Transect Analysis Using PHABSIM
Modeling available habitat for fish via PHABSIM: can a less intensive data collection effort yield equivalent results that a large data collection effort yields?
Need and Benefit
To help meet its obligations to implement Action 149 of the Biological Opinion (BiOp) on operation of the Federal Columbia River Power System (FCRPS) issued by National Oceanic and Atmospheric Administration (NOAA) Fisheries in 2000, Reclamation is required to conduct PHABSIM studies over a 10-year period in priority subbasins to address flow needs of listed fish species.
PHABSIM is a software program utilized to predict the amount of habitat available to specific fish species at varying flows in specified river systems. Data that are input into the program consist of stream width, depth, velocity, discharge, substrate, and cover data collected on a number of transects within a particular river. In many Instream Flow Incremental Methodology (IFIM) studies, a high number of transects are used to collect data under the misconception that a large number of transects is always necessary to yield the most accurate results. Given the importance of balance between accuracy and cost for IFIM and PHABSIM studies, there are very few analyses that have examined the question of sufficient number of transects.
The number of transects required to produce valid habitat indices, or Weighted Usable Area, is directly dependent on habitat variability. In rivers that have a high degree of habitat complexity, more transects may be required to accurately capture the hydraulic and physical variability of the stream segment. In stream segments with more uniform river channels, analyzing fewer transects may yield identical results as analyzing a higher number of transects. Several other studies have shown that fewer cross sections of data can yield equivalent results as many cross sections but at a lower investment of money and resources. Under the objective of cost savings, we propose to show that, relative to habitat complexity, the number of transects at which data are collected can be greatly reduced without compromising the accuracy of the PHABSIM results.
Within the John Day River Basin in Oregon, a high number of transects were surveyed during 2003 and 2004 for a PHABSIM study to model the amounts of habitat for threatened bull trout and endangered Chinook salmon and steelhead at varying flows. This river basin contains stream segments that exhibit a wide range of habitat complexity. Using data already collected, we propose to compare the PHABSIM output results for segments with a range of habitat complexity within the John Day River Basin using data from many transects with the habitat output results using incrementally fewer transects. We plan to categorize stream segments by level of fish habitat variability and conduct analyses within each segment to determine the number of transects that will yield equivalent results as the higher number of transects. By conducting repeated PHABSIM analyses with incrementally fewer transects, we will determine the number of transects at which weighted usable area (WUA) graph validity decreases for specific habitat complexity levels.
The resulting information from this research will provide guidance and save money and effort for future PHABSIM studies within the John Day Basin and other subbasins affected by the FCRPS BiOp. We are contracted to conduct PHABSIM studies on rivers within this basin for the next seven years. By showing that less effort will still produce valid results, we can conduct accurate PHABSIM analyses and save Reclamation time and money.
This information will be applicable to other PHABSIM studies in guiding the planning process by quantifying the habitat variability and setting a standard for the number of transects at which to measure depth and velocity. Ultimately, this will conserve time, effort, and money by eliminating redundant, unnecessary transects. This cost-saving measure will allow Reclamation to conduct PHABSIM studies in river basins throughout the Western United States. Fish habitat availability information resulting from PHABSIM analysis can positive
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