NO. 58 DATE 09/09/02

Indicators of Hydrologic Alteration

By Steve Swanson, River Modeling Specialist,
BLM, National Science and Technology Center

Hydrologists of the Bureau of Land Management have long been challenged to identify and record anthropogenic effects on the natural flow regimes of streams and rivers on public lands. Increased public concern and legislation over the past 20–30 years have resulted in heightened scrutiny of the consequences of human-caused alterations to these natural systems.

The historical tools available to answer the question “How much hydrologic alteration is too much for a riverine system?” have not been adequate. Historical methods may focus on specific portions of the ecosystem, such as aquatic biology or riparian functionality, rather than the system as a whole. Another limitation involves the fundamental differences in language and analytical methods that exist between hydrologists and ecologists or biologists. Hydrologists tend to be concerned with the statistics of monthly and annual averages and the frequencies of floods and flows. Ecologists and biologists are often concerned with flow durations, low-flow extremes, and the rates at which water levels rise and fall.

Hydrologic Parameters Indicate Anthropogenic Effects
The Nature Conservancy has developed an approach, led by Brian Richter, in which the gaps in communication and methods are bridged. This approach, termed Indicators of Hydrologic Alteration (IHA), is a suite of 33 hydrologic parameters that are ecologically meaningful and serve as sensitive indicators of anthropogenic effects on riverine systems. These parameters and their potential influences are shown in the Table.

Some of these parameters are difficult or impossible to calculate by using standard spreadsheet or statistical software. The National Science and Technology Center (NSTC), Denver, Colorado, uses an IHA software package (Smythe Scientific Software) that calculates the entire suite of parameters by using daily streamflow data obtained from the U.S. Geological Survey. The program also plots the results and provides two types of statistical analysis and three methods of parameter analysis and interpretation:

Statistical Analysis

Parameter Analysis

Staff of the NSTC are presently using the Indicators of Hydrologic Alteration approach on a project in southern Arizona. This approach also has significant application to other locations within the Bureau.

Steve Swanson, National Science and Technology Center,
Denver Federal Center,
Building 50, P.O. Box 25047
Denver, CO 80225-0047
phone: 303-236-8693
fax: 303-236-3508

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