Documenting success stories of management of phosphorus emissions at catchment scale: an example from the pilot river Odense, Denmark.

Documentation of the effects of different mitigation measures adopted at different scales to reduce phosphorus (P) loadings to surface waters is needed to help catchment managers select the best management practices. Water quality monitoring data from the outlets of two paired catchments (the river Odense catchment versus a neighbouring control catchment) on the island of Funen, Denmark, showed significantly different trends in annual flow-weighted P concentrations during the period 2000-2013. A significant downward trend in flow-weighted particulate P (PP) concentrations (0.051 mg P L-1) and loss (0.155 kg P ha-1) was detected for the river Odense catchment, whereas a similar trend did not emerge in the control catchment. The observed differences in PP reductions may be due to wetlands acting as P sinks since wetland restoration activities have been much more comprehensive in the river Odense catchment (1.8 ha wetlands km-2) than in the control catchment (0.5 ha wetland km-2). The excess downward trend in total P and PP in the river Odense catchment (5,600 kg P and 3,700 kg P) is corroborated by extrapolating the results from a mass-balance study and 10 years of in situ measurements of P storage (3,700 kg P and 15,000 kg P).

Leaching of dissolved phosphorus from tile-drained agricultural areas.

We investigated leaching of dissolved phosphorus (P) from 45 tile-drains representing animal husbandry farms in all regions of Denmark. Leaching of P via tile-drains exhibits a high degree of spatial heterogeneity with a low concentration in the majority of tile-drains and few tile-drains (15% in our investigation) having high to very high concentration of dissolved P. The share of dissolved organic P (DOP) was high (up to 96%). Leaching of DOP has hitherto been a somewhat overlooked P loss pathway in Danish soils and the mechanisms of mobilization and transport of DOP needs more investigation. We found a high correlation between Olsen-P and water extractable P. Water extractable P is regarded as an indicator of risk of loss of dissolved P. Our findings indicate that Olsen-P, which is measured routinely in Danish agricultural soils, may be a useful proxy for the P leaching potential of soils. However, we found no straight-forward correlation between leaching potential of the top soil layer (expressed as either degree of P saturation, Olsen-P or water extractable P) and the measured concentration of dissolved P in the tile-drain. This underlines that not only the source of P but also the P loss pathway must be taken into account when evaluating the risk of P loss.

Successful reduction of diffuse nitrogen emissions at catchment scale: example from the pilot River Odense, Denmark.

Land-based total nitrogen (N) loadings to Danish coastal waters have been markedly reduced since 2000. This has been achieved by general measures reducing discharges from all point sources and N leaching from farmed land supplemented with more local and targeted mitigation measures such as restoration of wetlands to increase the catchment-specific N retention. In the catchment of River Odense, restoration of wetlands has been extensive. Thus, in the major gauged catchment (485 km(2)) eleven wetlands (860 ha) have been restored since 2000. A comparison of data on N concentrations and loss from a gauging station in the River Odense with data from a control catchment (772 km(2)), in which a significantly less intensive wetland restoration programme has been undertaken, showed an excess downward trend in N, amounting to 124 t N yr(-1), which can be ascribed to the intensive wetland restoration programme carried out in the River Odense catchment. In total, the N load in the River Odense has been reduced by 377 t N yr(-1) (39%) since 2000. The observed downward trend is supported by monitoring data from two wetlands restored in 2001 and 2004 in the River Odense catchment.

Mitigation options to reduce phosphorus losses from the agricultural sector and improve surface water quality: a review.

The EU Water Framework Directive (WFD) obliges Member States to improve the quality of surface water and groundwater. The measures implemented to date have reduced the contribution of point sources of pollution, and hence diffuse pollution from agriculture has become more important. In many catchments the water quality remains poor. COST Action 869 was an EU initiative to improve surface water quality that ran from 2006 to 2011, in which 30 countries participated. Its main aim was a scientific evaluation of the suitability and cost-effectiveness of options for reducing nutrient loss from rural areas to surface waters at catchment scale, including the feasibility of the options under different climatic and geographical conditions. This paper gives an overview of various categories of mitigation options in relation to phosphorus (P). The individual measures are described in terms of their mode of action, applicability, effectiveness, time frame, environmental side-effects (N cycling) and cost. In total, 83 measures were evaluated in COST Action 869.

