Load Apportionment Model Research Articles

Quantifying groundwater phosphorus flux to rivers in a typical agricultural watershed in eastern China.

Increasing evidence indicates that groundwater can contain high dissolved phosphorus (P) concentrations, thereby contributing as a potential pollution source for surface waters. However, limited quantitative knowledge is available concerning groundwater P fluxes to rivers. Based on monthly hydrochemical monitoring data for rivers and groundwater in 2017-2020, this study combined baseflow separation methods and a load apportionment model (LAM) to quantify contributions from point sources, surface runoff, and groundwater/subsurface runoff to riverine P pollution in a typical agricultural watershed of eastern China. In the studied Shuanggang River, most total P (TP) and dissolved P (DP) concentrations exceeded targeted water quality standards (i.e., TP ≤ 0.2mg P L-1, DP ≤ 0.05mg P L-1), with DP (76 ± 20%) being the major riverine P form. Observed DP concentrations in groundwater were generally higher than those of river waters. There was a strong correlation between river and groundwater P concentrations, implying that groundwater might be a considerable P pollution source to rivers. The nonlinear reservoir algorithm estimated that baseflow/groundwater contributed 66-68% of monthly riverine water discharge on average, which was consistent with results estimated by an isotope-based sine-wave fitting method. The LAM incorporating point sources, surface runoff, and groundwater effectively predicted daily riverine TP [calibration: coefficient of determination (R2) = 0.76-0.82, Nash-Sutcliffe Efficiency (NSE) = 0.61-0.77; validation: R2 = 0.88-0.98, NSE = 0.54-0.64] and DP loads (calibration: R2 = 0.73-0.84, NSE = 0.67-0.72; validation: R2 = 0.88-0.97, NSE = 0.56-0.83). The LAM estimated point source, surface runoff, and groundwater contributions to riverine loads were 15-18%, 14-35%, and 46-70% for TP loads and 7-9%, 10-32%, and 59-82% for DP loads, respectively. Groundwater was the dominant riverine P source due to long-term accumulation of P from excess fertilizer and farmyard manure applications. The developed methodology provides an alternative method for quantifying P pollution loads from point sources, surface runoff, and groundwater to rivers. This study highlights the importance of controlling groundwater P pollution from agricultural lands to address riverine water quality objectives and further implies that decreasing fertilizer P application rates and utilizing legacy soil P for crop uptake are required to reduce groundwater P loads to rivers.

Evaluating diffuse and point source phosphorus inputs to streams in a cold climate region using a load apportionment model

Variation in the timing and quantity of diffuse versus point-source inputs of phosphorus (P) to streams can be evaluated by examining P concentration-flow relationships. Diffuse load inputs usually increase with stream flow (due to increased delivery caused by precipitation); whereas, point-source concentrations decrease with rising river flow (due to increased dilution). This study tested the suitability of a load apportionment model (LAM), a power-law function of flow, to estimate contributions of diffuse and point inputs to P loads for eight sub-watersheds in the Red River Valley, a cold-climate rural region of Manitoba, Canada. For all but two sub-watersheds, annual and seasonal (snowmelt and summer) models of P concentration versus flow best fit a strictly diffuse source contribution. The models identified significant point-source inputs (in addition to diffuse sources) in two sub-watersheds, during summer in both watersheds (consistent with the fact that wastewater from sewage lagoons is discharged to upstream reaches between June and September) and during snowmelt for one watershed. Application of a LAM proved to be a simple and rapid method for nutrient source apportionment as well as detection of unknown sources for cold-climate, rural sub-watersheds. Such information is critical for developing the most effective mitigation strategies to reduce P concentrations and eutrophication risk.

