Entire River System Research Articles

The role of spatial units in modelling freshwater fish distributions: Comparing a subcatchment and river network approach using MaxEnt

Species Distribution Models (SDM) are frequently used in ecological research, but the effects of model extent and granularity on model outcomes are rarely addressed. In freshwater SDMs, two different approaches are commonly used to define the granularity of the models, i.e. to subdivide entire river systems into appropriate spatial modelling units: river reaches on the actual river network or subcatchments. We built maximum entropy (MaxEnt) SDMs for barbel (Barbus barbus) and grayling (Thymallus thymallus) in the River Ruhr catchment in Germany with identical environmental predictor data and analysed the difference between these two approaches. All models performed well (AUC > 0.9) and geographic predictors (e.g. distance to the river source) dominated over hydromorphological predictors in explaining species distributions. There was high agreement between the two model setups in river parts of low and high habitat suitability but considerably lower agreement in river parts of medium habitat suitability. In these medium suitable river parts, model results were spatially very heterogeneous and alternated at fine spatial scales especially in models based on river reaches. Increasing model regularization, a setting to control overfitting, had a smoothing effect on the environmental variables in the river reach models similar to the coarser subcatchment granularity. A restriction of the spatial extent led to a shift in predictor contributions to the model and an increase of the importance of hydromorphologic predictors by ca. 45 %. Restricting the model extend to the natural core distributional area of a given fish species might therefore be considered beneficial for the application of SDMs in a management context. By decreasing the weight of fixed geographic predictors in the model, predictors relating to hydromorphological river structures (i.e. which are accessible for restoration projects) gain importance. We conclude that SDM setups based on river reaches and subcatchments can both give valuable complementary information about the distribution of species. The high-resolution model based on river reaches might better discover individual local habitat features, whereas the subcatchment model might better account for the minimum spatial requirements of a fish population.

Changing runoff due to temperature and precipitation variations in the dammed Jinsha River

Impacts of climatic variations and human activities on water discharge in the lower streams of dammed rivers have been widely studied due to the rich water resources and hydropower potential. However, the effects of climate variations in upper streams on the whole river system are always ignored. Therefore, integrated methods of hydrological modelling and numerical analysis were employed to identify runoff characteristics in response to climatic variations in conjunction with hydroelectric exploitation in the Jinsha River in the “Asian Water Tower” region. While there was no significant rise in annual precipitation for most areas from 1970 to 2017, highly significant upward trends in temperature were observed in areas along the mainstream. Snowmelt periods varied between upper and middle streams due to constantly varying geographic conditions. Peaking runoff of 56.67 m3 s−1, 1350.12 m3 s−1, 175.63 m3 s−1, 2343.89 m3 s−1, 1855.02 m3 s−1, 3285.61 m3 s−1, and 10,922.65 m3 s−1 occurred during July in Tuotuohe, Qingshuihe_Jinsha, Qingshuihe_Yalong, Batang, Kangding, Xianggelila, and Yibin, respectively, along the Jinsha River. During snowmelt periods, runoff in the upper stream was greatly influenced by temperature changes. However, during recent periods, runoff in the middle stream has also become sensitive to rising temperatures, as indicated by change points. The contribution of runoff from upper streams to the outlet increased from 10.57% (naturalised river period) to 13.01% (individual reservoir period), followed by a further increase of 13.37% (cascade reservoirs period). Seasonal differences were observed up to the middle streams, where more runoff occurred at the outlet during flood seasons due to hydroelectric exploitation after 1999. Annually, runoff contribution in different areas increased in response to climatic variations during 1999–2017. This study highlights the vulnerability of runoff variations in upper streams to climate change. Watershed management rather than river regulation alone is needed, which could help water resources management for the entire river system.

Estimating the eco-toxicological risk of estrogens in China's rivers using a high-resolution contaminant fate model

