Critical Source Areas Research Articles

Evaluation of Phosphorus Indices after Twenty Years of Science and Development

The P Index was proposed as a nutrient management tool in 1992 and has been implemented as such for the past decade. However, lack of water quality improvement in agricultural watersheds and discrepancies in P loss ratings between P indices have raised questions about continued use of the P Index. In response to these concerns, a symposium was held as part of the 2011 ASA, CSSA, SSSA annual meetings. This symposium produced a special collection of seven papers describing the role of P indices in P management, evaluation of P indices, new models for assessing P loss, methods to improve P indices, and changes in producer behavior resulting from P Index use. The objectives of this introductory paper are to provide background on the P Index concept, overviews of the special collection papers, and recommendations for future P Index evaluation and development research. The papers in this special collection conclude that P indices can provide accurate assessments of P loss but must be evaluated appropriately. Evaluation will require compiling large regional P loss datasets at field and small watershed scales. Simulation models may be used to generate P loss estimates; however, models must be calibrated and validated to ensure their accuracy. Further development of P indices will require coordinated regional efforts to identify common P Index frameworks and standardized interpretations. Stringent P Index evaluations will expand the utility of P indices for critical source area identification and strategic best management practice implementation by regulatory, education, and scientific communities alike.

Ca–Fe oxide granules as potential phosphate barrier material for critical source areas: a laboratory study of P retention and release

Phosphate barriers may mitigate dissolved P losses from critical source areas. We studied P retention of industrially produced Ca–Fe oxide as potential P barrier material. In batch tests with 1 mg l−1 P solution, P retention was 85% efficient in 5 min. In a flow-through system, the granule phosphate-retention capacity was 6–7 mg g−1, being largely unaffected by pre-leaching. Phosphate release from P-saturated granules was pH-dependant and suggested P association with Fe oxides, and as Ca-phosphate precipitates. In a sequential extraction of P-saturated granules, about 25% of retained P was released, whereas a separate anaerobic incubation resulted in negligible release of P. Immersion of unleached, P-saturated granules for 16 days in a low-P–concentration lake resulted in more than 80% Ca loss, but no loss of metals, and about 25% loss of P accumulated earlier in granules. These granules are promising for P retention and merit a field-scale study.

Dynamics of critical source areas: Does connectivity explain chemistry?

Critical source area approaches to catchment management are increasingly being recognised as effective tools to mitigate sediment and nutrient transfers. These approaches often assume hydrological connectivity as a driver for environmental risk, however this assumption has rarely been tested. Using high resolution monitoring, 14 rainfall events of contrasting intensity were examined in detail for spatial and temporal dynamics of overland flow generation at a hydrologically isolated grassland hillslope in Co. Down, Northern Ireland. Interactions between overland flow connectivity and nutrient transfers were studied to test the critical source area hypothesis. While total and soluble phosphorus loads were found to be representative of the size of the overland flow contributing area (P=<0.05), the dynamics of concentrations throughout storm hydrographs were found to be complex and storm dependant. Near linear relationships were observed between the contributing area and total overland flow volumes (R2=0.86). Export coefficients (kgha−1) calculated using plot size were found to under estimate annual losses of total phosphorus by a factor of 17, when compared to those calculated using the contributing area. This study shows that current critical source area definitions for implementing mitigation measures may be overlooking the importance of storm characteristics in determining nutrient transfers and hence may be insufficient in determining catchment scale risk.

Accounting for the role of uncertainty in declining water quality in an extensively farmed grassland catchment

Abstract Debate around the likelihood of achieving full compliance with Water Framework Directive objectives by 2027 has largely focused on the uncertainty in the lag-times between the implementation of mitigation measures and improvements in water quality. This paper examines uncertainties related to limited knowledge, predictability and ambiguity and their roles in declining water quality in Lough Melvin, a large lake in Ireland. Between 1991 and 2007 the total phosphorus (TP) concentration in the lake increased by 52%, raising concerns about eutrophication. Although agriculture in the catchment is extensive, operating at low stocking rates, it is estimated to contribute 62% of the TP load to the lake. Defining the reason(s) for this decline in water quality were hindered by sources of uncertainty in (1) understanding of farm practices; (2) understanding of the catchment hydrology; (3) monitoring and evaluation; and (4) farmer participation in agri-environmental schemes and policy implementation. Sources 1 and 2 contributed to the development of critical source areas of P in the catchment; source 3 reflects the accuracy both of water quality assessment and P source apportionment, and 4 played a part in the limited response to the observed changes in water quality. A number of approaches to overcoming these uncertainties are reviewed in the context of achieving the objectives of the WFD including: a catchment specific approach; stakeholder participation and critical appraisal of the decision – making processes for implementing the WFD.