Loose One-Way Coupling of Land Use and Nutrient Emission Models to Assess Effects of Regional Development Scenarios on Catchment Water Quality

Nutrient enrichment and eutrophication can increase when urbanisation and intensification of agriculture production occurs without accompanying mitigation measures to offset impacts from land use transitions. The identification of measures to protect or restore water quality is a challenging exercise, particularly in the context of increasing population and urbanisation. Hence, decision-makers need adequate tools to better understand and evaluate the effects of policy interventions on water management and quality control in urban regions. A model integration method was developed to assess future scenarios of urban development on water quality using land use model Monitoring Land Use/Cover Dynamics and nutrient emission model Source Load Apportionment Model. A case study application investigated how projected changes in urban land use in the most populated region of Ireland, the Greater Dublin Region, impact on water quality. Results for all scenarios indicate increasing losses to water for both phosphorus (18–25%) and nitrogen (9–12%). However, as these scenarios assume that wastewater treatment efficiencies remain static into the future, this study highlights that the magnitude and distribution of investment in urban wastewater collection and treatment will have the greatest impact on changes in future nutrient emissions to water in this urban region. The differences between the development scenarios for diffuse nutrient losses were small in comparison, even though the location of specific land uses varied broadly across scenarios. It was found that the decline of agricultural land cover and replacement with urban development in Dublin region by 2026 as represented by four modelled scenarios resulted in substantial increases in diffuse phosphorus emissions, but only slight changes in diffuse nitrogen emissions. It was shown that the scales of impact from sources of nutrients vary from scenario to scenario and that these should be considered alongside planned mitigation of point sources of nutrient emissions to water. Such information can support physical planners, catchment managers and policy makers to plan accordingly to get the best possible environmental outcomes. The case study application demonstrated that the loose one-way coupling of a land use model and a nutrient emission model can be an effective and inexpensive approach to improve understanding of the effects of urbanisation on water quality and assist in the strategic planning of catchment management and infrastructure investment.

Developing a comprehensive framework for eutrophication management in off-stream artificial lakes

In this paper, a comprehensive and interdisciplinary framework for management of eutrophication in off-stream artificial lakes in semi-arid and arid regions is proposed. Identification of the lake’s water resources system components and stakeholders, simulation of Phosphorus (P) export from upstream watershed, simulation of the lake water quality as well as simulation of water demands and supply, development of management scenarios for the lake and selecting the best scenario using social choice methods (i.e. discrete and fuzzy Borda counts) are the four main parts of the framework. The proposed framework is applied on Chitgar Artificial Lake (ChAL), the largest intra-urban artificial lake in Tehran which has been constructed in 2010–2013 for recreational purposes. The Load Apportionment Model (LAM) is used for the simulation of P loads from the point and non-point (diffusive) sources and the LakeMab model is used for the simulation of P dynamics in the lake. The management scenarios contain optimized rule curves for water intake/outtake blended with P management plans (i.e. removal of point sources of P load in the upstream watershed, construction of a hydroponic bio-filter or an advanced water treatment plant beside the lake for reduction of external loading of P and recycling lake water, alum treatment of lake sediments for controlling the internal loading of P as well as construction of a dry detention basin). The most preferred scenarios selected by the discrete Borda count are the low-cost alum treatment and dry detention basin, while the most preferred scenario according to fuzzy Borda count, which considers the uncertainty of model inputs, is the costly water treatment plant. In all preferred scenarios, water intake is conducted from flood flows in order to avoid conflict with downstream agricultural demands. In addition to decentralized decision making and stakeholders’ participation, the proposed framework promotes the integration of the technical aspects such as the role of internal loading in lake eutrophication and separation of flood and non-flood flows in the off-stream lakes’ systems.

Non-domestic phosphorus release in rivers during low-flow: Mechanisms and implications for sources identification

A common assumption in phosphorus (P) load apportionment studies is that P loads in rivers consist of flow independent point source emissions (mainly from domestic and industrial origins) and flow dependent diffuse source emissions (mainly from agricultural origin). Hence, rivers dominated by point sources will exhibit highest P concentration during low-flow, when flow dilution capacity is minimal, whereas rivers dominated by diffuse sources will exhibit highest P concentration during high-flow, when land-to-river hydrological connectivity is maximal. Here, we show that Soluble Reactive P (SRP) concentrations in three forested catchments free of point sources exhibited seasonal maxima during the summer low-flow period, i.e. a pattern expected in point source dominated areas. A load apportionment model (LAM) is used to show how point sources contribution may have been overestimated in previous studies, because of a biogeochemical process mimicking a point source signal. Almost twenty-two years (March 1995–September 2016) of monthly monitoring data of SRP, dissolved iron (Fe) and nitrate-N (NO3) were used to investigate the underlying mechanisms: SRP and Fe exhibited similar seasonal patterns and opposite to that of NO3. We hypothesise that Fe oxyhydroxide reductive dissolution might be the cause of SRP release during the summer period, and that NO3 might act as a redox buffer, controlling the seasonality of SRP release. We conclude that LAMs may overestimate the contribution of P point sources, especially during the summer low-flow period, when eutrophication risk is maximal.

