Physical habitat assessment techniques were originally developed as a decision-making framework for assessing the impacts of water development projects on aquatic ecosystems. However, the wider application of these techniques, at least in Europe, concerns the calculation of environmental flows. Spain is a good example of this, where these techniques have been implemented in a very specific legislation and have become a policy concerning water resources management criteria.

There are several problems associated with this application of physical habitat assessment techniques, especially in Mediterranean rivers: i) a few hydraulic variables (depth, water velocity, substrate type, ...) may not always properly describe the amount of available habitat for a given species; ii) results of physical habitat obtained in a specific river reach can hardly be extrapolated to other reaches; iii) besides spatial variability, there is also a temporal variability in physical habitat data for a river reach (i.e. changes before and after flood events); iv) habitat preferences are of course different depending on species and life stages, and the may also change throughout the day. Therefore, preference curves only partially reflect the ecological requirements of the species (which on the other hand never occur separately but combined), and do not integrate requirements for all present species.

In this study, environmental flows determination is analyzed on a real scenario based on Spanish legislation. Results show a high dispersion on the quantification of the physical habitat, and therefore how useless are this kind of techniques for the calculation of instream flow needs.