The EVHA software can be used to evaluate the physical habitat of river fish and is thus an invaluable aid for predicting flow rates or instream regimes downstream of dams.
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How can the consequences of changing the flow rate of a river to the fish populations downstream of a dam be predicted? At what flow rate will the habitat be most favourable to the fish at a given stage of their development? How can this flow rate be positioned in the reconstructed hydrologic chronology of the site?
To answer these questions, managers, planners and consultancy firms specialising in impact studies need a simulation tool that factors in the living component, particularly fish.
The EVHA software, developed by the Cemagref Lyon Laboratory of Quantitative Hydroecology, can be used to evaluate the physical habitat of river fish and is an invaluable aid for predicting flow rates and also instream regimes downstream of dams.
The chief concept underlying the software is the coupling of a hydraulic model with a biological model, wherein the former describes the physical characteristics of the river and the latter translates these characteristics into the carrying capacity for fish Hence for each intended flow rate, the simulations give the potential fish habitat for entire river segments.
Since 1985, the Cemagref Laboratory of Quantitative Hydroecology has been developing and adapting this method to French watercourses. The initial objective of this method was to help in determining instream flow rates. It is also an investigation and research tool for enabling a better understanding of the links between the physical habitat (amount of available habitat and temporal variations) and the fish (ranging from the spatial distribution of individuals to the population dynamics). We must point out, however, that this method is not designed for analysing factors other than the physical habitat, nor will it answer all questions that could be raised about aquatic ecosystems, such as questions regarding pollution, water quality, temperature, aquatic vegetation, etc.
Requested by the French Ministry for Regional Planning and the Environment, EVHA is currently available in version 2.0, which includes a hydrologic module that makes it possible to integrate the time dimension in the analyses. It is presently adapted to the trout streams that served as references for developing the models. It is now in the process of being developed so that it can be used on larger watercourses with multi-species communities.
The implementation of the method requires the collection of field data, the details of which are explained in the guide that comes with the software. A topographic and hydraulic measurement campaign is required. The latter consists of recording the topography of representative stations of the segments being studied as well as collecting data on velocities, depths, and particle sizes of the substrate in carefully selected cross-sectional profiles at these stations. After the data are input and verified, the depths and velocities at different flow rates can be calculated with the hydraulic model integrated in the EVHA software. The physical data are then linked to the biological model consisting of preference curves established for different life stages of the species being modelled. This linkage is used to calculate potentially exploitable surfaces as a function of the flow rate, translating the concept of carrying capacity into m² of favourable habitat. The new hydrologic module can then be used to analyse habitat chronologies linked to the natural hydrological regime and to instream regime scenarios relative to those stages and periods that are considered to be limiting factors.
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