Mapping eutrophication risk from climate change: Future phosphorus concentrations in English rivers

Climate change is expected to increase eutrophication risk in rivers yet few studies identify the timescale or spatial extent of such impacts. Phosphorus concentration, considered the primary driver of eutrophication risk in English rivers, may increase through reduced dilution particularly if river flows are lower in summer. Detailed models can indicate change in catchment phosphorus concentrations but targeted support for mitigation measures requires a national scale evaluation of risk.In this study, a load apportionment model is used to describe the current relationship between flow and total reactive phosphorus (TRP) at 115 river sites across England. These relationships are used to estimate TRP concentrations for the 2050s under 11 climate change driven scenarios of future river flows and under scenarios of both current and higher levels of sewage treatment.National maps of change indicate a small but inconsistent increase in annual average TRP concentrations with a greater change in summer. Reducing the TRP concentration of final sewage effluent to 0.5mg/L P for all upstream sewage treatment works was inadequate to meet existing P standards required through the EU Water Framework Directive, indicating that more needs to be done, including efforts to reduce diffuse pollution.

Sources of nitrogen and phosphorus emissions to Irish rivers and coastal waters: Estimates from a nutrient load apportionment framework

More than half of surface water bodies in Europe are at less than good ecological status according to Water Framework Directive assessments, and diffuse pollution from agriculture remains a major, but not the only, cause of this poor performance. Agri-environmental policy and land management practices have, in many areas, reduced nutrient emissions to water. However, additional measures may be required in Ireland to further decouple the relationship between agricultural productivity and emissions to water, which is of vital importance given on-going agricultural intensification.The Source Load Apportionment Model (SLAM) framework characterises sources of phosphorus (P) and nitrogen (N) emissions to water at a range of scales from sub-catchment to national. The SLAM synthesises land use and physical characteristics to predict emissions from point (wastewater, industry discharges and septic tank systems) and diffuse sources (agriculture, forestry, etc.). The predicted annual nutrient emissions were assessed against monitoring data for 16 major river catchments covering 50% of the area of Ireland. At national scale, results indicate that total average annual emissions to surface water in Ireland are over 2700tyr−1 of P and 82,000tyr−1 of N. The proportional contributions from individual sources show that the main sources of P are from municipal wastewater treatment plants and agriculture, with wide variations across the country related to local anthropogenic pressures and the hydrogeological setting. Agriculture is the main source of N emissions to water across all regions of Ireland. These policy-relevant results synthesised large amounts of information in order to identify the dominant sources of nutrients at regional and local scales, contributing to the national nutrient risk assessment of Irish water bodies.

Nutrient load apportionment to support the identification of appropriate water framework directive measures

A model for predicting the sources of nutrient loads (phosphorus and nitrogen) to water has been developed to support Water Framework Directive (WFD) implementation. The Source Load Apportionment Model (SLAM) framework described in this paper integrates catchment data and pressure information from point discharges and diffuse sources to enable characterisation of source–pathway–receptor relationships. Hydrogeological controls have a strong impact on nutrient fluxes, particularly in agricultural catchments, and have been incorporated into the diffuse agricultural model (the CCT). Results for the Suir catchment matched the measured loads of nitrogen and phosphorus well, and showed that pasture is the dominant source of nitrogen. The main sources of phosphorus in sub-catchments varied between diffuse agriculture, wastewater and industrial discharges. A relatively small proportion (13%) of the Suir catchment area requires a reduction in phosphorus emissions to achieve Good status. In these areas, model results can be used in conjunction with local knowledge gathered through the WFD characterisation process to identify significant pressures that contribute excessive nutrient loads. An example of assessing load reduction scenarios is presented to illustrate how modelling can support catchment scientists in identifying appropriate measures.