Evaluating nutrient source regulations at different scales in five agricultural catchments

The European Union (EU) Nitrates Directive constrains nitrogen (N) and phosphorus (P) use and management on agricultural land with the expectation that better nutrient source management will improve water quality. Linking the effects of agricultural practices to impacts on water quality is a challenging task in terms of deciding on appropriate measurement scales. At national scale in Ireland, P fertiliser use and the numbers of soils tested with excessive P levels have declined since the introduction of the Nitrates Directive policies. However, in a detailed study of five benchmark agricultural catchments, between 6 and 26% of the soil still had excessive soil test P levels, showing the legacy effect of over application of P fertilisers and manures in the past. At farm and field scale large spatial variability in soil P fertility levels due to poor manure and fertiliser distribution was evident. The range of soil test P levels on most farms shows scope to correct these imbalances with better nutrient management planning. The application of critical source area management also needs to be considered; significant differences were shown between soil P attenuation and loss for different soil types across these catchments, for which the regulations do not discriminate. The multi-scale approach employed in this study allows a full spatial and temporal realisation of source and an appreciation of the management constraints which underlie nutrient management decisions on farms. A better understanding of nutrient source management in relation to regulatory compliance standards is needed in order to establish whether current policy instruments will bring about reductions in nutrient losses that are expected to result in improved water quality. For this a better farm-scale nutrient auditing tool could be used to improve the spatial distribution of nutrients, accounting for the influence of soil type and landscape hydrology factors on nutrient source mobilisation and loss.

Effectiveness Assessment of Soil Erosion Critical Source Areas for Soil and Water Conservation

Critical source areas (CSAs), characterized by severe soil erosion and high sediment yield, are considered to have a high priority for conservation. How to identify CSAs and assess the effectiveness of conservation practices is a key issue in site—specific watershed management. The Soil and Water Assessment Tool (SWAT) model is a useful tool for site-specific conservation practices design and several studies have attempted to identify CSAs based on watershed models. However, limited research has reported about the effectiveness of conservation practices targeting CSAs. The aim of this study was to assess the effectiveness of conservation pracrices targeted on CSAs using the SWAT model. CSA was firstly identified based on the 4-year average yearly erosion of each HRU. Appropriate soil conservation practices were then designed for the CSAs. A scenario with conservation practices for the whole watershed was also established as the contrasting counter parts scheme and then compared to the outcome of CSA-targeted conservation pracrices. The result shows that SWAT can accurately simulate sediment yield in the study area. CSAs were mainly located in slope farmland areas and steep gullies, coinciding with the distribution of land use and slope. The identified CSA covered 20% of the HRUs and contributed on average 44% of sediment yield. Conservation practices targeting CSAs had higher sediment reduction effectiveness (24 115 t km-2 y-1) than conservation practice covering the whole watershed (20 290 t km-2 y-1). Thus conservation practices targeting CSAs are more effective than broad conservation practices. We conclude that soil conservation practices focusing on CSAs do increase sediment reduction effectiveness. Targeting the placement of soil conservation practices based on the CSAs concept will assist water quality control in watersheds.

Identification of critical source areas of soil erosion on moderate fine spatial scale in Loess Plateau in China

Critical Source Areas (CSAs) are considered as priority areas for soil conservation and it is essential to identify CSAs for effective watershed management. Soil and water assessment tool (SWAT) model is a useful tool in identifying CSAs. Previous studies that used SWAT for CSAs identification were almost carried out on the basis of sub-watershed level which was too coarse to capture spatial detail of soil erosion. This research identified CSAs of soil erosion at a moderate fine spatial detail scale in a small watershed of Loess Plateau in China using SWAT model. CSAs were identified based on the 4-year average annual sediment yield of hydrological response units (HRU). The result shows that CSAs were mainly located in steep slope farmland areas and gully dominated areas. CSAs covered 10% areas of watershed, and contributed 30% sediment yield to the watershed. Such a trend is more obvious under larger storms. This could imply that CSAs identification on HRUs level is suitable for site-specific management design. This study also confirms that CSAs identification could be a potential approach assisting water quality control.

