Idrogeologia del Lago Trasimeno: sintesi, problemi, aggiornamenti

Hydrogeology of Lake Trasimeno: up to day knowledge and problems

Abstract

The Trasimeno is a closed lake subject to strong pluriannual oscillations: the building of a large outlet has lowered the mean level of the lake but it hasn’t eliminated its level variations. The scarce depth of the lake and the flatness of its bottom and banks cause vast and repeated marches damaging heavily the ecosystem of the lake and the local Economy. In the time between 1959 and 1961 in order to find some remedies for such a state of things the drainage basin was enlarged artificially, but the results are not satisfactory. The Trasimeno basin is generally composed of formations with little permeability. The flysch known as “Arenarie del Monte Cervarola” of Oligo-Miocene age and river-lacustrial Villafranchian and present sediments are the main outcrops. The hydrogeologic characteristics of such formations and the general structural pattern show that there are scarce possibilities of underground outlets from the catchment basin towards external zones. Despite the scarce permeability a weak water body, which feeds the lake, has taken place. In this work a linear relation between level variations and annual rainfalls has been found. This has given the possibility to simulate with good approximation the lake levels, to calculate the mean annual loss of the water surface (1 500-1 600 mm), the needed rain to compensate it (about 710 mm) and the coefficient of run-off of the drainage basin (0,50). Furthermore, it has been possible to estimate the effects of the enlargement works of the basin already carried out and of eventual future enlargements and the ground-water balance has been roughly determined. Studies on the annual precipitations on the lake basin from 1921 to 1980 show also that the lake level variations are linked with pluriannual cyclic of high and low rainy periods: the actual hydric yield is not only insufficient to eliminate the effects of the low rainy periods, but even to limit them in an efficacious way.