Complex River Network Research Articles

Spatial and temporal variations of nutrition in representative river networks in Southwest China.

To control pollutants in rivers, we need to have an understanding of the spatial and temporal variations on nutrients and environmental processes in complex river networks. In this study, 177 sampling sites were located in Jinjiang River in 2017, 15 monitoring stations in Jinjiang River from 2011 to 2016 were also collected. According to the data from the monitoring station, the total phosphorus (TP) and total nitrogen (TN) were the dominant contaminants. By analyzing historical water quality data and up-to-date information about the nutrient concentrations, the secondary canals of the Jinjiang River system, most of which were black and odorous, were much more seriously polluted than the main channel and tributaries. Correlation analysis indicated that the NH3, CODcr, TN, and TP had similar sources (R2NH3-CODcr = 0.572, R2NH3-TN = 0.543, R2NH3-TP = 0.537, p < 0.01). The vertical banks of urban rivers and the inadequate and poorly maintained pipe network systems promote deterioration of water quality in these secondary canals. Overall, our results suggest that the river water quality could be improved if the municipal pipe network systems were better maintained and if the vertical banks were transformed into vegetated buffer strips. This study will support initiatives to improve the water quality and function of the river network ecosystem.

Steady-state distributed modeling of dissolved oxygen in data-poor, sewage dominated river systems using drainage networks

The determination of water quality along a river drainage network is of paramount importance for many environmental applications. Whereas several codes have been proposed to deal with this problem along a single reach of a river, no simple solution has been proposed for a complex river network. In this contribution, a methodology is presented for preliminary screening of river water quality at the watershed-scale and at steady state. The solution process exploits the information contained within the watershed's space-filling drainage network (SFDN), that is automatically computed by processing the raster map of the watershed's elevations. To this end, a visiting algorithm for non-binary trees is coupled to a Newton-Raphson method that locally solves the system of nonlinear water-quality mass balances. We believe that the resulting framework will be especially valuable for initial screening analyses in data-poor watersheds. A free code called Q2T that implements the described methods is provided.

Water Quality in Representative Tuojiang River Network in Southwest China

To control pollutants in rivers, we need to have an understanding of the spatial and temporal variations on water quality in complex river networks. In this study, 153 sampling sties were located in Chengdu segment of Tuojiang River in 2017, 23 monitoring stations in Tuojiang River in the Chengdu segment from 2011 to 2016 were also collected. According to the data from the monitoring station, the concentrations of total nitrogen (TN), total phosphorus (TP), ammonia nitrogen (NH4+), and chemical oxygen demand (CODcr) concentrations were exceeded the water quality targets at 22, 14, 10, and 3 of the 23 monitoring stations, respectively. The regression analysis showed the NH3, TN and TP was increased from 2011 to 2016 (R2NH4+ = 0.30, p &lt; 0.05; R2TN = 0.88, p &lt; 0.05; R2TP = 0.30, p &lt; 0.05), which indicated water quality was deteriorated from 2011 to 2016. In the 153 sample sites, the NH4+, CODcr, TP and TN concentrations exceeded the class III water quality threshold at 23%, 23%, 26%, and 99% of the total sampling sites, respectively. Correlation analysis showed that the main sources of NH4+, CODcr, TN, and TP (R2NH4+-CODcr = 0.572; R2NH4+-TN = 0.543; R2NH4+-TP = 0.537, p &lt; 0.01) were municipal domestic sewage. The main pollutants in Tuojiang river network were TN and TP, high P and N concentrations probably contribute to the deterioration of surface water quality at some sampling sites (R2TPs-TP = 0.183 and R2TNs-NH4+ = 0.472, p &lt; 0.01). TP, TN, TP in the sediments (TPs), and TN in the sediments (TNs) have accumulated in the river system because the river morphology has been changed by, for example, the damage of riparian zones and the construction of water conservation structures. The results from this study will support initiatives to improve the overall water quality and function of the river ecosystem.

Study of ecological compensation in complex river networks based on a mathematical model.

Transboundary water pollution has resulted in increasing conflicts between upstream and downstream administrative districts. Ecological compensation is an efficient means of restricting pollutant discharge and achieving sustainable utilization of water resources. The tri-provincial region of Taihu Basin is a typical river networks area. Pollutant flux across provincial boundaries in the Taihu Basin is hard to determine due to complex hydrologic and hydrodynamic conditions. In this study, ecological compensation estimation for the tri-provincial area based on a mathematical model is investigated for better environmental management. River discharge and water quality are predicted with the one-dimensional mathematical model and validated with field measurements. Different ecological compensation criteria are identified considering the notable regional discrepancy in sewage treatment costs. Finally, the total compensation payment is estimated. Our study indicates that Shanghai should be the receiver of payment from both Jiangsu and Zhenjiang in 2013, with 305 million and 300 million CNY, respectively. Zhejiang also contributes more pollutants to Jiangsu, and the compensation to Jiangsu is estimated as 9.3 million CNY. The proposed ecological compensation method provides an efficient way for solving the transboundary conflicts in a complex river networks area and is instructive for future policy-making.