The contamination of freshwater systems arises in many river basins due to industrialization and population growth, posing risks to ecosystems and human health. Despite these concerns, the fate and potential impact of many emerging pollutants are currently unknown, especially when the compounds are released into surface waters from populations distributed across large spatial scales. In order to address this shortcoming, a spatially-explicit contaminant fate model was developed as an extension of the global, vector-based river routing model HydroROUT. HydroROUT operates at very high spatial resolution (∼500 m), simulates river and stream chemical transport with in-stream removal, and contains links to a set of lakes and reservoirs, which act as a partial sink during the transport. The chemical fate model was applied to China and includes a consumption and release module based on county-level population demographics, considers point-source contributions from 2547 wastewater treatment plants, and accumulates contributions of rural and urban populations not connected to sewage treatment plants. As a case study, the sources and fates of the estrogens estrone (E1), 17β-estradiol (E2), estriol (E3), as well as the synthetic estrogenic steroid hormone 17α-ethinylestradiol (EE2) were modelled in Chinese surface water bodies. Preliminary validation of the results showed predictions to be within the ranges of concentrations reported in literature, with exception of EE2. The total estrogenic mass in the entire river and lake system amounted to 370 tonnes of estrogens, with about 1.3 tonnes per year discharged to the ocean, neighboring countries or to inland sinks. Under a selected baseline scenario, eco-toxicological risk—i.e., contaminant concentrations in excess of the predicted no effect concentration (PNEC)—is found in 23.6% of all analyzed rivers of China with an average flow > 0.1 m3/s. Out of these, about 4% of rivers showed a high level of risk of 10 times or more above PNEC. Medium-to-large rivers are disproportionally affected; for example, 23.6%, 37.3%, 29.0% and 21.6% of river length are at risk in rivers of 1–10, 10–100, 100–1,000, and 1,000–10,000 m3/s of discharge, respectively, whereas no risk was predicted in the largest rivers (i.e., >10,000 m3/s) of China. Wastewater treatment plants process 22.5% of the total hormone load and thus play an important role in water quality control by reducing the risk in substantial portions of the river network, which would otherwise show elevated risk. Releases from untreated population dominate by far the overall contribution to risk.

The importance of sediments in ecological quality assessment of stream headwaters: embryotoxicity along the Nidda River and its tributaries in Central Hesse, Germany

BackgroundAlthough the crucial importance of sediments in aquatic systems is well-known, sediments are often neglected as a factor in the evaluation of water quality assessment. To support and extend previous work in that field, this study was conducted to assess the impact of surface water and sediment on fish embryos in the case of a highly anthropogenically influenced river catchment in Central Hesse, Germany.ResultsThe results of 96 h post fertilisation fish embryo toxicity test with Danio rerio (according to OECD Guideline 236) revealed that river samples comprising both water and sediment exert pivotal effects in embryos, whereas surface water alone did not. The most prominent reactions were developmental delays and, to some extent, malformations of embryos. Developmental delays occurred at rates up to 100% in single runs. Malformation rates ranged mainly below 10% and never exceeded 25%.ConclusionA clear relationship between anthropogenic point sources and detected effects could not be established. However, the study illustrates the critical condition of the entire river system with respect to embryotoxic potentials present even at the most upstream test sites. In addition, the study stresses the necessity to take into account sediments for the evaluation of ecosystem health in industrialised areas.

Coordinated operations of multiple-reservoir cascaded hydropower plants with cooperation benefit allocation

The coordinated operations of multiple-reservoir cascaded hydropower plants provide opportunities to increase the benefits of the entire river system. However, it is very challenging to fairly allocate the incremental benefits of cooperation among all participant hydropower plants, which is critical to the implementation of operation polices in practice. A methodology that combines POA-DDDP–based multidimensional search algorithm(PDMSA) with game theory is proposed to address this challenge. The PDMSA is developed to determine optimal operation decisions and obtain the multi-yearly average revenue under all possible coalitions of plants. Thus, the cooperation benefit can be accurately calculated based on the differences of generation production revenue among various coalitions. The game-theoretic Shapley method is used to find the appropriate share of each cooperating plant from overall cooperation benefits. The cooperative core based on a set of necessary conditions helps select possibly stable allocation schemes, and their stability is evaluated by the propensity to disrupt(PTD). The proposed methodology is applied to a multiple-reservoir hydropower system on Lancang River, which is one of 14 large hydropower bases in China. This case shows that the method provides the most stable incremental allocation scheme by comparison with several commonly used methods.

