7410

• UCREFRP Catalog Number: 7410
• Author: Williams, O. R., S. L. Ponce and A. E. Johns
• Title: Use of Departure Analysis in Decision Making
• USFW Year: 1988
• USFW - Doc Type: American Water Resources Association
• Copyright Material: YES

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Other land management actions that are far less ambitious than large-scale dam
construction may have impacts upon water-related resource values. Por example,
withdrawals of water from a stream system for irrigation may alter a river's dynamic
equilibrium to the point of accelerated bar formation. This, in turn, can produce
favorable habitat for vegetation encroachment (Northrop, 1965) and consequent increases
in water consumption by phreatophytic plants. The reverse of this process can occur
with urbanization and vegetation eradication. Increases in impermeable surfaces (Park,
1977) and decreases in evapotranspiring surfaces (Troendle and King, 1987) can each
increase surface water yield, and thus kinetic energy in the channel.

IHPACr EVALUArION

It seems, as described by Cabu (1978), that any action affecting the environment
produces unending study and review. This trend is likely to continue as resource
scarcity becomes more commonplace. With this the case, it is necessary to develop
approaches that will produce the needed information at reasonable cost while providing
adequate statistical and managerial certainty. Managers require a means wq.ereby they
may examine the trade-offs which inevitably occur as a result of their decisions and
which may visit economic or environmental cost upon other segments of the economy and
even future generations. However, to fully understand all of the impacts of a deciSion,
a manager would have to be truly omniscient. In fact, the current state of the science
does not even provide the means for a complete understanding of environmental effects,
let alone long-term interactions.

However, scientific techniques for examining the responses of a limited number of
facets of the ecosystem do exist. These techniques provide useful estimates of the
response of certain segments of the ecosystem to stresses imposed by humans. Further,
these techniques can be utilized in combination to develop at least a portion of the
information needed to adequately assess decision impacts. A decision assessment
methodology which employs this approach has been developed by the National Park Service
and is called Departure Analysis.

The Departure Analysis indirectly describes the complex response of water-related
resource attributes to land use management decisions affecting hydrologic regimes by
closely examining, and then predicting the response of, one or more dependent
variables. Consider a case in which the perpetuation of indigenous fish is a management
objective. In this case the responses of fish to changes in hydrologic regime are of
concern to management. However, these responses are difficult to directly predict. An
alternative approach is to predict changes in those physical and/or biotic components of
the aquatic environment most closely related to the well-being of fish populations.
Such a component could be fish habitat as measured in units of square feet of channel
bed area covered by a minimum amount of water during specified life stages. This
variable can be measured in an objective manner and reasonably predicted for
hypothetical regimes of discharge.

For hypothetical flow regimes the Departure Analysis estimates habitat area which
can be compared to that measured under existing hydrologic conditions. This information
can be used to demonstrate departures, if any, from the existing condition. This allows
an impartial determination of whether or not the water-related resource attribute __ in
this case indigenous fish - would be affected by a regime of flow which differs from
existing in terms of total amount or rate of delivery.

This technique has much in common with the USDI Fish and Wildlife Service's Instream
Flow Incremental Methodology (UIM) which examines potential impacts to fisheries from
proposed water developments and/or operations (Bovee, 1982). 1'he Departure Analysis,
however, addresses a broader spectrum of resource values, impacts, and potential water
development. Further, the Departure Analysis attempts only to identify change, it does
not evaluate that change as positive or negative.

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