Modelling mitigation options to reduce diffuse nitrogen water pollution from agriculture

Highlights

• We review models used for assessing nitrogen mitigation measures.

• Models can consider lag time, pollution swapping, and targeting.

• Models are key elements for successful implementation of the Water Framework Directive.

Abstract

Agriculture is responsible for large scale water quality degradation and is estimated to contribute around 55% of the nitrogen entering the European Seas. The key policy instrument for protecting inland, transitional and coastal water resources is the Water Framework Directive (WFD). Reducing nutrient losses from agriculture is crucial to the successful implementation of the WFD. There are several mitigation measures that can be implemented to reduce nitrogen losses from agricultural areas to surface and ground waters. For the selection of appropriate measures, models are useful for quantifying the expected impacts and the associated costs. In this article we review some of the models used in Europe to assess the effectiveness of nitrogen mitigation measures, ranging from fertilizer management to the construction of riparian areas and wetlands. We highlight how the complexity of models is correlated with the type of scenarios that can be tested, with conceptual models mostly used to evaluate the impact of reduced fertilizer application, and the physically-based models used to evaluate the timing and location of mitigation options and the response times. We underline the importance of considering the lag time between the implementation of measures and effects on water quality. Models can be effective tools for targeting mitigation measures (identifying critical areas and timing), for evaluating their cost effectiveness, for taking into consideration pollution swapping and considering potential trade-offs in contrasting environmental objectives. Models are also useful for involving stakeholders during the development of catchments mitigation plans, increasing their acceptability.

Introduction

Protecting European water resources is high on the agenda of the European Commission because of their ecological and economic importance. One major environmental issue affecting the quality of inland and coastal waters is eutrophication caused by excessive presence of dissolved inorganic nutrients (Deflandre and Jarvie, 2006). Algal bloom and oxygen depletion are still reported in many European water bodies (Voss et al., 2011, Sutton et al., 2011). In 1990, nitrogen emissions to European seas amounted to 4 Tg, with the contribution from agriculture at ~ 55%, but with large regional differences (Bouraoui et al., 2011, Grizzetti et al., 2012) ranging from 80% in Denmark to < 30% in Finland (OECD, 2001). Similar figures were reported for 2005 (Grizzetti et al., 2012).

To combat these high nitrogen loads entering European seas, the European Union has been setting various directives since the beginning of the '90s. The legislation for controlling eutrophication and nutrient loading into surface and ground waters started in 1991 with the implementation of two major Directives: the Nitrate Directive (Directive 91/676/EEC), to control nitrate pollution from agricultural activities (diffuse sources), and the Urban Waste Water Treatment Directive (Directive 91/271/EEC), to reduce pollution from waste water treatment plants (point sources). To rationalise and update the existing legislation, the Water Framework Directive (WFD) was adopted in 2000 (Directive, 2000/60/EC) with the aim of achieving good ecological and chemical status for all water bodies by 2015 by implementing Programmes of Measures included in River Basin Management Plans. The protection of water was then completed by the Groundwater Directive (a daughter Directive of the WFD), which limits the amount of pollution in groundwater (including nitrate), and the Marine Strategy Framework Directive (Directive, 2008/56/EC) that aims at preventing the deterioration of, or where practicable, restoration of marine waters in areas where they have been adversely affected. At the same time, the reform of the Common Agricultural Policy (CAP) by decoupling subsidies from production levels and linking them to the protection of the environment has been promoting more environmentally-friendly agriculture, supporting the reduction of nitrogen loss to water, and more sustainable use of resources. Agricultural subsidies are now linked to the application of statutory minimum requirements (SMR) and cross compliance. Farmers willing to go beyond SMR can get additional payments through Rural Development Programmes by implementing “Good Farming Practices”. These measures should lead to a decrease in fertilizer inputs and consequent nitrogen leaching to waters.

Despite this large body of legislation and financial incentives, progress in improving water quality has not been as good as expected. When fully implemented, the key Directives have helped decrease the amount of nutrient input and nitrogen in particular (Bouraoui and Grizzetti, 2011). However, the implementation is still lagging behind in some countries and water quality has not reached the expected levels of improvement. In addition, inertia and the delay in the response of the environment to the mitigation measures for reducing nitrogen losses are responsible in many areas for subdued or no improvement in water quality (Jackson et al., 2008, Behrendt et al., 2000). Overall, the implementation of the Waste Water Directive has led to a very significant decrease of point source emissions of nitrogen and phosphorus, while the Nitrate Directive has had less obvious positive outcomes (Bouraoui and Grizzetti, 2011). As the contribution of point sources to the total load of nutrients has severely dropped in many countries, more emphasis is now being placed on controlling key diffuse sources.

Evaluating correctly the efficiency of measures to combat diffuse pollution is difficult due to the temporal and spatial lags between the management actions taken at the local scale and the environmental response, which can be measured at the local or regional scale (Bouraoui et al., 2011, Jackson et al., 2008; Schröder et al., 2004; Behrendt et al., 2000). Besides the correct identification and quantification of sources, cost-effective nutrient mitigation requires the delineation of critical source areas that contribute disproportionate amounts of nutrients to receiving waters (White et al., 2009, Heathwaite et al., 2005). Targeting and prioritising diffuse pollution control has the potential to increase the efficiency of pollutant reduction, is economically attractive, and minimises the extent of areas that are affected by restrictive land use practices.

Models have been widely used to predict the effects of mitigation measures on water quality and have proved useful during the implementation of the WFD (European Commission. COM(2012) 673 final, 2012, Hartnett et al., 2007). Models were used in the initial stages of WFD implementation to perform a status review through a pressure and impact analysis as required by the Directive. They were also applied during the design and setting up of the surveillance, operational and investigative monitoring networks. Models are currently used for the development of the River Basin Management Plans to demonstrate how the water status improvement will be achieved and to help develop the Programmes of Measures needed to achieve the good status objectives. The use of models is thus of key importance in the implementation of the WFD, in the selection and localisation of the appropriate management and policy measures, in predicting the efficiency of such measures and the time needed before any sustained improvement in the ecological and chemical status of waters will be observed.

In the above context, the aim of this paper was to analyse how models have been used in Europe for assessing mitigation measures for reducing nitrogen pollution of water and to reflect on the main challenges of their applicability considering the needs of the current European environmental policies. The analysis is based mainly on the information published in scientific literature. The overarching goal was to provide a review and reflection on the applicability of river basin modelling of mitigation measures with regard to the challenging objectives and needs of current environmental policies in Europe. The synthesis focused on agricultural measures, also referred to as practices or methods (Novotny and Olem, 1994) which are used to reduce nitrogen losses linked to farming practices for achieving a water quality objective. The synthesis and review did not consider in stream measures, nor measures dealing with animal breeding practices or those used to reduce greenhouse gas emissions. In the first part of the paper, we describe models currently used in Europe to assess the transformations and fate of nitrogen. The review does not aim at providing an extensive list of studies but rather to give a flavour based on a literature search of the most commonly used models in relation to the local or regional implementation of policies to reduce nitrogen diffuse pollution, providing examples of their application in Europe. In the second part of the paper, we discuss the main challenges for the applicability of models, considering the needs of current European environmental policies.