Evaluation of targeting methods for implementation of best management practices in the Saginaw River Watershed

Increasing concerns regarding water quality in the Great Lakes region are mainly due to changes in urban and agricultural landscapes. Both point and non-point sources contribute pollution to Great Lakes surface waters. Best management practices (BMPs) are a common tool used to reduce both point and non-point source pollution and improve water quality. Meanwhile, identification of critical source areas of pollution and placement of BMPs plays an important role in pollution reduction. The goal of this study is to evaluate the performance of different targeting methods in 1) identifying priority areas (high, medium, and low) based on various factors such as pollutant concentration, load, and yield, 2) comparing pollutant (sediment, total nitrogen-TN, and total phosphorus-TP) reduction in priority areas defined by all targeting methods, 3) determine the BMP relative sensitivity index among all targeting methods. Ten BMPs were implemented in the Saginaw River Watershed using the Soil and Water Assessment Tool (SWAT) model following identification of priority areas. Each targeting method selected distinct high priority areas based on the methodology of implementation. The concentration based targeting method was most effective at reduction of TN and TP, likely because it selected the greatest area of high priority for BMP implementation. The subbasin load targeting method was most effective at reducing sediment because it tended to select large, highly agricultural subbasins for BMP implementation. When implementing BMPs, native grass and terraces were generally the most effective, while conservation tillage and residue management had limited effectiveness. The BMP relative sensitivity index revealed that most combinations of targeting methods and priority areas resulted in a proportional decrease in pollutant load from the subbasin level and watershed outlet. However, the concentration and yield methods were more effective at subbasin reduction, while the stream load method was more effective at reducing pollutants at the watershed outlet. The results of this study indicate that emphasis should be placed on selection of the proper targeting method and BMP to meet the needs and goals of a BMP implementation project because different targeting methods produce varying results.

A Simple Model to Assess Nitrogen and Phosphorus Contamination in Ungauged Surface Drainage Networks: Application to the Massaciuccoli Lake Catchment, Italy

Modeling is a common practice to evaluate factors affecting water quality in environmental systems impaired by point and nonpoint losses of N and P. Nevertheless, in situations with inadequate information, such as ungauged basins, a balance between model complexity and data availability is necessary. In this paper, we applied a simplified analytical model to an artificially drained floodplain in central-western Italy to evaluate the importance of different nutrient sources and in-stream retention processes and to identify critical source areas. We first considered only a set of chemical concentrations in water measured from February through May 2008 and from November 2008 through February 2009. We then broadened available data to include water discharge and hydraulic-head measurements to construct a hydrogeological model using MODFLOW-2000 and to evaluate the reliability of the simplified method. The simplified model provided acceptable estimates of discharge (ranging from 0.03-0.75 m s) and diffuse nutrient inputs from water table discharge and in-stream retention phenomena. Estimates of PO-P and total P retention (ranging from 1.0 to 0.6 μg m s and from 1.18 to 0.95 μg m s for PO-P and total P, respectively) were consistent with the range of variability in literature data. In contrast, the higher temporal variability of nitrate concentrations decreased model accuracy, suggesting the need for more intensive monitoring. The model also separated the dynamics of different reaches of the drainage network and identified zones considered critical source areas and buffer zones where pollutant transport is reduced.

Mining simulation data by rule induction to determine critical source areas of stream water pollution by herbicides

Modelling in environmental sciences is becoming increasingly complex because ever-increasing numbers of processes are combined, thus making model-based decision aids both more relevant but more difficult to develop. Our approach focused on water quality and aimed to identify spatial tree patterns that represent surface flow and pollutant pathways from plot to plot involved in water pollution by herbicides. First, a simulation model predicted herbicide transfer rate, the proportion of applied herbicide that reaches water courses, based on the spatial and temporal distribution of weed-control activities. These predictions were used as a set of learning examples for symbolic learning techniques to induce rules based on qualitative and quantitative attributes and explain two classes of transfer rate. In this study we compared two automatic symbolic learning techniques applied to a set of simulations: (1) a relational-inductive method using the inductive logic programming (ILP) approach to induce spatial tree patterns; and (2) an attribute-value method to induce aggregated attributes of the trees. Twenty-eight and thirty-three rules were learnt by the ILP and attribute-value approaches which explained 81% and 88% of the examples, respectively. The ILP approach provided relevant indicators of plot-to-plot connectivity. The integrated attribute-value approach is simpler and quicker, but the ILP patterns are more useful for stakeholders.

