MAP Technical Reports Series No. 106 UNEP

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confounded with source emission. E.g., the amount of phosphorus and nitrogen in faeces
produced per a 250 kg cow/day, determined under controlled rearing conditions, is rather
constant, but the amount of phosphorus and nitrogen discharged to waterways from a herd of
cows may vary according to forage conditions, age composition, rearing environment,
zootechnical practice, etc. In a similar fashion, the unit amount of a nitrogen fertilizer applied to
agricultural fields represent the unit source strength, but the amount lost from different fields
depends on a variety of factors, such as the exact chemical nature and composition of the
fertilizer used, soil properties, slop and drainage conditions, crop produced, etc. Accordingly, in
order to assess the relative importance of all nitrogen and phosphorus sources, each single
source has to be evaluated separately, and its fractional emission determined. The composite
total is then obtained by summation. For reference cf. e.g., Vollenweider (1968), Porter (1975).
In practice, a complete source assessment for each single case and basin is a tedious
task, and certain shortcuts may be unavoidable. Assessment of point sources at the outlet point
(normally discharge pipes) is generally straight forward on the condition that temporal variations
in source discharges are duly taken into account. Quantities may be expressed as e.g., kg/day,
t/year, etc., but in some cases it may desirable to refer quantities also to certain units, such as
e.g., kg/inhabitant/year. Diffused sources cannot be measured directly, but may be assessed
indirectly. This is possible, if basin conditions are reasonably well known, and experience gained
from pilot studies or appropriately selected sub-basins can justifiably be transferred to and/or
extended to the basin in question as a whole. Unit source emission of diffused sources is often
expressed in terms of export coefficients, e.g., as kg/ha.year. Export coefficients are estimated
experimentally in appropriately selected sub-basins, and careful analysis of the exported nutrient
load at a calibrated river station.
Table 1 provides some ideas about the magnitude and variation of such export
coefficients as obtained in some 40 agricultural basins in Europe and North America. Values
very much larger than the means resulted from highly fertilized fields.
Phosphorus:
Table 1
Export Coefficients for Phosphorus and Nitrogen from Agricultural Areas,
kg/ha.year
AVG STD Nr GeoMean ±1 STD ±2 STD
USA/Canada 0.26 0.29 15 0.15 0.43 0.05
Europe 0.45 0.58 11 0.21 0.79 0.06
All 0.34 0.45 26 0.17 0.57 0.05 1.89 0.02
Nitrogen:
USA/Canada 23.32 34.59 17 10.17 34.64 2.98
Europe 13.11 11.35 25 6.84 30.21 1.55
Max/Min 2.14 0.01
All 17.39 24.52 42 8.08 32.62 2.00 131.8 0.50
Max/Min 120 0.10