Determining Ecological Flow Needs in North Carolina
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The Division of Water Resources (DWR) has adopted a river basin approach for the
long-range planning that is necessary to guide the use of North Carolina's water
resources in a sustainable manner. Put simply, this means that for
planning purposes DWR will evaluate the current and projected uses of surface
waters against the amount of water available in each of the
17 major river basins
in North Carolina. The analysis of this
information, and whether there is enough water to meet instream needs and support
existing and projected offstream uses, will be presented in individual plans
specifically prepared for each of the major river basins.
A critical component of this planning effort is the determination of how much flow
is needed to support and sustain the aquatic life present in specific
watercourses. The amount of water needed to sustain the diversity of
aquatic life and the functioning ecosystem in a river or stream is referred to as
the "ecological flow
The challenge facing DWR is how to determine the appropriate ecological
flows for the variety of waters in North Carolina, so these values can be
included in our comprehensive river basin planning efforts. DWR presented
a Water Allocation Research Seminar on this topic on June 29, 2010.
to view this slide show.
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The Division of Water Resources' (DWR) overall planning efforts rely heavily on
river basin hydrologic models
that are prepared
within or for DWR. These models, when completed, will allow evaluation
of water availability at any specified location. Scenarios that would
be routinely analyzed include stream flows as influenced by water withdrawals and
wastewater discharges under existing conditions. DWR will also analyze withdrawals needed
to meet the projected demands for the 20- and 50-year time horizons.
To simulate declining stream flow conditions in the event of increasing atmospheric
temperatures and accelerated evaporation, one or more additional scenarios will
be analyzed that simulate climate change for the 50-year time horizon.
Each river basin model will also include a set of hydrologic data for the unaltered
flow condition with no reservoirs, withdrawals or discharges.
During river basin modeling discussions, it became clear that a key piece of the
overall approach is how to quantify the amount of flow needed at a specified location
to maintain the ecological integrity of the water body - or the "ecological flow."
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Ecological Flows Defined
As defined in the NC General Assembly's
Water Allocation Study report
is "the ability to support and maintain a balanced, integrated,
adaptive community of organisms having a species composition, diversity, and functional
organization comparable to that of the natural habitat."
A living system exhibits ecological integrity if
it recovers from a disruption and continues to provide the natural goods and
services that normally accrue from that system.
encompasses the following characteristics of stream
flow and their interactions: magnitude, timing, frequency, duration and rate of change.
A flow regime that protects ecological integrity is often referred to as an ecological
, although the latter term is sometimes used in a more general
sense to refer to the amount of water flowing in a stream - without providing any
particular level of protection.
 Karr, J.R. and D.R. Dudley. (1981).
Ecological Perspectives on Water Quality Goals. Environ. Manage. 5: 55-68.
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Earlier efforts to maintain instream flows focused on minimum flows or minimum releases from
dams. This approach can work as long as the withdrawals of water from a stream
are relatively small in the context of the naturally occurring flows in that stream.
However, withdrawals often increase over time as various demands for offstream
water use expand, including public drinking water supplies, industrial uses, irrigation and
use in power generation
. This is especially true in areas experiencing significant
and land development.
If withdrawals are increasing and the only protection for instream flows is the
specified minimum value, more and more of the stream flow present on a given day
is withdrawn (or stored in a reservoir) for offstream use. This results in increasingly
frequent and longer periods when the only flow remaining in the stream is the specified
minimum value. There are two negative aspects to such a trend.
First, minimum flows are intended to be only occasional short-term events that
maintain stream conditions at a survivable level for aquatic life. One
example of such a minimum flow requirement is the "7Q10 flow" - the lowest flow
occurring for seven consecutive days, with a probability of occurring once every 10 years.
This is a drought flow statistic that is used to determine wastewater discharge
effluent limits such that the pollutant load can still be assimilated and chemical
water quality standards can still be maintained during the driest week occurring once
every 10 years. This type of minimum flow will not protect ecological integrity
if it is frequently the only flow in the stream, and/or occurs for long periods of time.