Reconstructing historical changes in phosphorus inputs to rivers from point and nonpoint sources in a rapidly developing watershed in eastern China, 1980–2010

Quantifying point (PS) and nonpoint source (NPS) phosphorus inputs to rivers is critical for developing effective watershed remediation strategies. This study reconstructed PS and NPS total phosphorus (TP) inputs to the Yongan River in eastern China in 1980–2010 using a load apportionment model (LAM) from paired riverine TP concentrations and river discharge records. Based on the fundamental hydrological differences between PS and NPS pollution, the LAM statistically quantified their individual inputs as a power-law function of river discharge. The LAM-estimated monthly/annual riverine TP loads were in good agreement with results derived from a regression model, Load Estimator (LOADEST). The annual TP load increased from 18.4 to 357.0Mgyr−1 between 1980 and 2010. The PS input contributed 7–45% of annual total TP load and increased 23-fold, consistent with a 20-fold increase in flow-adjusted average chloride concentration during the low flow regime (a proxy for wastewater inputs), as well as measured increases in population, poultry, and industrial production. Inferring from observed TP and chloride ratios, as well as total suspended solids (TSS) and river discharge dynamics, temporally retained P load within the river during the low flow regime was estimated to contribute 18–65% of the annual PS input load. NPS inputs consistently dominated the annual riverine TP load (55–93%) and increased 19-fold, consistent with the strong correlation between riverine TP and TSS concentrations, increasing developed land area, improved agricultural drainage systems, and phosphorus accumulation in agricultural soils. Based on our analysis, TP pollution control strategies should be preferentially directed at reductions in NPS loads, especially during summer high-flow periods when the greatest eutrophication risk occurs.

Identifying priorities for nutrient mitigation using river concentration–flow relationships: The Thames basin, UK

Summary The introduction of tertiary treatment to many of the sewage treatment works (STW) across the Thames basin in southern England has resulted in major reductions in river phosphorus (P) concentrations. Despite this, excessive phytoplankton growth is still a problem in the River Thames and many of its tributaries. There is an urgent need to determine if future resources should focus on P removal from the remaining STW, or on reducing agricultural inputs, to improve ecological status. Nutrient concentration–flow relationships for monitoring sites along the River Thames and 15 of its major tributaries were used to estimate the relative inputs of phosphorus and nitrogen from continuous (sewage point sources) and rain-related (diffuse and within-channel) sources, using the Load Apportionment Model (LAM). The model showed that diffuse sources and remobilisation of within-channel phosphorus contributed the majority of the annual P load at all monitoring sites. However, the majority of rivers in the Thames basin are still dominated by STW P inputs during the ecologically-sensitive spring-autumn growing season. Therefore, further STW improvements would be the most effective way of improving water quality and ecological status along the length of the River Thames, and 12 of the 15 tributaries. The LAM outputs were in agreement with other indicators of sewage input, such as sewered population density, phosphorus speciation and boron concentration. The majority of N inputs were from diffuse sources, and LAM suggests that introducing mitigation measures to reduce inputs from agriculture and groundwater would be most appropriate for all but one monitoring site in this study. The utilisation of nutrient concentration–flow data and LAM provide a simple, rapid and effective screening tool for determining nutrient sources and most effective mitigation options.

A modified load apportionment model for identifying point and diffuse source nutrient inputs to rivers from stream monitoring data

Determining point (PS) and diffuse source (DS) nutrient inputs to rivers is essential for assessing and developing mitigation strategies to reduce excessive nutrient loads that induce eutrophication. However, application of watershed mechanistic models to assess nutrient inputs is limited by large data requirements and intensive model calibration efforts. Simple export coefficient models and statistical models also require extensive primary watershed attribute information and further they cannot address seasonal patterns of nutrient delivery. In practice, monitoring efforts to identify all PSs within a watershed are very difficult due to time and economic limitations. To overcome these issues, based on the fundamental hydrological differences between PS and DS pollution, a modified load apportionment model (LAM) was developed relating the river nutrient load to nutrient inputs from PS, DS and upstream inflow sources while adjusting for in-stream nutrient retention processes. Estimates of PS and DS inputs can be easily achieved through Bayesian calibration of the five model parameters from commonly available stream monitoring data. It considers in-stream nutrient retention processes, temporal changes of PS and DS inputs, and nutrient contributions from upstream inflow waters, as well as the uncertainty associated with load estimations. The efficacy of this modified LAM was demonstrated for total nitrogen (TN) source apportionment using a 6-year record of monthly water quality data for the ChangLe River in eastern China. Aimed at attaining the targeted river TN concentration (2mgL-1), required input load reductions for PS, DS and upstream inflow were estimated. This modified LAM is applicable for both district-based and catchment-based water quality management strategies with limited data requirements, providing a simple, effective and economical tool for apportioning PS and DS nutrient inputs to rivers. © 2013 Elsevier B.V.