A geomorphological characterisation of river systems in South Africa: A case study of the Sabie River

Fluvial geomorphology affects river character, behaviour, evolution, trajectory of change and recovery potential, and as such affects biophysical interactions within a catchment. Water bodies in South Africa, in common with many other water-stressed parts of the world, are generally under threat due to increasing natural and anthropogenic influences including aridity, siltation and pollution, as well as climate and environmental change. This study reports on a case study to characterise the geomorphology of different river systems in South Africa, with the aim of better understanding their properties, controls, and implications for biophysical interactions including water quality, biodiversity (aquatic and riparian), and human activity within the catchment. The approach adopted is based on the River Styles® framework (RSF), a geomorphology-based approach developed for rivers in New Zealand and Australia, but applied here for the first time to South Africa. Based on analysis of remote sensing imagery, SRTM-2 digital topographic data and field observations on sites through the entire river system, six geomorphic elements were identified along the Sabie River, northeast South Africa (gorge, bedrock-forced meander, low-moderate sinuosity planform controlled sand bed, meandering sand bed, low sinuosity fine grained sand bed, and floodouts), using the RSF classification scheme and based on the RSF procedural tree of Brierley and Fryirs (2005). Previous geomorphological studies along the Sabie River have shown that different reaches respond differently to episodic floods; we use these data to link river geomorphological character (as defined by the RSF) to the hydrodynamic conditions and processes giving rise to such character. This RSF approach can be used to develop a new management approach for river systems that considers their functional biophysical behaviour within individual reaches, rather than considering them as homogeneous and uniform systems.

Spatial and temporal variability in the nutritional quality of basal resources along a temperate river/estuary continuum

The nature and quality of basal resources within aquatic food webs are complex and have the potential to shift over space and time. We used fatty acid analysis to assess variations in the nutritional structure of the suspended and basal pools along an entire river system, and to assess the contributions of vascular plant (i.e., mainly of terrestrial origin, but could include aquatic macrophytes) vs algal (i.e., aquatic origin) sources to the suspended particulate matter (SPM) pools. Samples were collected in a temperate South African river on four occasions between September 2012 and June 2013. We found orderly patterns in the fatty acid composition of the basal resources at the sites during most seasons. Regardless of site or season, the benthic algal pools (epiphyton, epipelon and epilithon) were the most nutritionally rich resources based on essential fatty acid contents and diatom indices. During early and late spring, proportions of essential fatty acids in the epiphyton decreased downstream where increased light was available, consistent with predictions from the light:nutrient hypothesis (but inconsistent with epilithon and epipelon results). There were substantial changes in vascular plant contributions to the SPM pools along the river, but the same patterns were not produced in all seasons. Of all the river models considered, the data were more consistent with the riverine productivity model, which particularly emphasises the importance of autotrophic production in rivers. Our study provides new detail on the complexity of basal resource nutritional quality and how it can shift along a lotic system over time.

Sources of nitrogen heterocyclic PAHs (N-HETs) along a riverine course

For the first time the occurrence of 26 nitrogen heterocyclic polyaromatic hydrocarbons (N-HETs) and the antiepileptic drug carbamazepine (CBZ) as a marker for anthropogenic influence was determined in an entire river system and its influents. Besides the investigation of diurnal and annual variations in concentrations, dilution and load calculations, the aims were to facilitate transferability to other rivers by identifying sources of the different substances. To create a sufficient database over 180 aqueous samples from the River Fuhse (Lower Saxony, Germany), its catchment area, effluents of municipal and industrial wastewater treatment plants (WWTPs) and rainwater were analyzed. Typical substances found in background water samples and rain samples were quinoline and isoquinoline, whereas 1-/3-methylisoquinoline were traced back to an industrial WWTP. Due to similar concentration levels in the effluents of municipal WWTPs, their impact on the river correlated with their nominal loads. In municipal WWTP effluents CBZ (cmedian=1693ngL−1), 2-methylquinoline (cmedian=64ngL−1), and acridine (cmedian=62ngL−1) prevailed. Although the occurrence of N-HETs at contaminated sites is a widely discussed scientific topic, this study showed that the investigated site was a source with only small volumetric contribution to N-HET concentrations in the adjacent river. In total the River Fuhse discharges 20kga−1 N-HETs and 21kga−1 CBZ into the receiving river.

Modeling of River Width Variations Based on Hydrological, Morphological, and Biological Dynamics

AbstractOne-dimensional (1D) models can be applied, for engineering purposes, to long-term and large-scale morphodynamic simulations of entire river systems only if appropriate simplifications are introduced. This paper proposes an improvement of an existing simplified 1D model based on the local uniform flow hypothesis, coupled with a synthetic description of the transverse profile, which provides the active river width by analyzing the total and the vegetated widths of a watercourse, with assumed variables of the water flow. The overall density of the riparian vegetation, expressed in terms of biological carrying capacity, is predicted as a function of the local climate and some stresses due to interaction between hydrology, morphology, and biology. The constitutive equations have been deduced and the relevant parameters have been calibrated and validated against various hydrological and geometrical data, taken by satellite imagery covering two large watercourses located in tropical and subtropical area...