Description of nine nutrient loss models: capabilities and suitability based on their characteristics.

In EUROHARP, an EC Framework V project, which started in 2002 with 21 partners in 17 countries across Europe, a detailed intercomparison of contemporary catchment-scale modelling approaches was undertaken to characterise the relative importance of point and diffuse pollution of nutrients in surface freshwater systems. The study focused on the scientific evaluation of different modelling approaches, which were validated on three core catchments (the Ouse, UK; the Vansjo-Hobøl, Norway; and the Enza, Italy), and the application of each tool to three additional, randomly chosen catchments across Europe. The tools involved differ profoundly in their complexity, level of process representation and data requirements. The tools include simple loading models, statistical, conceptual and empirical model approaches, and physics-based (mechanistic) models. The results of a scientific intercomparison of the characteristics of these different model approaches are described. This includes an analysis of potential strengths and weaknesses of the nutrient models.

Basin characteristics and nutrient losses: the EUROHARP catchment network perspective.

The EC-funded EUROHARP project studies the harmonisation of modelling tools to quantify nutrient losses from diffuse sources. This paper describes a set of study areas used in the project from geographical conditions, to land use and land management, geological and hydro-geological perspectives. The status of data availability throughout Europe in relation to the modelling requirements is presented. The relationships between the catchment characteristics and the nutrient export are investigated, using simple data available for all the catchments. In addition, this study also analyses the hydrological representativity of the time series utilised in the EUROHARP project.

Ensemble modelling of nutrient loads and nutrient load partitioning in 17 European catchments.

An ensemble of nutrient models was applied in 17 European catchments to analyse the variation that appears after simulation of net nutrient loads and partitioning of nutrient loads at catchment scale. Eight models for N and five models for P were applied in three core catchments covering European-wide gradients in climate, topography, soil types and land use (Vansjø-Hobøl (Norway), Ouse (Yorkshire, UK) and Enza (Italy)). Moreover, each of the models was applied in 3-14 other EUROHARP catchments in order to inter-compare the outcome of the nutrient load partitioning at a wider European scale. The results of the nutrient load partitioning show a variation in the computed average annual nitrogen and phosphorus loss from agricultural land within the 17 catchments between 19.1-34.6 kg N ha(-1) and 0.12-1.67 kg P ha(-1). All the applied nutrient models show that the catchment specific variation (range and standard deviation) in the model results is lowest when simulating the net nutrient load and becomes increasingly higher for simulation of the gross nutrient loss from agricultural land and highest for the simulations of the gross nutrient loss from other diffuse sources in the core catchments. The average coefficient of variation for the model simulations of gross P loss from agricultural land is nearly twice as high (67%) as for the model simulations of gross N loss from agricultural land (40%). The variation involved in model simulations of net nutrient load and gross nutrient losses in European catchments was due to regional factors and the presence or absence of large lakes within the catchment.

Nitrogen and phosphorus retention in surface waters: an inter-comparison of predictions by catchment models of different complexity.

Nitrogen and phosphorus retention estimates in streams and standing water bodies were compared for four European catchments by a series of catchment-scale modelling tools of different complexity, ranging from a simple, equilibrium input-output type to dynamic, physical-based models: source apportionment, MONERIS, EveNFlow, TRK, SWAT, and NL-CAT. The four catchments represent diverse climate, hydrology, and nutrient loads from diffuse and point sources in Norway, the UK, Italy, and the Czech Republic. The models' retention values varied largely, with tendencies towards higher scatters for phosphorus than for nitrogen, and for catchments with lakes (Vansjø-Hobøl, Zelivka) compared to mostly or entirely lakeless catchments (Ouse or Enza, respectively). A comparison of retention values with the size of nutrient sources showed that the modelled nutrient export from diffuse sources was directly proportional to retention estimates, hence implying that the uncertainty in quantification of diffuse catchment sources of nutrients was also related to the uncertainty in nutrient retention determination. This study demonstrates that realistic modelling of nutrient export from large catchments is very difficult without a certain level of measured data. In particular, even complex process oriented models require information on the retention capabilities of water bodies within the receiving surface water system and on the nutrient export from micro-catchments representing the major types of diffuse sources to surface waters.