An evaluation of catchment-scale phosphorus mitigation using load apportionment modelling

Functional relationships between phosphorus (P) discharge and concentration mechanisms were explored using a load apportionment model (LAM) developed for use in a freshwater catchment in Ireland with fourteen years of data (1995–2008). The aim of model conceptualisation was to infer changes in point and diffuse sources from catchment P loading during P mitigation, based upon a dataset comprising geospatial and water quality data from a 256 km 2 lake catchment in an intensively farmed drumlin region of the midlands of Ireland. The model was calibrated using river total P (TP), molybdate reactive P (MRP) and runoff data from seven subcatchments. Temporal and spatial heterogeneity of P sources existed within and between subcatchments; these were attributed to differences in agricultural intensity, soil type and anthropogenically-sourced effluent P loading. Catchment rivers were sensitive to flow regime, which can result in eutrophication of rivers during summer and lake enrichment from frequent flood events. For one sewage impacted river, the LAM estimated that point sourced P contributed up to of 90% of annual MRP load delivered during a hydrological year and in this river point P sources dominated flows up to 92% of days. In the other rivers, despite diffuse P forming a majority of the annual P exports, point sources of P dominated flows for up to 64% of a hydrological year. The calibrated model demonstrated that lower P export rates followed specific P mitigation measures. The LAM estimated up to 80% decreases in point MRP load after enhanced P removal at waste water treatments plants in urban subcatchments and the implementation of septic tank and agricultural bye-laws in rural subcatchments. The LAM approach provides a way to assess the long-term effectiveness of further measures to reduce P loadings in EU (International) River Basin Districts and subcatchments.

Predicting phosphorus concentrations in British rivers resulting from the introduction of improved phosphorus removal from sewage effluent

Phosphorus (P) concentration and flow data gathered during the 1990s for a range of British rivers were used to determine the relative contributions of point and diffuse inputs to the total P load, using the Load Apportionment Model (LAM). Heavily urbanised catchments were dominated by sewage inputs, but the majority of the study catchments received most of their annual phosphorus load from diffuse sources. Despite this, almost 80% of the study sites were dominated by point source inputs for the majority of the year, particularly during summer periods when eutrophication risk is greatest. This highlights the need to reduce sewage P inputs to improve the ecological status of British rivers. These modelled source apportionment estimates were validated against land-use data and boron load (a chemical marker for sewage). The LAM was applied to river flow data in subsequent years, to give predicted P concentrations (assuming no change in P source inputs), and these estimates were compared with observed concentration data. This showed that there had been significant reductions in P concentration in the River Thames, Aire and Ouse in the period 1999 to 2002, which were attributable to the introduction of P stripping at sewage treatment works (STW). The model was then used to forecast P concentrations resulting from the introduction of P removal at STW to a 2 or 1 mg l − 1 consent limit. For the urbanised rivers in this study, the introduction of phosphorus stripping to a 1 mg l − 1 consent level at all STW in the catchment would not reduce P concentrations in the rivers to potentially limiting concentrations. Therefore, further sewage P stripping will be required to comply with the Water Framework Directive. Diffuse P inputs may also need to be reduced before some of the highly nutrient-enriched rivers achieve good ecological status.