In-time source tracking of watershed loads of Taihu Lake Basin, China based on spatial relationship modeling.

Influent river carrying cumulative watershed load plays a significant role in promoting nuisance algal bloom in river-fed lake. It is most relevant to discern in-stream water quality exceedance and evaluate the spatial relationship between risk location and potential pollution sources. However, no comprehensive studies of source tracking in watershed based on management grid have been conducted for refined water quality management, particularly for plain terrain with complex river network. In this study, field investigations were implemented during 2014 in Taige Canal watershed of Taihu Lake Basin. A Geographical Information System (GIS)-based spatial relationship model was established to characterize the spatial relationships of "point (point-source location and monitoring site)-line (river segment)-plane (catchment)." As a practical exemplification, in-time source tracking was triggered on April 15, 2015 at Huangnianqiao station, where TN and TP concentration violated the water quality standard (TN 4.0mg/L, TP 0.15mg/L). Of the target grid cells, 53 and 46 were identified as crucial areas having high pollution intensity for TN and TP pollution, respectively. The estimated non-point source load in each grid cell could be apportioned into different source types based on spatial pollution-related entity objects. We found that the non-point source load derived from rural sewage and livestock and poultry breeding accounted for more than 80% of total TN or TP load than another source type of crop farming. The approach in this study would be of great benefit to local authorities for identifying the serious polluted regions and efficiently making environmental policies to reduce watershed load.

Characterization of River Networks: A GIS Approach and Its Applications.

Fluvial geomorphology provides the basis for characterizing complex river networks and evaluating biophysical processes within watersheds. Understanding the spatial organization of morphological features, their influencing processes, and resultant geomorphic diversity in stream networks are important for efficient restoration, river health assessment, and improving our knowledge of the resilience of riverine landscapes. River characterization is a means to determine the biophysical character of river networks but many methods are fraught with pitfalls, such as the use of incorrect variables and limited acknowledgment of the hierarchical organization of rivers. In this paper, a top-down geographic information system-based approach for determining the physical typology of river networks is outlined. A suite of multivariate analyses are used to develop a nomenclature for functional process zones (FPZs) - large tracts of the river network with similar hydro-geomorphological character. Applied to the Little Miami River, Ohio, six distinct FPZs emerged, which had a nonuniform distribution along the river network. Some FPZs repeated downstream; others were rare in terms of total length and number of FPZ segments. The physical structure of the Little Miami River network was analyzed using a series of community metrics. Application of this approach for river monitoring, establishing reference conditions, as well as management of threatened and endangered species and asset trading is highlighted.

Study on water environmental dispatch scheme for complex river network in Pearl River Estuary

In order to promote the construction of Guangdong-Hong Kong-Macao Greater Bay Area, we should take into account the improvement of regional water environment while developing economy in Pearl River Estuary area. The water environmental dispatch scheme of river network is an important part of water environment improvement, and the key technology of dispatch is to determine the appropriate dispatch path, timing, scale and mode. The Hengqin Central River is taken as the study area in the paper. Based on the analysis of the characteristics of Hengqin Central River, the characteristics of nearby tides and water environment, this paper determines the dispatching path of “diversion from the east to the west”, the time of diversion during flood tide and drainage during ebb tide, the dispatching mode of sluice control, the dispatching scale of more water exchange during spring tide and less water exchange during neap tides, and puts forward the landscape dispatching scheme and the dilution dispatching scheme. In this paper, the semiexchange period and the rate of water exchange are taken as the evaluation indexes, and the dispatching effects of different schemes are analyzed by simulating different control schemes through water quality and quantity models. The results show that the water exchange capacity of Hengqin Central River network will be greatly enhanced when the diversion and drainage amplitude increases. The research results provide reference for water quality protection and control in the Pearl River Estuary complex river network.