Phosphorus losses from agricultural areas in river basins: effects and uncertainties of targeted mitigation measures.

In this paper we show the quantitative and relative importance of phosphorus (P) losses from agricultural areas within European river basins and demonstrate the importance of P pathways, linking agricultural source areas to surface water at different scales. Agricultural P losses are increasingly important for the P concentration in most European rivers, lakes, and estuaries, even though the quantity of P lost from agricultural areas in European catchments varies at least one order of magnitude (<0.2 kg P ha(-1) to >2.1 kg P ha(-1)). We focus on the importance of P for the implementation of the EU Water Framework Directive and discuss the benefits, uncertainties, and side effects of the different targeted mitigation measures that can be adopted to combat P losses from agricultural areas in river basins. Experimental evidence of the effects of some of the main targeted mitigation measures hitherto implemented is demonstrated, including: (i) soil tillage changes, (ii) treatment of soils near ditches and streams with iron to reduce P transport from source areas to surface waters, (iii) establishment of buffer zones for retaining P from surface runoff, (iv) restoration of river-floodplain systems to allow natural inundation of riparian areas and deposition of P, and (v) inundation of riparian areas with tile drainage water for P retention. Furthermore, we show how river basin managers can map and analyze the extent and importance of P risk areas, exemplified by four catchments differing in size in Norway, Denmark, and the Netherlands. Finally, we discuss the factors and mechanisms that may delay and/or counteract the responses of mitigation measures for combating P losses from agricultural areas when monitored at the catchment scale.

Buffer zones as a sink for sediment and phosphorus between the field and stream: Danish field experiences.

Rill erosion, surface runoff and storage of sediment and phosphorus in buffer zones were investigated during three winters on ca. 140 arable field slope units in twenty places in Denmark covering all landscape types, climate gradients and dominant soil types. The dominant soils are Alfisols and Spodosols, with textural composition typically ranging from sand to loam. The average slope is 7% (range: 2-20%) and median buffer zone width was 8.3 m (range: 0.6-125 m). The geometric mean annual rill erosion was 0.33 m3 ha(-1) equalling to 495 kg sediment ha(-1) and 0.25 kg P ha(-1). The deposition of sediment on the field, in the buffer zone and delivery of soil to the stream was surveyed within the 140 slope units following three winters. Deposition of sediment in the buffer zone was observed in 31% (1997/98), 31% (1998/99) and 29% (1999/2000) of the slope units. Delivery of soil across the edge of the stream was observed in 23% (1997/98), 17% (1998/99) and 25% (1999/2000) of the 140 slope units. Median dissolved P concentration in surface runoff was 0.18 mg Pl(-1) (range: 0.029-16.294 mg Pl(-1)). A probability model was developed to predict the efficiency of different widths of buffer zones.

Subsurface tile drainage loss of modern pesticides: field experiment results.

The concentration and loss of two herbicides (bentazone and MCPA), two fungicides (fenpropimorph and propiconazole) and two insecticides (dimethoate and pirimicarb) were measured in tile drainage water from a 2,813 m2 experimental grass field from May to August 2001. Three different pesticides were applied to the field and subsequently a rainfall of 10 mm was simulated during the first experiment and 16 mm during the second experiment. Bromide was applied as a conservative tracer in the first experiment and the concentration of bromide and suspended sediment was also measured in tile drainage water. In the first experiment, maximum concentrations of bentazone, fenpropimorph and dimethoate in drainage water were 5.8 microg l(-1), 0.33 microg l(-1) and 2.29 microg l(-1), respectively. In the second experiment, maximum concentrations for MCPA, propiconazole and pirimicarb were 3.6 microg l(-1), 0.065 microg l(-1), 2.3 microg l(-1), respectively. The loss:applied ratio was highest for bentazone (0.088%) and declined in the order of dimethoate (0.057%), pirimicarb (0.050%), propiconazole (0.0031%) and fenpropimorph (0.00042%). Exposure of the macroinvertebrate species Gammarus pulex to pesticides in the drainage water during the second experiment (exposure time: 7 hours) showed significant mortality/inactivity as compared to an upstream and downstream control.

