New perspectives for ecological flow determination in semi-arid regions: a preliminary approach

Abstract

Streams in semi-arid regions, such as the south of Portugal, have highly seasonal flow regimes with a marked pattern of low or zero flow during summer and early autumn. As a result, throughout these months, most rivers have long dry reaches with occasional pools. The biotic communities have evolved specific adaptive strategies to face those alternating lotic–lentic conditions, but when the dry season becomes longer and/or more extreme, the environmental stress can be lethal for important groups of the biota. Most ecological flow methodologies have been developed for permanent streams, namely salmonid streams. Ecological flow determination for temporary rivers requires a different approach, as an essential issue is the maintenance of the pools until the end of the dry season in such conditions that make possible the survival of the aquatic communities. Groundwater plays an important role in this process because of the hydric connection with the surface water, and so it should be considered. In this paper, the limitations of the most common ecological flow methods to the unique Mediterranean temporary rivers are discussed, particularly for the summer period. Because of the utmost importance of summer conditions, an empirical approach was developed. Three types of procedures were involved: (1) characterisation of fish assemblages along the river in order to identify the most important reaches, (2) analysis of the existing aerial photography to identify interannual variation of summer water availability for the river in general and especially for the more relevant reaches, (3) development of a precipitation–run-off model to relate river run-off to the persistence of the summer pools. The purpose was to define the lowest possible flow regime compatible with favourable water conditions during summer. Copyright © 1999 John Wiley & Sons, Ltd.