Interactions between groundwater and river marta (central italy) and problems related to the definition of the instream flow

Abstract

An analysis of the interactions between surface and ground waters has been proposed for the upper basin of the River Marta (about 578 km2) in order to examine the problems related to the instream flow definition for some significant river sections.The River Marta, outlet of the Lake Bolsena, presents along its master course five hydroelectric power stations and the waste of some towns and little industries. The groundwater resources have been extracted from several wells. In addition, the basin is characterized by some areas of environmental value.The analysis included the collection of the streamflow records of the River Marta, new measurements of the streamflow, processing of the meteorological data and acquisition of the data concerning wells and springs.It emerged that the streamflow is fed from groundwater, being the river the main discharge of the volcanic aquifer, as well as from the effluent of the Lake Bolsena and surface runoff. A first average annual evaluation of the water budget of the basin points to an amount of water resources of about 200u106 m3/year, among which 51% is constituted by effective infiltration, 30% by surface runoff and 19% by groundwater inflow from outside the watershed. The flow leaving the system discharges mainly in the river. Actual water loss from the system is constituted by the withdrawals for irrigation (about 20% of the overall water resources).The determination of the instream flow of the River Marta applied to some significant sections with different methods leads to various values. The experience showed that the evaluation of instream flow cannot be based only on the investigation of the water-course. The whole hydrogeological system connected with the river must be considered too, particularly when the river is fed from groundwater. It is necessary to take into consideration not only the direct withdrawals from the river but also the pumping from the aquifer which impoverishes the river baseflow.