Streamwater phosphorus and nitrogen across a gradient in rural–agricultural land use intensity

This paper provides an overview of the impacts of rural land use on lowland streamwater phosphorus (P) and nitrogen (N) concentrations and P loads and sources in lowland streams. Based on weekly water quality monitoring, the impacts of agriculture on streamwater P and N hydrochemistry were examined along a gradient of rural–agricultural land use, by monitoring three sets of ‘paired’ (near-adjacent) rural headwater streams, draining catchments which are representative of the major geology, soil types and rural/agricultural land use types of large areas of lowland Britain. The magnitude and timing of P and N inputs were assessed and the load apportionment model (LAM) was applied to quantify ‘continuous’ (point) source and ‘flow-dependent’ (diffuse) source contributions of P to these headwater streams. The results show that intensive arable farming had only a comparatively small impact on streamwater total phosphorus (TP loads), with highly consistent stream diffuse-source TP yields of ca. 0.5 kg-P ha −1 year −1 for the predominantly arable catchments with both clay and loam soils, compared with 0.4 kg-P ha −1 year −1 for low agricultural intensity grassland/woodland on similar soil types. In contrast, intensive livestock farming on heavy clay soils resulted in dramatically higher stream diffuse-source TP yields of 2 kg-P ha −1 year −1. The streamwater hydrochemistry of the livestock-dominated catchment was characterised by high concentrations of organic P, C and N fractions, associated with manure and slurry sources. Across the study sites, the impacts of human settlement were clearly identifiable with effluent inputs from septic tanks and sewage treatment works resulting in large-scale increases in soluble reactive phosphorus (SRP) loads and concentrations. At sites heavily impacted by rural settlements, SRP concentrations under baseflow conditions reached several hundred μg-P L −1. Load apportionment modelling demonstrated significant ‘point-source’ P inputs to the streams even where there were no sewage treatment works within the upstream catchment. This indicates that, even in sparsely populated rural headwater catchments, small settlements and even isolated groups of houses are sufficient to cause significant nutrient pollution and that septic tank systems serving these rural communities are actually operating as multiple point sources, rather than a diffuse input.

Changes in point and diffuse source phosphorus inputs to the River Frome (Dorset, UK) from 1966 to 2006

Changes in the relationship between soluble reactive phosphorus (SRP) concentration and river flow between 1966 and 2006 were assessed for the River Frome, UK using the recently developed Load Apportionment Model. The resulting source load estimates gave good agreement with known changes within the catchment. The model indicated an increase in point source contribution to the total river load from 46% to 62% between 1970 and 1985. This corresponded with the population increase within the catchment during that time. The predicted mean SRP load was highest between 1996 and 2000 (30 t y − 1 ), with 49% coming from point sources. Despite no lowering in population or major changes in agricultural practice, the model predicted a reduced load of 18.1 t y − 1 for the period 2001 to 2005, due mainly to a decrease in point source inputs from 14.6 t y − 1 to 6.1 t y − 1 (equivalent to 34% of the total load). This prediction matches the major improvements in sewage treatment that occurred within the catchment in 2002. This study thus provides a major validation of the Load Apportionment Model. The model provides an effective and rapid method of determining past changes in phosphorus sources, based entirely on the P concentration – flow relationship: critically, it does not require any historical information on land use, fertiliser application rates, topography, soil types and sewage inputs. Further decreases in SRP concentration in the River Frome during the algal growing season would be best achieved by further reductions of STW inputs.

Modelling of phosphorus inputs to rivers from diffuse and point sources

The difference in timing of point and diffuse phosphorus (P) delivery to a river produces clear differences in the P concentration–flow relationship. Point inputs decrease in concentration with increasing river flow, due to dilution of a relatively constant input, whereas diffuse (non-point) load usually increases with river flow. This study developed a simple model, based on this fundamental difference, which allowed point and diffuse inputs to be quantified by modelling their contribution to river P concentration as a power-law function of flow. The relationships between total phosphorus (TP) concentration and river flow were investigated for three contrasting UK river catchments; the Swale (Yorkshire), the Frome (Dorset) and the Avon (Warwickshire). A load apportionment model was fitted to this empirical data to give estimates of point and diffuse load inputs at each monitoring site, at high temporal resolution. The model produced TP source apportionments that were similar to those derived from an export coefficient approach. For many diffuse-dominated sites within this study (with up to 75% of the annual TP load derived from diffuse sources), the model showed that reductions of point inputs would be most effective in order to reduce eutrophication risk, due to point source dominance during the plant and algae growing period. This modelling approach should provide simple, robust and rapid TP source apportionment from most concentration–flow datasets. It does not require GIS, information on land use, catchment size, population or livestock density, and could provide a valuable and versatile tool to catchment managers for determining suitable river mitigation options.