The potential for global climate change
to change the patterns of water availability adds
to the importance of protecting ecological flows, not just maintaining minimum flows of
Second, a minimum flow approach does not incorporate critical characteristics
of a flow regime (magnitude, timing, frequency, duration, variability and rate of change)
needed to protect ecological integrity. Stream biota have life
cycles that are adapted to a flow regime, not a constant minimum flow
(Poff and Allan 1995)
Minimum flows lack the variability between different times of year (monthly and seasonal),
as well as the inter-annual variability between different types of years (wet, dry, average).
When this variability is reduced or lost, aquatic species
diversity is often diminished
Species that are most tolerant of degraded ecosystems can predominate - analogous
to the way that weeds and invasive exotic species can colonize cleared land.
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More Comprehensive Approaches Used to Determine Ecological Flows
The issue of how to best determine and protect ecological flows is not unique to
North Carolina. In fact, there are ongoing efforts in other states and
other countries with the same goal. Some good background information
and examples can be found at the:
At this point in the process for North Carolina, the state Division of Water Resources
is focused on identifying the best technical approach
to determine ecological flows
throughout the state. Any eventual application would undergo a policy
review that would consider a whole range of issues - a process that would involve
the affected stakeholders.
Much can be learned from investigations conducted in other states.
The Southeastern Instream Flow Network
and Instream Flow Council
(IFC) have also compiled a great deal of useful information.
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Protecting Instream Flows in North Carolina
At present, the only statute that directly regulates withdrawals in North Carolina
is the Water Use Act of 1967 - and this only applies to designated
Capacity Use Areas
. The N.C. Division of Water Resources (DWR), however,
may include conditions regulating withdrawals and instream flows when reviewing
various other permits that might be required. These permits include sections
401 and 404 of the Clean Water Act, the N.C. Dam Safety Act, federal hydropower licensing,
and reviews and environmental studies required by the National and N.C. (State) Environmental
Policy Acts (NEPA and SEPA). A slide show overview of how instream flows
are currently managed in North Carolina can be seen at 2007 WRRI conference
DWR's instream flow program
approach is project-specific, and may involve in-depth field studies and aquatic
habitat modeling to determine instream flow recommendations.
DWR's comprehensive river basin planning program, however, requires an approach
that can be applied throughout the state for planning purposes. Ecological
flow values are needed at every location of interest in river basin hydrologic models
to assess water availability for both instream and offstream needs - now and in the future.
The current challenge is to develop an approach that can determine ecological flow
values for a multitude of locations in a relatively short time. Such an
approach also needs to reflect the flow regime characteristics needed to maintain
ecological integrity, and recognize the diversity of stream types and ecosystems in
North Carolina. A one-size-fits-all minimum flow standard for the entire
state would not accomplish this objective.
It should be noted that any new approach is not intended to replace in-depth,
site-specific studies for particular water project proposals - especially those larger
projects with more complex environmental concerns.
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Hydrologic Stream Classification
A first step often used in determining ecological flows is to develop a classification
system for streams that subdivides surface waters into a manageable number of distinct
groups. The approach for quantifying ecological flows can then be tailored
to the hydrologic and ecological characteristics that define each group.
A good example of statewide stream classification is the system developed in
In the summer of 2009, staff from DWR,
the Wildlife Resources Commission (WRC)
the Environmental Defense Fund (EDF)
began discussing what technical work was needed
to develop a technical approach for determining ecological flows in North Carolina.
Funding from EDF was used to contract with
Environmental Flow Specialists
to develop a hydrologic stream classification
system specifically for North Carolina. DWR, WRC and EDF jointly reviewed the progress of this work.
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Hydrologic Stream Classification System for North Carolina
The consultant's work was completed in December 2009 and culminated in a two-day
workshop for professionals selected for their expertise in various aspects of stream
ecology. The workshop first introduced the hydrologic stream classification
system developed for North Carolina - the technical analysis behind the classification
system and how to use the software. The consultant also performed additional
analysis comparing USGS stream gage data and flows simulated by the OASIS model for the
Neuse basin. The workshop notes, slide shows and
final study report can be viewed here.
The hydrologic classification software for North Carolina is a proprietary product.
