Erasing the boundaries

Posted on June 19, 2017

The new discipline of ecohydraulics considers the effects of water movement in aquatic ecosystems and blends ideas and techniques from aquatic ecology and engineering hydraulics. A central challenge for ecohydraulics is to reconcile differences between hydraulic engineers and ecologists in terms of the spatial and/or temporal scale of the processes that they tend to deal with. In the first paper of the Journal of Ecohydraulics, the authors propose that guiding principles based on reference frameworks can help to integrate the emerging discipline.  They note that, starting in the late 1970s, computerised, cell-based hydraulic models were introduced to simulate stream depth and velocity conditions, and that these were soon combined with information on habitat suitability to produce the first recognizable ecohydraulics tool (PHABSIM, short for physical habitat simulation). Critical assumptions of PHABSIM, and other similar methods, are that the scales used to quantify river discharge and define habitat in a given cell are similar, and that the influence of flow on fish populations is represented by local habitat conditions. However, although PHABSIM has been widely used, it’s been criticized for its lack of biological realism and its inability to deal with causes of population decline unrelated to physical habitat degradation. Single reference frameworks like those underlying PHABSIM are inadequate if their simplifying assumptions are violated – for example, if organisms such as migrating fish respond to flow conditions across multiple locations rather than just the conditions in their specific cell, or if their use of habitat is motivated by non-hydraulic factors, such as hunger, memory, parasite load, water quality or the presence of predators or competitors. In such cases, the use of multiple reference frameworks, made possible by advances in computational fluid dynamics, allows for much greater levels of realism.  Typically, this approach integrates three frameworks – a stationary framework that describe fluxes of water through a fixed grid, a mobile framework that follows the movements of individual organisms through space, and a third framework with a cognition algorithm that allows a “smart particle” to gather information from its environment and respond accordingly. One successful test of this technique has been its ability to describe the downstream movements of migrating juvenile salmon in the U.S.A.

 Reference: Nestler, J.M. et al.  2016.  Ecohydraulics exemplifies the emerging “paradigm of the interdisciplines”. Journal of Ecohydraulics 1, 5-15.