3.5. minimum and ecological flow
The ecological quality of rivers must be maintained by maintaining a minimum flow. Rivers must not dry-up or have their physical regimes significantly altered in order to conserve the hydrological and ecological functions of their drainage networks. This question must be borne in mind when planning and managing the water resources, especially in semi-arid zones. Ecological discharges, which take place as a result of the aquifer discharges in a natural regime, can be artificially maintained by reservoir management. The determination and mapping of ecological flows for semi-arid areas of EEA is, therefore, considered to be of paramount importance (INAG, 1995a).
Some of the Spanish River Authorities have decided that ecological use is the next highest priority after water supply to cities and towns. The Spanish Water Act (BOE, 1986) refers to the need of maintaining a minimum discharge with a view to guaranteeing the conservation of natural environments. However, neither the Act nor the regulations that develop it are explicit about how to calculate such a discharge or its relationship with the ecological discharge, these questions being left to the discretion and responsibility of the various River Authorities (Ruiz, 1993). One of the few existing ways to approach the problem of determining the ecological flow, is included in the legislation for the Principality of Asturias (Consejo Gobierno del Principado de Asturias, 1987), where river reaches are divided into three types (trout fishing, of fishing interest and salmon fishing); different ecological discharges are established as a function of the discharges guaranteed for 347 days of the year.