Lakes and Watercourses

from fields in lowland areas and class 5 represents leaching from cultivated
sandy soils, often combined with manure use.
Since nitrogen losses over fairly large agricultural areas exceed 16 kg
N/ha, there is reason to make particular note of areas where nitrogen
losses are extreme (over 32 kg N/ha, year). This is particularly called for
when setting priorities for remedial measures.
The different classes are matched by various flow-weighted annual
mean concentrations, depending on the flow per unit surface area. Figure 1
shows the correlations between area-specific loss and concentration at
four different levels of flow per unit surface area.
TABLE 6.
CURRENT CONDITIONS: area-specific loss of total phosphorus,
watercourses (kg P/ha, year)
Class Description Area-specific loss
1 Very low losses ≤ 0.04
2 Low losses 0.04 – 0.08
3 Moderately high losses 0.08 – 0.16
4 High losses 0.16 – 0.32
5 Very high losses > 0.32
Area-specific losses refer to the monitoring of concentrations 12 times
a year over three years and recorded or modelled water flow per 24-hour
period. It may be necessary to monitor concentrations more frequently in
small watercourses. 24-hour water flow figures are multiplied by the
corresponding concentrations obtained using linear interpolation
between readings. The 24-hour transport figures thus obtained are
accumulated to give annual figures and show area-specific losses after
division by the area of the catchment.
Phosphorus loss includes input from all sources upstream of the
monitoring point, which classifies the total area-specific input from the
catchment to lakes and seas, for example. The scale is also intended to be
used to assess losses from all types of soil in comparison with normal
losses from different types of land use. Known input from point sources
can be deducted to gain a better picture of diffuse phosphorus losses.
Class 1 represents the lowest leaching on record from unaffected
forest soils. Class 2 shows normal leaching from normal forest soils in
Sweden. Class 3 contains losses from clear-cut areas, bog/peat land,
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