Modelling nutrient retention function of ecosystem – a case study in Baoxing County, China

Non-point source pollution is one of the crucial causes leading to degradation of rivers and lake water quality; therefore, it has been studied in depth worldwide. However, few studies exploring the effect of terrestrial ecosystem nutrient retention function on water quality have been conducted to date. Natural ecosystems such as forests and grassland can hold runoff and sediment, thereby reducing the nutrients entering nearby aquatic systems, which is very important to improving water quality. Based on the landscape source-sink theory, this study assessed the nutrient retention function of ecosystems. The results showed that terrestrial ecosystems retained 78% of the Total Nitrogen (TN) within the system, thereby significantly reducing the TN entering the water. The nutrient retention function of different ecosystems varied significantly. Forests had the greatest potential capacity to maintain TN, but the actual retained TN was only about 10% of that applied to cropland. Because of irrational land use patterns, the potential forest intercepting nutrient function was not fully utilized. Specifically, nutrient retention function varied with altitude and slope. When the altitude was in the range of 1000 m or between 4000 m and 4500 m, the highest function could be attained, resulting to increases or decreases between regions. The nutrient retention function decreased as slope increased. The critical source area (CSA) was primarily distributed along the river bank. However, the critical sink area which made a high contribution to nutrient reduction did not appear that similar distribution like CSA. The critical sink area scatted in the whole basin mainly and only overlapped with the critical source areas in some place.

A review of phosphorus and sediment release from Irish tillage soils, the methods used to quantify losses and the current state of mitigation practice

Throughout the European Union (EU), agricultural soils with high phosphorus (P) status due to surplus fertiliser input have been identifi ed as a landscape pressure impacting on water quality. In Republic of Ireland, approximately 80% of agricultural land is devoted to grass, 11% to rough grazing, and 9% to arable cereal and crop production. Consequently, the majority of erosion research has focused on quantifying nutrient and sediment losses from grassland. Tillage soils are, however, more susceptible to erosion than grassland soils and, in general, have higher levels of soil P. This paper reviews the current state of research and the regulatory regime relating to diffuse P and sediment loss for tillage soils. It identifi es the key threats to soil quality associated with cultivated soils, and proposes the targeting and remediation of critical source areas for effective mitigation of P losses from tillage soils. A multiscaled approach is recommended, in which catchment and fi eld-scale monitoring is complemented with controlled laboratory and small plot-scale rainfall simulation experiments to identify areas where P loss and soil erosion are at critical levels and may pose a threat to water quality. Catchment scale research will help to link critical source areas of sediment and P loss with hydrological pathways to surface waters in the catchment area. These areas can then be targeted for remediation in the river basin management plans.

A REVIEW OF PHOSPHORUS AND SEDIMENT RELEASE FROM IRISH TILLAGE SOILS, THE METHODS USED TO QUANTIFY LOSSES AND THE CURRENT STATE OF MITIGATION PRACTICE

Throughout the European Union (EU), agricultural soils with high phosphorus (P) status due to surplus fertiliser input have been identified as a landscape pressure impacting on water quality. In Republic of Ireland, approximately 80% of agricultural land is devoted to grass, 11% to rough grazing, and 9% to arable cereal and crop production. Consequently, the majority of erosion research has focused on quantifying nutrient and sediment losses from grassland. Tillage soils are, however, more susceptible to erosion than grassland soils and, in general, have higher levels of soil P. This paper reviews the current state of research and the regulatory regime relating to diffuse P and sediment loss for tillage soils. It identifies the key threats to soil quality associated with cultivated soils, and proposes the targeting and remediation of critical source areas for effective mitigation of P losses from tillage soils. A multiscaled approach is recommended, in which catchment and field-scale monitoring is complemented with controlled laboratory and small plot-scale rainfall simulation experiments to identify areas where P loss and soil erosion are at critical levels and may pose a threat to water quality. Catchment scale research will help to link critical source areas of sediment and P loss with hydrological pathways to surface waters in the catchment area. These areas can then be targeted for remediation in the river basin management plans.