Modelling flow in the middle and lower Yangtze River, China

A one-dimensional numerical model was developed to simulate the hydrodynamic processes in a large and complex river network located in the middle and lower Yangtze River basin, China. The main stem of the river is connected to two large lakes, Dongting and Poyang, through many branches. A mass conservation method was used to determine the lateral inflows from local basins along the river. The model was verified against measured data at a large number of hydrological stations over a 2-year period. The water levels and discharges predicted by the model were found to generally agree well with the measured data. The model was then used to reproduce the flow process during a large flood event that occurred in 1998. It was found that the two large lakes could store a large volume of water and thus played an important role in attenuating the flood impact.

Modelling Approaches for Minimally Gauged Sg Similajau, Bintulu

Minimally gauged river basins are common in Sarawak. Many river basins are lacking in hydrological monitoring mechanism mostly due to the vast land mass and complex river network, hence causing isolation of certain locations. This paper is describing the approaches to model Sg Similajau of Bintulu, Sarawak with little information to support sound analysis. A river model is developed to reconstruct missing flows of the river. In order to have reliable results, a short-term monitoring program is strongly suggested to collect data pertaining to river processes in the field. Pre-processing processes are demonstrated to obtain inflow data for upstream boundaries, tide/King Tide data for downstream boundary and a weir at Samalaju Water Intake for middle boundary. Particularly the weir, its presence convinces the verification of the river model. Post-processing on the impacts of the weir in flow patterns, in this case the water head produced by the weir and with such a head to flush out saltwater brought about by tides are demonstrated as ideal tool to test the model.

Sieved Transport and Redistribution of Bioavailable Phosphorus from Watershed with Complex River Networks to Lake.

An innovative approach was developed to reveal phosphorus (P) transport and redistribution in large and complex river networks in the Lake Taihu basin by establishing the relations between sediment P spatial distribution and P sorption behavior on particles with different grain size, sorted by hydrodynamics. The sediment P fractionation composition changed greatly across the basin, where 69% consisted of acid-soluble fractions (HCl-P) in upstream rivers while 70% was in fractions associated with reducible metal hydroxides (BD-P) and amorphous hydroxides (NaOH25-P) in downstream rivers. Fine particles enriched in BD-P and NaOH25-P fractions tended to sorb liberated P during the resuspension process, and fine particles were more easily delivered downstream toward the lake, forming a sieved transport of P in the river networks. This will lead to a great potential for sediment P release when environmental anoxia develops in the sediments or high pH occurs in the sediment surface during intensive algal blooms in the shallow lake. Reduction of external P from point sources from urbanized areas is an important requirement at the basin scale; however, long-term efforts are needed to restore aquatic macrophytes in the lake, which would decrease P recycling rates at the water-sediment interface.

Tracking Nitrogen Sources, Transformation, and Transport at a Basin Scale with Complex Plain River Networks.

This research developed an innovative approach to reveal nitrogen sources, transformation, and transport in large and complex river networks in the Taihu Lake basin using measurement of dual stable isotopes of nitrate. The spatial patterns of δ15N corresponded to the urbanization level, and the nitrogen cycle was associated with the hydrological regime at the basin level. During the high flow season of summer, nonpoint sources from fertilizer/soils and atmospheric deposition constituted the highest proportion of the total nitrogen load. The point sources from sewage/manure, with high ammonium concentrations and high δ15N and δ18O contents in the form of nitrate, accounted for the largest inputs among all sources during the low flow season of winter. Hot spot areas with heavy point source pollution were identified, and the pollutant transport routes were revealed. Nitrification occurred widely during the warm seasons, with decreased δ18O values; whereas great potential for denitrification existed during the low flow seasons of autumn and spring. The study showed that point source reduction could have effects over the short-term; however, long-term efforts to substantially control agriculture nonpoint sources are essential to eutrophication alleviation for the receiving lake, which clarifies the relationship between point and nonpoint source control.

Patterns of streamflow regimes along the river network: The case of the Thur river

A modeling framework for point-wise prediction of the probability density function and flow duration curve of streamflows along complex river networks is presented. The predictions are based on catchment-scale climatic and morphological features, without calibration on observed discharge time-series. The framework was applied to a test basin in north-eastern Switzerland, and relevant flow statistics were validated at six sub-catchment outlets with satisfactory results. Spatial patterns of flow regime exhibit a strong climatic signature, mostly driven by reduced rainfall depths and increasing effective rainfall frequency in the downstream areas. The increasing non-linearity of the catchment response with contributing area is reflected by the observed increase in the recession parameters along the main river channel. This framework offers a novel approach for assessing the spatial patterns of streamflows based on limited information, which is important for evaluation of human and ecological functions in riverine systems.