水位低下による土砂スルーシング時の超音波を用いた SS 濃度と粒度分布の連続計測

In recent years, ultrasonic attenuation spectroscopy has been frequently used for the measurement of suspended sediment concentration and particle size distribution. Sediments are derived from the rock masses at the upper basin and transported downstream owing to floods, and then deposited in the reservoir. The reservoir sediments affect the entire river system because the sediment transport is blocked by dams. For this reason, sediment sluicing has been performed in several rivers. In such cases, it is important to monitor the suspended sediment concentration and particle size distribution. Therefore, we have measured the suspended sediment concentration and particle size distribution using an ultrasonic spectrometer during sediment sluicing by drawdown operation. The applicability of our measurement technology using ultrasonic attenuation spectroscopy for monitoring of suspended sediment concentration and particle size distribution was verified.

Synthetic events for flood risk calculation by using a nested Copula structure

Risk analysis requires considering the entire frequency domain of flood consequences. Synthetic events were generated for the entire river system of the Scheldt estuary. This estuary contains multiple navigable waterways and is situated in Belgium and the Netherlands. Extreme water levels are influenced by rainfall-runoff discharges, tiding, storm surges, and wind speed and direction. For the generation of hydraulic boundary conditions for flood risk assessment, these influences and their mutual dependencies and correlations are taken into account by means of a nested extreme value copula structure. The variation in time is taken into account by standardized profiles, computed by normalizing all recorded extreme events and fitting a probability distribution to the variation of the standardized events, yielding 5 profile classes through another stratification. Eventually this resulted in a total of 1920 sets of synthetic events. All events were run through the hydrodynamic model of the river system. The frequency distribution of the resulting water levels are calculated by accumulation of the corresponding probabilities of occurrence of the synthetic events at each location. The methodology has the advantage that it determines a statistical distribution of consequences, rather than assigning frequencies to hydrodynamic boundary conditions.

TMDL development for the Taihu Lake’s influent rivers, China using variable daily load expressions

Influent river carrying nutrient pollutants from watershed loads makes a great contribution to the eutrophication in river-fed lake. It is scientific standard to make policies on river pollution control based on loading capacity of the river of interest. To control eutrophication in Taihu Lake has been the focal point of “The Twelfth Five-Year Guideline” proposed by Chinese government. The Taigeyunhe, Caoqiaohe and Yincungang Rivers which were the most polluted influent rivers in Taihu Lake Basin were scheduled for nutrient total maximum daily load (TMDL). A variety of mechanistic and empirical models are applied worldwide for TMDL development. However, model selection depends on management objectives, site-specific characteristics and availability of data resource. In this study, based on watershed characteristics and limited data, a nutrient TMDL is developed using flow and temporally variable daily load expressions. The simple and effective approaches specify allowable daily maximum loads for controlling on instantaneous high load and allowable daily median loads for achieving long-term TMDL allocation. For the entire river system, loading capacities are much lower during low flows. The maximum percent load reductions for biochemical oxygen demand, ammonia nitrogen, total nitrogen in spring and total phosphorus in winter can be obtained when pollution source inputs seasonally vary. This study provides local authority with two different alternatives in decision-making for pollution control on influent rivers and then to reduce external loads to the lake.

Assessing the impacts of climate change and socio-economic changes on flow and phosphorus flux in the Ganga river system.

Anthropogenic climate change has impacted and will continue to impact the natural environment and people around the world. Increasing temperatures and altered rainfall patterns combined with socio-economic factors such as population changes, land use changes and water transfers will affect flows and nutrient fluxes in river systems. The Ganga river, one of the largest river systems in the world, supports approximately 10% global population and more than 700 cities. Changes in the Ganga river system are likely to have a significant impact on water availability, water quality, aquatic habitats and people. In order to investigate these potential changes on the flow and water quality of the Ganga river, a multi-branch version of INCA Phosphorus (INCA-P) model has been applied to the entire river system. The model is used to quantify the impacts from a changing climate, population growth, additional agricultural land, pollution control and water transfers for 2041-2060 and 2080-2099. The results provide valuable information about potential effects of different management strategies on catchment water quality.