Phosphorus source areas in a dairy catchment in Otago, New Zealand

It is important to recognise source areas of phosphorus (P) in agricultural catchments and to understand how they contribute to catchment losses of P in order to effectively target mitigation strategies to decrease losses to surface waters. In a small dairy catchment (4.1 ha), soil physical properties and overland flow from pasture, a laneway, and around a watering trough were measured, together with subsurface flows from pasture and catchment discharge. Soil measured around the trough and in the laneway was found to be enriched in Olsen P (56 and 201 mg P/kg, respectively) compared with the pasture (24 mg P/kg), as well as having a greater bulk density resulting from more frequent use by animals. Dissolved P losses from lane and trough plots were greatly enhanced via dung. At the catchment scale, sources and transport processes resulted in losses mainly in the particulate P form (0.21 mg/L), while dissolved reactive P (DRP) concentrations were enriched during storm events (0.08 mg/L). Subsurface flow was found to be an important contributor of discharge and likely P losses, and this warrants further investigation. The scaling up of overland-flow plot data suggested that the laneway contributed up to 89% of the DRP load when surface overland flow was likely. This represents a substantial source of P loss on dairy farms. Additionally, the variation of sources and transport processes with season adds another aspect to the critical source area concept, and suggests that given the loss during summer and high algal availability of dissolved P, mitigation strategies should target decreasing dissolved P loss from the laneway.

Detection of Hot Spots of Soil Erosion and Reservoir Siltation in Ungauged Mediterranean Catchments

The long-term average, distributed parameter PhosFate model was applied for erosion hot spot identification in unmonitored Mediterranean catchments. Based on experiences of previous case studies in well-monitored watersheds, the model was tested in a study over the entire area of Albania with success. The model results were validated by the sporadically available data on river discharge and sediment loads. With an optimization method cost effective interventions were planned to efficiently reduce soil erosion. The optimization algorithm is based on a simple sediment transport model and it minimizes the necessary area of interventions. In accordance with the critical source area concept, it was shown that intervention with better management practices on properly selected, a few percent of the total area is sufficient to reach a significant reduction in soil loss, reservoir siltation and consequently an improvement in water quality.

An improved SWAT-based computational framework for identifying critical source areas for agricultural pollution at the lake basin scale

Soil and Water Assessment Tool (SWAT) model has been successfully used to identify critical source areas (CSAs) at the lake basin scale. However, when employing the well-mixed river channel model, SWAT has difficulty in decomposing the pollutant load at watershed outlet in terms of origin location. In previous studies, the river migration process is not included in the CSAs identification framework. The identification criterion for CSAs mainly based pollutant load releasing into river rather than the final pollutant load flowing into lake. This study is used to develop a new computational framework to address this problem. The SWAT model was used to offer a linkage between pollution input in the upstream watershed and pollutant load response at the watershed outlet. Simulation test was utilized to investigate the pollution contributions from different agricultural activities and from different villages. Combined with the agricultural production statistic data, the pollution contribution from each village was separately calculated, and the cumulative curve of pollution distribution was thus drawn to identify CSAs. We illustrated this method in the Lake Erhai basin, Yunnan province, China. CSAs for N pollution were identified, and a comparison with the existing method was made. The quantitative results indicate that there is a large gap between the pollutant loads released to river and flowed to lake. The river migration process was vital for N degradation so as to significantly influence the final CSAs map. The new method improved the ability of supplying a more comprehensive delineation of CSAs.

Using discharge data to reduce structural deficits in a hydrological model with a Bayesian inference approach and the implications for the prediction of critical source areas

A distributed hydrological model was used to simulate the distribution of fast runoff formation as a proxy for critical source areas for herbicide pollution in a small agricultural catchment in Switzerland. We tested to what degree predictions based on prior knowledge without local measurements could be improved upon relying on observed discharge. This learning process consisted of five steps: For the prior prediction (step 1), knowledge of the model parameters was coarse and predictions were fairly uncertain. In the second step, discharge data were used to update the prior parameter distribution. Effects of uncertainty in input data and model structure were accounted for by an autoregressive error model. This step decreased the width of the marginal distributions of parameters describing the lower boundary (percolation rates) but hardly affected soil hydraulic parameters. Residual analysis (step 3) revealed model structure deficits. We modified the model, and in the subsequent Bayesian updating (step 4) the widths of the posterior marginal distributions were reduced for most parameters compared to those of the prior. This incremental procedure led to a strong reduction in the uncertainty of the spatial prediction. Thus, despite only using spatially integrated data (discharge), the spatially distributed effect of the improved model structure can be expected to improve the spatially distributed predictions also. The fifth step consisted of a test with independent spatial data on herbicide losses and revealed ambiguous results. The comparison depended critically on the ratio of event to preevent water that was discharged. This ratio cannot be estimated from hydrological data only. The results demonstrate that the value of local data is strongly dependent on a correct model structure. An iterative procedure of Bayesian updating, model testing, and model modification is suggested.

