Lakes and Watercourses

TP ref (kg P/ha, year) =
0.002 · x 1 + 0.015 (1)
(0.10 · x 2 + 1.2) / (5 · x 2 + 12) (2)
0.91 · x 3 · 10 –3 + 0.02 (3)
2.45 · x 4 · 10 –3 + 0.024 (4)
3.15 · x 1 · 10 –4 · (5 + 60 · x 5 ) (5)
TN ref (kg N/ha, year) =
0.018 · x 1 + 0.85 (6)
–0.023 · x 2 + 1.25 (7)
0.008 · x 3 + 0.85 (8)
0.03 · x 4 + 0. 90 (9)
3.15 · x 1 · 10 –4 · (125 + 500 · x 5 ) (10)
where x 1 = specific flow (l/km 2 , sec)
x 2 = lake percentage in catchment
x 3 = area-specific loss COD Mn (kg/ha, year)
x 4 = area-specific loss silicon (kg/ha, year)
x 5 = flow-weighted mean absorbency 420 nm (abs f 420/5 )
The new function (5, 10) uses the absorbency of the water measured
using filtered water (0.45 µm membrane filter) in a 5 cm cuvette at a
wavelength of 420 nm. Taking account of the element of uncertainty
introduced, absorbency (abs f 420/5 ) can be calculated by multiplying
water colour (mg Pt/l) by 0.002.
COD Mn is derived by dividing the permanganate number by 3.95.
One of these equations may be unsuitable for use in some situations.
Hence, organic matter may raise the concentration of oxygen-consuming
substances, which will render use of COD inappropriate. If the pollutant
is largely uncoloured matter, the absorbency function (5, 10) will be a
better guide. This should in turn be avoided where it is suspected that
water colour is anthropogenically elevated, eg, as a result of discharges
from pulp and paper mills, leachate from rubbish tips or because of
increased humus losses caused by forestry practices. Silica (4, 9) may also
be anthropogenically affected, eg, in the form of lower concentrations
due to eutrophication, which will particularly impact on water systems
containing many lakes.
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