Operational multi-sensor monitoring of turbidity for the entire Mekong Delta

An operational satellite-based approach was implemented to monitor turbidity and organic absorption in the Mekong river system. Using physics-based algorithms linked together in a fully automated processing chain, more than 300 Landsat Enhanced Thematic Mapper (ETM) scenes and 1000 MODIS scenes, representing five years of data, were used to produce standardized, quantitative time series of turbidity and organic absorption across Vietnam, Thailand, Cambodia, Laos, and China. To set up this system, the specific inherent optical properties (SIOPs) of the Mekong river system were determined through three separate field campaigns, laboratory analysis, and subsequent optical closure calculations. Following this, a range of satellite data types was tested using the derived Mekong-specific inherent optical properties, including Moderate Resolution Imaging Spectroradiometer (MODIS) 500 m data, Landsat ETM, Medium Resolution Imaging Spectrometer (MERIS), Satellite Pour l’Observation de la Terre (SPOT) 5, RapidEye, Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), and QuickBird. The satellite-based turbidity estimates were coincident with available field data, and comparisons showed them to be in good agreement. Overall, the derived SIOPs were suitable for water-quality monitoring of the Mekong, and the MODIS, MERIS, Landsat, and RapidEye sensors were found to be the most radiometrically stable and thereby suitable for ongoing operational processing. The implemented system delivers consistent results across the different satellite sensors and over time, but is limited to where the spatial resolution of the sensor is still able to resolve the river width. The system is currently applicable for the entire Mekong river system, both for near-real-time monitoring and for analysis of historical data archive.

A Riverscape Analysis Tool Developed to Assist Wild Salmon Conservation Across the North Pacific Rim

ABSTRACTA major constraint for management and conservation of wild salmon is the large geographic area and diversity of rivers that provide critical freshwater habitats for salmon production and sustainability. These habitats span lengths of entire river systems, crossing international borders and management jurisdictions, while encompassing a range of climate and landscape conditions and human impacts. We developed the Riverscape Analysis Project (RAP) to provide a consistent and comprehensive geospatial database to document, assess, and compare the physical habitats of large salmon rivers of the North Pacific Rim (NPR). Here, we introduce and summarize a web‐based GIS and decision support system (DSS) to assist salmon conservation around the NPR. The foundation of the RAP database is a seamless mosaic of moderate (30 m) resolution, multispectral satellite imagery from the Landsat TM instrument series, mapped with coincident 90‐m resolution digital terrain Digital Evaluation Model (DEM) information to a consistent global projection; these data produced a set of watershed, river, and floodplain physical features and derived riverine freshwater habitat metrics important for salmon. The RAP DSS is publicly available online (http://rap.ntsg.umt.edu) and includes user‐friendly tools and tutorials to allow users to compare, query, and download geospatial summary data across a suite of physical metrics.

Design of sampling locations for mountainous river monitoring

River water quality monitoring data are essential to water basin development and decision making. The location of sampling stations is one of the most critical factors in monitoring network design. Most previous studies have focused on mathematical aspects of monitoring network design, whereas human activities within a river basin are often not considered comprehensively. In this study, a new water monitoring network design procedure is introduced to identify representative river water quality sampling locations. The new procedure combines river mixing length, human activities, and geographic information systems (GIS) to locate position of sampling points. An implementation of a new potential pollution score (PPS) model of land use helps to classify the importance of each sampling point prior to selecting the most appropriate locations for an entire river system. With the study area, urban and agriculture are mainly concentrated in the downstream area and were the primary non-point pollution sources in this region. Seven sampling points are proposed to monitor river water quality in the study area.

Survival of Migrating Atlantic Salmon Smolts through the Penobscot River, Maine: a Prerestoration Assessment

AbstractSurvival, distribution, and behavior of hatchery (n = 493) and naturally reared (n = 133) smolts of Atlantic salmon Salmo salar migrating through the Penobscot River and estuary in Maine were evaluated with acoustic telemetry in 2005 and 2006. Survival and use of a secondary migration path (the Stillwater Branch) were estimated with a multistate mark–recapture model. Higher rates of mortality per kilometer (range = 0.01–0.22) were observed near release sites and within reaches that contained three particular dams: Howland, West Enfield, and Milford dams. Estimated total survival of tagged hatchery smolts through entire individual reaches containing those dams ranged from 0.52 ( 0.18) to 0.94 ( 0.09), whereas survival through most of the reaches without dams exceeded 0.95. Of those smolts that survived to the Penobscot River–Stillwater Branch split at Marsh Island, most (≥74%) remained in the main stem around Marsh Island, where they experienced lower survival than fish that used the Stillwater Branch. Movement rates of hatchery‐reared smolts were significantly lower through reaches containing dams than through reaches that lacked dams. Smolts arriving at dams during the day experienced longer delays than smolts arriving at night. Planned removal of two dams in this system is expected to enhance the passage of smolts through the main‐stem corridor. However, the dams currently scheduled for removal (Great Works and Veazie dams) had less influence on smolt survival than some of the dams that will remain. This case study shows that by examining prerestoration migration dynamics throughout entire river systems rather than just in the vicinity of particular dams, tracking studies can help prioritize restoration efforts or predict the costs and benefits of future hydrosystem changes.

