse distance weighting method was applied to integrate the extended meteorological variables from five stations adjacent to and within the Lhasa River basin. Finally, using the modified Dalton model and the integrated meteorological variables, we computed the spatially distributed monthly PE. This study indicated that spatially distributed PE can be obtained using data from sparse meteorological stations, even if only one station is available; the result showed that in the Lhasa River basin PE decreases when elevation increases. The new algorithm, including the modified model and the method for spatially extending meteorological variables, developed in this paper can provide the basic inputs for distributed hydrological models.
Full-text · Article · Oct 2014 · Hydrological Sciences Journal/Journal des Sciences Hydrologiques
[Show abstract][Hide abstract] ABSTRACT: Forecasting the level of waterborne bacterial pathogens (E. coli as indicator) in recreational waters using a deterministic model has been a very effective tool for water quality prediction and management. The fate and transport of pathogens in water is a complex process controlled by various factors of hydrodynamics, hydrology, chemistry, and microbiology. To better understand the importance of these factors and their roles in the inactivation, transport, and removal of pathogens, it is extremely important to enhance the reliability and effectiveness of a model by increasing the accuracy of simulation and prediction. This paper reports the results of sensitivity analyses on each of these factors using a calibrated hydrodynamic model coupled with a water quality model for temperature variation and E. coli transport. A nearshore region in southern Lake Michigan was used as the modeling domain in this research. Based on the sensitivity analysis method of differential analyses coupled with one-at-a-time design, the results show that the sensitivity of the different parameters can be ranked in decreasing order as follows: solar insolation, temperature correction factor, dispersion coefficients, tributary loading, wind velocity, and settling velocity. More detailed investigation of sunlight-related parameters using Chapra's formula shows that t90 is predominant over other factors on E. coli inactivation caused by insolation. The sensitivity of sunlight-related parameters can be ranked in decreasing order as follows: t90, θd, ke, α, and θl. The model used in this study, together with the sensitivity analysis results, can be used as a reference for similar pathogen transport investigations in other freshwater bodies.
No preview · Article · Sep 2014 · Journal of Environmental Engineering