Lakes and Watercourses

Mean concentration of total phosphorus (µg P/l)
200
180
160
140
120
100
q = 5 l/s,km 2
q = 10 l/s,km 2 10000
9000
8000
7000
6000
5000
q = 5 l/s,km 2
q = 10 l/s,km 2
80
q = 15 l/s,km 2
4000
q = 15 l/s,km 2
60
40
q = 20 l/s,km 2
3000
2000
q = 20 l/s,km 2
20
1000
0
0 0.04 0.08 0.12 0.16 0.20 0.24 0.28 0.32
0
0 2 4 6 8 10 12 14 16
Area-specific phosphorus loss (kg P/ha, year)
Area-specific nitrogen loss (kg N/ha, year)
Mean concentration of total nitrogen (µg N/l)
Figure 1. Annual mean concentration of phosphorus and nitrogen as a function of annual area-specific loss.
arable soils less susceptible to erosion, often with seeded grass cultivation.
Class 4 represents losses from fields under open cultivation and class 5
shows leaching from arable soils susceptible to erosion.
Since phosphorus losses over fairly large agricultural areas exceed 0.32
kg P/ha, year, there is reason to make particular note the subgroup of
areas within class 5 where phosphorus losses are extreme (over 0.64 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.
Assessment of deviation from reference values
TABLE 7.
DEVIATION from reference value, concentration of total
phosphorus in lakes
Class Description Recorded concentration/ reference value
1 No or insignificant deviation ≤ 1.5
2 Significant deviation 1.5 – 2.0
3 Large deviation 2.0 – 3.0
4 Very large deviation 3.0 – 6.0
5 Extreme deviation > 6.0
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