Spatio-temporal variations of nitrogen in an agricultural watershed in eastern China: Catchment export, stream attenuation and discharge

Using the monthly hydrogeochemical data of ChangLe River system from 2004 to 2008, total nitrogen (TN) export load (Sn) from nonpoint sources (NPS) to stream and in-stream attenuation load (AL) was estimated by the inverse and forward format of an existing in-stream nutrient transport equation, respectively. Estimated Sn contributed 96±2% of TN entering the river system, while AL reduced the input TN by 23±14% in average. In-stream TN attenuation efficiency in high flow periods (10±5% in average for the entire river system) was much lower than that in low flow periods (39±17%). TN attenuation efficiency in tributaries (28±16% in average) was much higher than that in mainstream (11±8%). Hydrological conditions are important in determining the spatio-temporal distributions of NPS TN export, stream attenuation and discharge. Increasing the water residence time might be a practical method for mitigating stream TN.

Assessing spatial and temporal connectivity between surface water and groundwater in a regional catchment: Implications for regional scale water quantity and quality

It is common for surface water–groundwater interaction assessments to investigate river reaches at a local scale and as discrete individual systems, which are generally classified as connected (gaining and losing type systems) or disconnected (transitional or completely disconnected type systems). While these classifications are valid at any point in space and time, studies often fail to consider how individual river reaches function in the context of the entire regional river system (comprising multiple river reaches) and what implications this can have on water quantity and quality. In this study, spatial and temporal assessments were made in a regional catchment using hydraulic, hydrochemical, and tracer-based techniques to determine the source and loss terms of the river and groundwater system and how their relative magnitude changes along the river from the catchment headwaters towards the sea. Applying an entire regional river system assessment we demonstrate that the state of connection can change along river reaches, as well as take place concurrently at the same location. Water level data, together with salinity and stable isotope results showed that the relatively low salinity of the fresh water river system was maintained in an otherwise saline regional groundwater system by virtue of the lack of saline groundwater inputs in a river with a dominantly losing connectivity state. This losing state was strongly influenced by the high evapotranspiration of the native vegetation. By determining the state of connection between surface water and groundwater and understanding the variable and complex nature of contiguous river reaches of an entire regional river system more appropriate management practices can be employed.

Combined inverse modeling approach and load duration curve method for variable nitrogen total maximum daily load development in an agricultural watershed

Nonpoint sources (NPS) pollution has been an important cause for water quality impairment worldwide. To take the temporal variations of both NPS pollution and in-stream attenuation into consideration, an inverse modeling approach and the load duration curve (LDC) method were combined for variable nutrient total maximum daily load (TMDL) development. Water quality and hydrological parameters were monitored monthly along the ChangLe River system in 2004-2008. The catchment NPS export load (EL) and TMDL for total nitrogen (TN) were estimated by the inverse format of an existing stream nutrient transport equation. The LDC method was used to describe the variability of EL, TMDL, requiring load (RLR) and percent (the ratio between the RLR and the EL, RPR) reduction, and then to set the variable requiring reductions under different uncertainties. Although both EL and TMDL for TN increased with stream flow, the increments of EL became larger than that of TMDL with increasing stream flow. Thus, RLR also increased with stream flow. The contribution of in-stream attenuation capacity for TN TMDL, which decreased with stream flow, occupied 37.3 ± 10.4% of the TMDL for the entire river system. To assure 90% compliance with the target in-stream TN level, the RLR and RPR was 1.16 × 10(3)-19.02 × 10(3)kgday(-1) and 53.6-59.9% for different flow regimes, respectively. For the NPS pollution-dominated watershed, temporal variable expressions of TMDL and requiring reduction are both necessary. This combined approach provides researchers and managers with a simple but efficient tool for variable TMDL development.