MAP Technical Reports Series No. 106 UNEP

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"active" 5 size of this basin, and known river inputs. Conceptually, the amounts of nutrients that
flow into a body of water are a function of natural losses augmented by anthropogenic inputs that
in some way are proportional to human activity, which latter in turn is proportional to population
density. Accordingly, the total loads cannot exceed a fixed upper boundary; hence, the question
arises how to estimate this boundary, as well as how to evaluate within some degree of
confidence the probable range of the likely real load.
1) Upper Limits. The 1985 population of the 18 countries bordering directly the
Mediterranean (i.e. excluding all Black Sea bound countries, and the Upper Nile African
countries) totals 355.6 millions, and the total surface of these countries amounts to 8.5 million
sqkm (cf. Table 7). Accordingly, the mean population density would be 39/sqkm.
However, even for approximate estimates of nutrient loads these values are unusable
in this form. Actual drainage basin size and resident population would be a better starting point.
UNEP (1989) provides figures for the resident Mediterranean population, country by country, but
no figures for the respective actual basin surfaces. Correcting the MAP-UNEP data given for
France and Italy, the population directly Mediterranean bound would sum up to 153.9 millions,
or 43% of the population of the 18 countries. Further, according to UNEP (1984) of the 154
millions about 40 to 45% would reside in coastal areas (UNEP, 1976 data report 44 millions).
Using the % population figures to approximate the corresponding basin size, a first
correction reduces the basin estimate to 3.9 million sqkm. Still, in considering the approximate
land needed to provide the food and other resources to maintain a self sustaining population of
150 Millions, the figure of 3.9 Million sqkm seems too high for the "active" basin size to use as
reference for estimating the nitrogen and phosphorus load from land that may reach the sea.
Taking also the extent of unproductive areas, country by country, into account, the figure for the
Mediterranean bound "active" basin would reduce to between 1 to 1.5 million sqkm and increase
the mean density to about 125 inh./sqkm. These values are somewhat lower than the UNEP
(1983) value of 1.8 million sqkm for the Mediterranean region, and slightly larger than the
estimated catchment area of 0.85 million sqkm (excluding the River Nile basin) estimated for 69
rivers with a total discharge of some 8840 m 3 /sec draining into the Mediterranean.
Using the regression model valid for the Po (cf. Table 5a), the expected nitrogen load
from land-based sources could be in the order of 1.5 million t/y, while the phosphorus load would
amount to some 0.1 million t/y (Table 8). Yet, this latter figure appears to be on the low side.
Using instead the model figures of Table 4, the estimated nitrogen export for a 1 million sqkm
area might range from 1.2 to 3 million t/year, and that of phosphorus from 0.16 to 0.25 million
t/y; taking 1.5 million sqkm area as reference, the maximum loads would be in the order of 4.5
and 0.38 million tonnes, respectively, which likely represent upper boundary values.
The difficulty lies now in the attempt to narrowing down these figures to values that are
consistent with the known nutritional and dynamic conditions of the Mediterranean.
5 The term "active" basin is used here as the size of the area that includes population
settlements plus all more or less intensively managed land areas to sustain the needs of the
population. Accordingly, barren areas, or areas utilized that do not essentially contribute to nitrogen and
phosphorus exports are not considered.