Impact of critical source area on AnnAGNPS simulation

The objective of this paper is to study the impact of critical source area (CSA) within an Annualized AGricultural Non-Point Source pollution models (AnnAGNPS) simulation at medium- large watershed scale. The impact of CSA on terrain attributes is examined by comparing six sets of CSA (0.5, 1, 2, 4, 6 and 8 km(2)). The accuracy of AnnAGNPS stimulation on runoff, sediment and nutrient loads on these sets of CSA is further suggested in this paper. The results are as followed: (1) CSA has little effect on watershed area, and terrain altitude. The number of cell and reach decreases with the increase of CSA in power function regression curve. (2) The variation of CSA will lead to the uncertainty of average slope which increase the generalization of land characteristics. At the CSA range of 0.5-1 km(2), there is little impact of CSA on slope. (3) Runoff amount does not vary so much with the variation of CSA whereas soil erosion and total nitrogen (TN) load change prominently. An increase of sediment yield is observed firstly then a decrease following later. There is evident decrease of TN load, especially when CSA is bigger than 6 km(2). Total phosphorus load has little variation with the change of CSA. Results for Dage watershed show that CSA of 1 km(2) is desired to avoid large underestimates of loads. Increasing the CSA beyond this threshold will affect the computed runoff flux but generate prediction errors for nitrogen yields. So the appropriate CSA will control error and make simulation at acceptable level.

Identifying critical agricultural areas with three-meter LiDAR elevation data for precision conservation

Determining which portions of agricultural landscapes are major sources of pollution within a watershed is time consuming and labor intensive. Small critical areas of the landscape contribute disproportionate amounts of sediment and phosphorus to nearby waterways. Critical areas are defined here as areas of accumulated overland runoff that are hydrologically connected to surface waters. With advancements in light detection and ranging (LiDAR) technologies, landscape topography can be represented with highly accurate terrain data. The objective of this study is to determine the effectiveness of using LiDAR–based terrain attributes to identify fine-scale critical areas in selected Minnesota watersheds and to analyze cost efficiency of this type of analysis. The LiDAR digital elevation model data were acquired for two south central Minnesota watersheds, and the terrain attributes slope, flow accumulation, and stream power index were calculated with a 3 m (9.8 ft) spatial resolution. Field surveys were conducted in these watersheds along the riparian corridor to identify side inlets and active gullies that contribute to surface water quality degradation. Terrain attributes were able to identify 80% of field-verified gullies in the study watersheds. Furthermore, an even higher percentage of gullies with a high sediment delivery potential were identified using terrain attributes. Gully size was ranked during field surveys, and 31 of the 32 largest gullies ranked in the field were successfully identified with LiDAR–based terrain attributes. In contrast, only 7 of these gullies could be identified using 30 m (98 ft) digital elevation model terrain attributes. The LiDAR approach for identifying critical source areas using terrain attributes has a large potential for cost savings relative to time-consuming field surveys. With an ever-increasing availability of LiDAR data, terrain analysis may prove very useful in the future for targeting best management practices to critical areas for reductions in nonpoint source pollution.

Approaches to the implementation of the Water Framework Directive: Targeting mitigation measures at critical source areas of diffuse phosphorus in Irish catchments

The Water Framework Directive (WFD) has initiated a shift towards a targeted approach to implementation through its focus on river basin districts as management units and the natural ecological characteristics of waterbodies. Due to its role in eutrophication, phosphorus (P) has received considerable attention, resulting in a significant body of research, which now forms the evidence base for the programme of measures (POMs) adopted in WFD River Basin Management Plans (RBMP). Targeting POMs at critical sources areas (CSAs) of P could significantly improve environmental efficiency and cost effectiveness of proposed mitigation strategies. This paper summarises the progress made towards targeting mitigation measures at CSAs in Irish catchments. A review of current research highlights that knowledge related to P export at field scale is relatively comprehensive however; the availability of site-specific data and tools limits widespread identification of CSA at this scale. Increasing complexity of hydrological processes at larger scales limits accurate identification of CSA at catchment scale. Implementation of a tiered approach, using catchment scale tools in conjunction with field-by-field surveys could decrease uncertainty and provide a more practical and cost effective method of delineating CSA in a range of catchments. Despite scientific and practical uncertainties, development of a tiered CSA-based approach to assist in the development of supplementary measures would provide a means of developing catchment-specific and cost-effective programmes of measures for diffuse P. The paper presents a conceptual framework for such an approach, which would have particular relevance for the development of supplementary measures in High Status Waterbodies (HSW). The cost and resources necessary for implementation are justified based on HSWs' value as undisturbed reference condition ecosystems.