Joint optimization scheduling for water conservancy projects in complex river networks

In this study, we simulated water flow in a water conservancy project consisting of various hydraulic structures, such as sluices, pumping stations, hydropower stations, ship locks, and culverts, and developed a multi-period and multi-variable joint optimization scheduling model for flood control, drainage, and irrigation. In this model, the number of sluice holes, pump units, and hydropower station units to be opened were used as decision variables, and different optimization objectives and constraints were considered. This model was solved with improved genetic algorithms and verified using the Huaian Water Conservancy Project as an example. The results show that the use of the joint optimization scheduling led to a 10% increase in the power generation capacity and a 15% reduction in the total energy consumption. The change in the water level was reduced by 0.25 m upstream of the Yundong Sluice, and by 50% downstream of pumping stations No. 1, No. 2, and No. 4. It is clear that the joint optimization scheduling proposed in this study can effectively improve power generation capacity of the project, minimize operating costs and energy consumption, and enable more stable operation of various hydraulic structures. The results may provide references for the management of water conservancy projects in complex river networks.

A cost path and network analysis methodology to calculate distances along a complex river network in the Peruvian Amazon

Distance is a key variable in explicating environmental, social, and economic conditions and in defining spatial and temporal patterns. Prior research has primarily focused on using simple to complex algorithms for calculating distances along road networks. In contrast, few algorithms are available for distance calculations along fluvial networks which are often more erratic, divergent and transient than road networks. Fluvial transportation is relied upon worldwide, particularly in developing regions, where communities use river networks for transportation, access to natural resources and for trade and commerce. This paper presents a methodology developed for mapping complex fluvial networks for travel distance analysis. The methodology was applied in four major river basins in western Amazonia over some 35,000 km of river length and incorporating 919 communities as origins/destinations. A cost path and network analysis methodology was created using vector and raster datasets in a Geographic Information System (GIS) to assess interactions among communities and the distances traveled by river to reach district capitals, major urban centres and marketplaces. An accuracy assessment using distance values calculated from a previous study using a different methodological approach in the region as well as Google Earth Pro, found a high degree of concordance for distance calculations. Our methodology creates a very flexible approach for complex river systems that can be use to calculate river distances in an adaptive and efficient manner and that can be use in other regions of the world where rural communities must rely on rivers for transportation.

Regulation causes nitrogen cycling discontinuities in Mediterranean rivers

River regulation has fundamentally altered large sections of the world's river networks. The effects of dams on the structural properties of downstream reaches are well documented, but less is known about their effect on river ecosystem processes. We investigated the effect of dams on river nutrient cycling by comparing net uptake of total dissolved nitrogen (TDN), phosphorus (TDP) and organic carbon (DOC) in river reaches located upstream and downstream from three reservoir systems in the Ebro River basin (NE Iberian Peninsula). Increased hydromorphological stability, organic matter standing stocks and ecosystem metabolism below dams enhanced the whole-reach net uptake of TDN, but not that of TDP or DOC. Upstream from dams, river reaches tended to be at biogeochemical equilibrium (uptake≈release) for all nutrients, whereas river reaches below dams acted as net sinks of TDN. Overall, our results suggest that flow regulation by dams may cause relevant N cycling discontinuities in rivers. Higher net N uptake capacity below dams could lead to reduced N export to downstream ecosystems. Incorporating these discontinuities could significantly improve predictive models of N cycling and transport in complex river networks.

Watershed delineation using hydrographic features and a DEM in plain river network region

Watershed delineation is a required step when conducting any spatially distributed hydrological modelling. Automated approaches are often proposed to delineate a watershed based on a river network extracted from the digital elevation model (DEM) using the deterministic eight-neighbour (D8) method. However, a realistic river network cannot be derived from conventional DEM processing methods for a large flat area with a complex network of rivers, lakes, reservoirs, and polders, referred to as a plain river network region (PRNR). In this study, a new approach, which uses both hydrographic features and DEM, has been developed to address the problems of watershed delineation in PRNR. It extracts the river nodes and determines the flow directions of the river network based on a vector-based hydrographic feature data model. The river network, lakes, reservoirs, and polders are then used to modify the flow directions of grid cells determined by D8 approach. The watershed is eventually delineated into four types of catchments including lakes, reservoirs, polders, and overland catchments based on the flow direction matrix and the location of river nodes. Multiple flow directions of grid cells are represented using a multi-direction encoding method, and multiple outflows of catchments are also reflected in the topology of catchments. The proposed approach is applied to the western Taihu watershed in China. Comparisons between the results obtained from the D8 approach, the ‘stream burning’ approach, and those from the proposed approach clearly demonstrate an improvement of the new approach over the conventional approaches. This approach will benefit the development of distributed hydrological models in PRNR for the consideration of different types and multiple inlets and outlets of catchments. Copyright © 2015 John Wiley & Sons, Ltd.

