Box 14D How to obtain better estimates of emissions from agriculture to water

In order to improve our knowledge about the emissions of nutrients from agriculture and from other sectors to the aquatic environment, two approaches seem appropriate: the National Network Approach and the Type Catchment Approach. Common to both approaches (exemplified below using nitrogen) is the establishment of a permanent river-monitoring network or the adaptation of existing networks to include continuous measurements and estimates of the discharge of flow and frequent water sampling for subsequent analysis of nitrogen species and phosphorus.

The National Network Approach

This mass-balance approach relies on setting up a monitoring network with the aim at determining the total flux of nitrogen (Ntot) through major rivers from country to country and to the sea. Knowing the nitrogen contributions from point sources, Npoint, either from measurements or by applying emission factors; the contribution, Nscat, from scattered houses and dwellings (if important); the retention of N in rivers and lakes, Nret, by sedimentation and denitrification; the area coefficient for N from uncultivated land, Nback, and the atmospheric deposition, Ndep, directly on freshwater surfaces, the contribution from agriculture, Nagri, can be obtained by solving the mass-balance equation:

Nagri = Ntot ­ Npoint ­ Nback ­ Nscat ­ Ndep ­ Nret

The reliability of this estimate is heavily dependent on the accuracy with which it is possible to measure or estimate the other important sources or sinks. If these are properly estimated, this approach applied to all major rivers of a country allows the calculation of (i) the total nitrogen load to the sea, and (ii) the partition of this load among the major nitrogen contributors (eg, agriculture, industry, domestic sewage). With minor extensions of the list of determinants to include, for example, other nutrients and heavy metals, the National Network Approach may serve multiple purposes with only limited additional costs.

The Type Catchment Approach

The agricultural nitrogen input to surface waters can also be estimated by comparing fluxes in watercourses draining well-defined small catchments on the same soil type but with different degrees of utilisation. Heath or forest covered catchments may serve as baseline catchments and provide estimates of the background concentration of nitrogen. The nitrogen flux in watercourses draining catchments dominated by agriculture is composed of the background and the agricultural inputs and subtraction of these contributions provides an estimate of the agricultural input. For the purpose of catchment selection 'baseline' and 'type' catchments might be defined respectively as catchments with more than 90 per cent natural areas and 90 per cent agriculture.

These approaches are recommended for use in UNEP/WHO's Global Environment Monitoring System, GEMS/WATER programme, and have, for example, been successfully applied for source partitioning of nutrient emissions to water in several countries bordering the North and Baltic seas.