Non-profit organizations and governmental agencies can purchase the software by
Environmental Flow Specialists
The initial hydrologic classification system consists of seven distinct types of
streams. It is important to recognize that these streams were grouped
only according to a statistical analysis of daily stream flows - using data from
231 U.S. Geological Survey stream gauging stations with at least 18 years of record. The seven classifications are:
|A. Coastal Streams||E. Large Piedmont Rivers|
|B. Small Stable Streams||F. Medium Stable Streams|
|C. Large Stable Streams||G. Small Seasonal Streams|
|D. Small Flashy Streams|| |
The different classifications show some geographic orientation to differing geologic
regions across the state, but geographic location and geology were not factors in the
consultant's analysis. Rather, geology affects stream flows that
drive the classification system. Click here for a map of the 231 gaging
stations and their stream classification
. The left
side of the screen will display a menu box labeled "Places." In this, scroll down and click on "Stream Classes" to display a checked box for each of the original seven classifications.
These boxes can be checked or unchecked to display one or more types of stream at a time.
The most important task for the workshop participants was to review the stream classifications
to consider whether any needed to be modified, combined or sub-divided. Mapping
of the gauging stations, classifications and underlying geology
reinforced that the seven classifications made sense from a hydrologic
standpoint. Based on knowledge of stream biota and different habitat needs,
the participants agreed to modify the initial seven classifications as follows:
|B. Small Stable Streams - subdivide into cool and cold water habitat|
|D. Small Flashy Streams - subdivide into natural and "accidental"|
|F. Medium Stable Streams - subdivide into cool and warm water habitat|
|G. Small Seasonal Streams - subdivide into natural and "accidental"|
Classifications D and G can reflect hydrologic characteristics influenced by particular
underlying geology - most notably the Carolina slate belt formation. However,
these two classifications can also reflect hydrology affected by land use, impervious surfaces,
numerous small impoundments and other alterations. This latter case would fall
into the "accidental" category of the subdivision of these two classifications.
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The Next Step - Investigating Alternate Approaches
Ultimately, DWR intends to develop a specific technical approach for determining ecological
flows for each of the 11 different stream classifications. For streams and
locations where U.S. Geological Survey (USGS) stream flow data is lacking, river basin
hydrologic models will be used to simulate a record of daily stream flows that can then
be analyzed with the stream classification software to determine the hydrologic classification.
The first step was to identify a suitable test site for a demonstration and evaluation
of alternative approaches for determining ecological flows. The site
needed to have either USGS stream gauge data or a completed river basin hydrologic
model. The other criterion for selecting a demonstration site was the availability
of a model of physical instream habitat conditions as a function of stream flow.
With this in mind, two sites on the Eno River between Hillsborough and
Durham were selected.
The Neuse River Basin hydrologic model
is ready for use and high quality habitat modeling studies already exist for the
Eno River at Hillsborough and the Eno River State Park on the west side of Durham.
The Eno River
is also a voluntary capacity use area.
Click here for a map of all aquatic habitat studies DWR has performed in the Neuse River Basin
The two Eno River demonstration sites being evaluated are numbers 3 and 5 on this map. Clicking on any of the sites will open additional information about that site.
By combining stream flow data generated by the hydrologic model with the habitat
response from the physical habitat model, DWR will be able to evaluate the effects
of different flow management approaches. General types of ecological flow approaches
to evaluate include:
- Setting the threshold for allowable withdrawals as the amount that results in
a change in the hydrologic stream classification.
- Allowing withdrawals at a certain percentage of the ambient flow adjusted on a daily to weekly basis.
The Community Water Supply Plan for the City of Charlottesville, VA contains an
example of this approach. A description of the ecological flow component of the
plan, as well as other case studies, can be viewed by clicking here.
- Setting a flow target that varies seasonally or monthly, and allowing some variation
within bounds above and below this target.
- Other approaches suggested by the analysis.
The Neuse River Basin model will be used to generate daily flows for 80 years of
record at the two Eno River demonstration sites. By trying different flow targets and percentages,
DWR will be able to test the effects on aquatic habitat at these two locations.
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