Demonstration Flow Assessment and 2-D Modeling: Perspectives Based on Instream Flow Studies and Evaluation of Restoration Projects

Railsback and Kadvany (2008) make a compelling case for the use of demonstration flow assessments (DFA) for instream flow assessments as a more robust method than traditional one-dimensional habitat simulation techniques, such as the Physical Habitat Simulation System. But, based on experience with DFAs used for instream flow assessments and evaluation of stream restoration projects, the methods presented by Railsback and Kadvany (2008) may not give reproducible results, and DFAs have significant drawbacks relative to two-dimensional (2-D) hydraulic and habitat modeling. Application of the DFA methodology presented in Railsback and Kadvany (2008) to assess a stream restoration project on the Trinity River, California, did not give reproducible results, with substantial disparity between replicate surveys in the total quantity and spatial distribution of habitat. As an empirical two-dimensional habitat modeling method, DFAs have several drawbacks. Compared to 2-D models, DFAs require interpolation and extrapolation of results from the observed flows, and lack biological realism because they use categorical (binary) habitat suitability criteria. Further, 2-D modeling offers significant benefits over DFA in the context of adaptive management. Advantages of DFAs cited by Railsback and Kadvany (2008), namely the feasibility of assessing habitat with complex hydraulics, the ease in using mechanistic and theoretical conceptual models, and the ability to assess long reaches, also apply to 2-D models. Modification of the DFA methods presented in Railsback and Kadvany (2008) to give reproducible results may increase the cost of DFAs or reduce the length of stream that can be assessed with DFAs to the point where 2-D modeling may be a more cost-effective technique than DFAs.