MAP Technical Reports Series No. 106 UNEP

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A generalized Model. Essentially the same order of magnitude is obtained from a
modified model approach. In this, the assumption is made that the generated total load is a
function proportional to the human population and its agro-industrial potential. To this end it is
assumed that the load contribution from agricultural activities (livestock and crop farming, etc.)
is correlated with population density and the level of agricultural development, and the index for
this latter in turn would be the amount of fertilizers used per unit of cultivated land. This avoids
giving to high weight to areas of prevalently extensive agricultural utilization (e.g., sheep and
goats raising), emphasizing instead intensive animal husbandry and crop farming. Indeed, there
is loose inverse proportionality between sheep and goats/inh. and fertilizer use/ha (cf. Table 11).
To this end the following model assumptions have been made:
i) estimation of nitrogen and phosphorus generation from population is reasonably
accurate;
ii) of the total domestic and industrial load generated, a certain minimum fraction is
retained and recycled in the basin;
iii) the percent estimates of population generated loads vary within certain limits of the
total basin load generated;
iv) it is reasonable to assume the percent contribution of population lies within the
range of 20 to 60% for nitrogen, and within 40 to 80% for phosphorus (cf. above).
v) the loss of nitrogen and phosphorus from land use (including both crop production
and livestock raising) is a function of the level of agricultural development, which
is assumed to be proportional to the intensity of fertilizer used.
Under i) the population figures in Table 6 are used assuming 4.4 and 0.54 kg per
capita/year metabolic production of nitrogen and phosphorus, respectively. Under ii) the further
assumption is made that the total nitrogen generated by domestic, industrial and related sources
is 1.2 times and that of phosphorus 2.3 times the metabolic value (which includes
polyphosphates in detergents 6 and from other sources, and that of both, nitrogen and
phosphorus, 50% is retained in the basin either in treatment plants, septic tanks, or otherwise.
With this simplified assumption the population values can directly be used for further calculation.
iii), iv) and i) are then combined for calculating a table using different assumptions for the basic
percentage to attribute to exports from the basin according to the following simplified formula:
Total export = M * ex * {1 + [basic %/(100 - basic %) ] * F.f.},
where M = nitrogen or phosphorus generated by population;
ex = export factor: 0.6 for nitrogen, 1.15 for phosphorus;
basic % = assumption for all diffused load regardless of country;
F.f. = fertilizer use factor by country as calculated in Table 9.
6 For some countries that introduced legislation to limit the polyphosphate content in
detergents, like Italy, the figure 2.3 may be somewhat in excess of the present conditions.