Hydrological projections based on the coupled hydrological–hydraulic modeling in the complex river network region: a case study in the Taihu basin, China

Water resources in the Taihu basin, China, are not only facing the effects of a changing climate but also consequences of an intensive urbanization process with the abandonment of rural activities and the resulting changes in land use/land-cover. In the present work, the impact of climate change and urbanization on hydrological processes was assessed using an integrated modeling system, coupling the distributed hydrological model variable infiltration capacity and the hydraulic model ISIS, while future climate scenarios were projected using the regional climate model providing regional climate for impact studies. Results show a significant increasing trend of impervious surface area, while other types of land cover exhibit decreasing trends in 2021–2050. Furthermore, mean annual runoff under different future climate scenarios will increase, especially during flood seasons, consistent with the changes in precipitation and evapotranspiration for both spatial and temporal distribution. Maximum and mean flood water levels under two future scenarios will be higher than levels under the baseline scenario (1961–1990), and the return periods of storms resulting in the same flood water level will decrease significantly in comparison to the baseline scenario, implying more frequent occurrence of extreme floods in future. These results are significant to future flood control efforts and waterlog drainage planning in the Taihu basin.

Calibrating pollutant dispersion in 1-D hydraulic models of river networks

The objective of this article is to investigate the major issues associated with the calibration of the pollutant dispersion in 1-D hydraulic models applied to river networks, especially large, complex, artificialized ones where ecological and socio-economical threats are important. Such issues are illustrated and discussed using the results of five fluorescent tracer experiments conducted in contrasted open-channel systems, ranging from a simple trapezoidal canal to a more complex river network. Experimental dispersion values were quantified using both the change of moment method and a simple fit-by-eye procedure for eight river reaches with homogeneous hydraulic conditions and an achieved tracer mixing and dispersive equilibrium. Since dispersion coefficient values depend on the assumed dispersion model, ideally they should be calibrated using the same model in which they are to be used, as was done in this study. We also derived concurrent longitudinal dispersion values using the velocity field measured by hydro-acoustic profilers (ADCP), which appears as a promising and cost-efficient technique for documenting dispersion in large river systems. It appears that the formulae for which the fit was mainly based on the cross-sectional aspect ratio are generally more appropriate for field data than those which are sensitive to the velocity to shear velocity ratio. The interpretation of complex dispersion and mixing processes, along with the selection of relevant dispersion coefficient predictors are key to minimizing errors in the numerical simulation of pollution dynamics in river networks.

Stochastic vs deterministic programming in water management: the value of flexibility

In the paper we develop a two stage scenario-based stochastic programming model for water management in the Indus Basin Irrigation System (IBIS). We present a comparison between the deterministic and scenario-based stochastic programming model. Our model takes stochastic inputs on hydrologic data i.e. inflow and rainfall. We divide the basin into three rainfall zones which overlap on 44 canal commands. Data on crop characteristics are taken on canal command levels. We then use ten-daily and monthly time intervals to analyze the policies. This system has two major reservoirs and a complex network of rivers, canal head works, canals, sub canals and distributaries. All the decisions on hydrologic aspects are governed by irrigation and agricultural development policies. Storage levels are maintained within the minimum and maximum bounds for every time interval according to a power generation policy. The objective function is to maximize the expected revenue from crops production. We discuss the flexibility of two stochastic optimization models with varying time horizon.

Macrobenthos Diversity Response to Hydrological Connectivity Gradient

The abundant riparian wetlands supported by the complex river network in Pearl River Delta (PRD) are under an intensely anthropogenic disturbance that threatens the local ecological environment. This paper examined the benthic invertebrate diversity and biomass within the Pearl River through a field study. A fieldwork was conducted to assess a hydrological connectivity gradient among the main channel of the Pearl River. The responses of macroinvertebrate metrics to the connectivity gradient and the restoration history in the river channel were examined. The Shannon diversity index ranged from 0.75 to 2.08, which was the highest among most connectivity river channels. Statistical analyses (PCA and CCA) revealed that the benthic abundance in high hydrological connectivity riverine habitats is clearly distinct from the low connectivity habitats. Results demonstrated that the diversity and density of macrobenthos are determined by hydrological connectivity and time span of the restoration (river channels). Therefore, conservation and restoration efforts should focus on the macroinvertebrate response to the hydrological connectivity of the river channels in PRD.