Application of the Indicators of Hydrologic Alteration Software in Environmental Flow Setting1

Abstract

Abstract: River scientists are increasingly asked to provide recommendations about the amount and timing of water flows needed to support ecosystem health. The need for scientifically credible environmental flow assessments and the limited availability of resources to conduct in‐depth studies place a premium on methods that can be readily applied at low cost. The Indicators of Hydrologic Alteration (IHA) software program was originally developed by The Nature Conservancy in the 1990s to quickly process daily hydrologic records to enable characterization of natural water conditions and facilitate evaluations of human‐induced changes to flow regimes. The evolution of the IHA software is discussed, including recent revisions and additions to the IHA that have improved its utility in environmental flow‐setting processes. Drawing from holistic methodologies developed around the world, the ability to calculate characteristics of five components of flow important to river ecosystem health – extreme low flows, low flows, high‐flow pulses, small floods and large floods – has been added to the IHA. A practical advantage of these environmental flow components is that an environmental flow prescription based upon them can be readily implemented in most water management settings. Frequently used as a one‐time generator of flow statistics, the value of the IHA increases when used interactively with ecological models. A process for linking IHA output to an ecological model to explore flow‐ecology relationships within an adaptive management context is presented and demonstrated through its application on the Green River, Washington. The IHA used in conjunction with ecological models facilitates the creation and testing of flow‐ecology hypotheses, formulation of water and land protection or restoration goals, and development of a focused research and monitoring program, all important components of environmental flow‐setting processes.