The SOIL-N/WEKU model system--a GIS-supported tool for the assessment and management of diffuse nitrogen leaching at the scale of river basins.

The SOIL-N/WEKU model system was developed to estimate groundwater-borne nitrogen inputs into river systems. The core of this model system is composed of a soil nitrogen leaching model (SOIL-N) and a groundwater residence time/denitrification model (WEKU). The application of the model system was carried out in the framework of the EU-project RANR (Regional analysis of subsurface nitrogen retention and its impact on the nitrogen export from land to sea) for a macroscale study river basin in Germany (the Uecker basin, ca. 2,400 km2) and a mesoscale study catchment area in Denmark (the Gjern basin, ca. 200 km2). For both catchment areas, the modelled average nitrogen loads leached into the groundwater were about 40 kg N/ha a, while the remaining groundwater-borne nitrogen intake to rivers was quantified to an average of about 2 kg/ha a. The comparison with observed groundwater-borne riverine nitrogen loads showed a very good agreement, proving the key role nitrogen retention in groundwater plays in the two catchment areas.

Analysis of the hydrology and flow of nitrogen in 17 Danish catchments.

In the search for tools for evaluating the effects of national action plans combating diffuse nutrient pollution causing eutrophication of surface waters, a study of the nitrogen (N) flow in 17 Danish agricultural catchments was carried out. Data on N input and N harvest for the agricultural year of 1993/1994 was obtained from questionnaire surveys facilitating the set up of N balances. Net export of N from the catchments measured at the outlet was obtained from time-series of stream water chemistry and discharge from 1993-1997. N leaching from the root zone of each field was calculated using an empirical model. A physically based lumped rainfall-runoff model was used for separating the hydrograph time-series into three runoff components: baseflow, interflow and overland flow. Large regional variations in net N input were found ranging from 62 kg N ha(-1) yr(-1) in the loamy eastern part of the country dominated by cereal production to 137 kg N ha(-1) yr(-1) in the western part characterised by less fertile sandy soils and dominated by animal husbandry. N leaching from the root zone showed a corresponding variation with regional averages ranging from 34.5 kg N ha(-1) yr(-1) to 90.9 kg N ha(-1) yr(-1). No similar regional pattern could however be found regarding net N export, and no relationship could be established between net N export and root zone N leaching. This finding was ascribed to a varying and in some catchments very high (>80%) N retention during subsurface transport to the stream. The hydrological modelling revealed that loamy catchments had a high proportion of quick flow (overland flow + interflow), whereas baseflow dominated the sandy catchments. Further, a highly significant relationship between N retention and proportion of quick flow was found emphasising the importance of understanding the hydrological pathways. This should be taken into consideration when evaluating the N loading of surface waters resulting from a given agricultural practice and the effects of possible changes in this practice.

Model system for the management of nitrogen leaching at the scale of river basins and regions.

In the framework of the EU-project RANR (Regional analysis of subsurface nitrogen retention and its impact on the nitrogen export from land to sea) a model system was developed to estimate groundwaterborne nitrogen inputs into river systems. The core of this model system is composed of a soil nitrogen leaching model (SOIL-N) and a groundwater residence time/denitrification model (WEKU). The application of the model system was carried out for the study catchment areas of the Uecker basin (ca. 2400 km2, Germany) and the Gjern basin (ca. 200 km2, Denmark). For both catchment areas, the modelled average nitrogen loads leached into the groundwater were about 40 kg N/ha a, while the remaining groundwaterborne nitrogen intake to rivers was quantified to an average of about 2 kg/ha a. The comparision with observed groundwaterborne riverine nitrogen loads showed a very good agreement, proofing the key role nitrogen retention in groundwater plays in the two catchment areas. With regard to the generalisation and transfer of the SOIL-N/WEKU model concept we assume that the model can be applicated in catchment areas in the European Pleistocene Lowland, which ranges from the Netherlands in the west to the Baltic States and the Ukraine in the east.