Lakes and Watercourses

TABLE 9.
DEVIATION from reference value, area-specific loss of total
nitrogen in watercourses
Class Description Recorded area-specific loss/reference value
1 No or insignificant deviation ≤ 2.5
2 Significant deviation 2.5 – 5
3 Large deviation 5 – 20
4 Very large deviation 20 – 60
5 Extreme deviation > 60
This classification is based on an overall assessment taking into account
the area-specific losses occurring in Swedish watercourses affected by
man to varying degrees. In some catchments, the ratio between recorded
concentration and reference value may be less than 1, which may
indicate a phosphorus deficit. Conditions of this kind should be particularly
noted so as to allow further study of possible acidification-related
effects. In some catchments, the ratio of recorded concentration to
reference value for nitrogen may be less than 1, which may indicate a
nitrogen deficit.
Recorded area-specific loss refers to a mean figure for a 3-year period,
calculated as shown above for classification of current conditions for
area-specific losses.
Reference values can be calculated or estimated in several ways. They
may be estimated on the basis of historical studies of the area in question
or studies of similar but unaffected watercourses in the vicinity. In the
absence of other data, reference values can also be calculated using the
characteristics of the catchment and other features of the watercourse.
The equations specified in the previous Quality criteria for lakes and
watercourses (Swedish EPA 1991) can be used for this purpose, together
with the additional equation shown below. All these relationships are
expected to yield low estimates and the highest figure obtained using
equations (1) – (5) and (6) – (10) should be used as the reference value.
One exception is where the lake percentage is less than or equal to 2,
where equations (2) and (7) should not be used.
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