Case Studies - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 391

400

Rainfall
Evaporation

deficit of the net annual hydric
balance > 600,000 m 3 /km 2

300

200

100

0

J-1981 J-82 J-83 J-84 J-85 J-86 J-87 J-88

years

FIGURE 9.3.3 Hydric balance in the Mar Menor area over a 10-year period. Rainfall is
scarce and usually concentrated in brief, torrential downpours during the year. (Data from
the San Javier meteorological station.)

evaporate and are lost through infiltration and do not reach the lagoon except in
instances of strong torrential rainfall. 5 Most of the watercourses discharge in the
southern half of the lagoon but that is determined by the sporadic and torrential
rainfall. Among these, the Albujón watercourse, the main collector in the drainage
basin, is an exception at present because it maintains a regular flux of water due to
changes in agricultural practices, as described below.
Annual total inflow of fresh water through run-off and rainfall into the lagoon
ranges from 27.9 to 122 Hm 3 while 155 to 205 Hm 3 evaporates, resulting in a hydric
deficit ranging from 38 to 115 Hm 3 per year. The net loss of fresh water is compen-
sated for by saltwater inputs from the adjacent sea, 6,7 and it is regulated by differences
in the sea level between the lagoon and the Mediterranean Sea. 8 The lagoon water
budget with the relative contribution of different components to the lagoon volume
is shown in Figure 9.3.5.

9.3.1.2 Hydrodynamics

The lagoon hydrodynamics is mainly driven by winds and thermohaline circulation
(Figure 9.3.1) . The exchange of water between the lagoon and the adjacent Medi-
terranean Sea is mainly driven by differences in sea level phases, 8 with the El Estacio
Channel playing the most significant role. The resulting exchange rates lead to
residence times of the water bodies in the basin that change from year to year and
are responsible for the development of the lagoon water characteristics and hence
of the long-term ecological balance in the lagoon. The salinity of the lagoon waters
ranges at present from 42 to 46 (with an annual mean value of 44.4) showing a
north-south gradient ( Figure 9.3.6) . 9 Three main gyres can also be identified in a
general circulatory pattern along this axis allowing us to differentiate three basins