 Flow
101
This page provides background information
on the definition and characteristics of low flows, the relationship between
low flows and aquatic life criteria, and design flows.
Low Flow and Droughts: Definitions and Characteristics
Low Flows and Aquatic Life Criteria
Design Flows: Definitions and Methods
References
Low Flow and Droughts: Definitions and Characteristics
What is low flow?
Low flow is the "flow of water in a stream during prolonged dry weather,"
according to the World Meteorological Organization. Many states use
design flow statistics such as the 7Q10 (the lowest 7-day average flow
that occurs on average once every 10 years) to define low flow for the
purpose of setting permit discharge limits.
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What is a drought?
A drought is a more general phenomenon than low flow and can be characterized
by more than low stream flows. Droughts can be classified as meteorological,
atmospheric, agricultural, hydrological, and water management. Typically,
a drought is defined in terms of water availability for various designated
uses.
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What is a hydrological drought?
Hydrological droughts are typically described by a reduction in lake
storage, a decrease of stream flow discharge, and a lowering of groundwater
levels over large areas, over one year or several consecutive years.
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What is the difference between low flow
and drought?
A low flow is a seasonal phenomenon (e.g., the "dry season") and is
an important component of the flow regime in any river or stream. By
contrast, a drought is a natural event that results from an extended
period of below average precipitation. While droughts include low flows,
a continuous seasonal low-flow event is not necessarily a drought.
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What factors affect variations in flow?
- Rainfall and snowmelt
- Land use/land cover (e.g., the permeability or imperviousness of surrounding
land)
- Water control structures (e.g., reservoirs and dams)
- Water intakes (e.g., for drinking water and industrial cooling)
- Water discharges (e.g., from industries, utilities, and wastewater
treatment plants)
- Geological characteristics (e.g., groundwater flow and stream slope)
Do low flows occur at the same time each year?
Most streams will illustrate annual variation that can be explained
by seasonal changes in snowmelt, rainfall, and other factors. For many
areas in the country, the lowest flows often occur near the end of the
summer or beginning of fall. However, each stream is different and any
particular year can be an anomaly in terms of if and when low flows
occur. The magnitude and duration of low flows can vary significantly
from year to year.
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Why does a low flow year begin in April while
a water year begins in October?
The U.S. Geological Survey defines a "water year" as the 12-month period
from October 1 through September 30 of the following year. This definition
is appropriate for dealing with water supply and high flow statistics
since typically the lowest flows of the year occur in the fall months.
However, for low flow calculations, April 1 through March 31 is typically
used.
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Low Flows and Aquatic Life Criteria
What are water quality criteria?
The Clean Water Act uses the term water quality criteria in two separate
ways. In section 303(c) the term is part of the definition of a water
quality standard, which are developed and adopted by States and Tribes
and require EPA approval. However, in section 304(a) the term "criteria"
is used in the scientific sense. Under section 304(a), EPA develops
scientifically sound criteria guidance which may form the basis for
State, Tribal or Federal adoption of water quality standards pursuant
to section 303(c). Section 304(a) criteria are based solely on data
and scientific judgments on pollutant concentrations and environmental
or human health effects. Criteria are developed for the protection of
aquatic life as well as for human health.
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What are aquatic life criteria and how do they
relate to water quality standards, TMDLs, and NPDES permits?
Aquatic life criteria list chemical (e.g., ammonia, copper, MTBE) and
other water quality (e.g., dissolved oxygen) concentration goals to
protect surface water for aquatic life. States use aquatic life criteria
in developing their water quality standards. Each water quality standard
is based upon a suite of aquatic life and other water quality (e.g.,
human health, microbial) criteria. State water quality standards are
used by states and EPA in establishing Total Maximum Daily Loads (TMDLs)
and issuing National Pollution Discharge Elimination System (NPDES)
permit limits.
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How do low flows affect water quality criteria
for aquatic life?
Low flows typically aggravate the effects of water pollution. Dilution
is the primary mechanism by which the concentrations of contaminants
(e.g., copper, lead) discharged from industrial facilities and other
point and some non-point sources are reduced. However, during a low
flow event, there is less water available to dilute effluent loadings,
resulting in higher in-stream concentration of pollutants. Additionally,
winds, bank storage, spring seepage, tributary streams, and the warming
effect of the sun have greater impacts on stream water temperatures
during low-flow periods. The exaggerated effects of these factors could
be additional stressors on aquatic life.
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How are water quality criteria for aquatic
life determined?
National water quality criteria for aquatic life are derived on the
basis of the best available biological, ecological, and toxicological
information concerning the effects of pollutants on aquatic organisms.
In addition to the national criteria, site-specific criteria may be
necessary to account for local conditions.
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How are aquatic life criteria expressed?
Aquatic life criteria are expressed in terms of the intensity of concentration,
duration of averaging period, and average frequency of allowed excursions.
This concentration-duration-frequency format takes into account the
fact that aquatic organisms can tolerate higher concentrations of pollutants
for shorter periods of time than they can tolerate throughout a complete
life cycle. Two concentrations, a continuous and a maximum, are used
to express aquatic life criteria.
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What is the difference between the two
concentrations used in aquatic life criteria?
- The lower concentration is called the Criterion Continuous Concentration
(CCC). The CCC is a multi-day average concentration of a pollutant in
ambient water that should not be exceeded more than once every three
years on the average. The CCC is usually four days (hence the biologically-based
design flow, 4B3) but can be as long as 30 days. This criterion is used
to protect aquatic life from chronic effects.
- The higher concentration is called the Criterion Maximum Concentration
(CMC). The one-hour average concentration in the ambient water should
not exceed the CMC more than once every three years. This criterion
is used to protect aquatic life from acute effects.
What is a wasteload allocation and how does
it relate to protecting aquatic health?
A wasteload allocation is the portion of a Total Maximum Daily Load
(TMDL) allocated to a point source of a pollutant. A TMDL is established
to ensure that the waterbody will attain and maintain water quality
standards (to protect human health, aquatic life, and designated uses).
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How do low flows affect the determination
of wasteload allocations?
Methods to determine wasteload allocations must account for the variability
in stream flow. The most common method for calculating wasteload allocations
in the United States is the critical-low-flow method. Under this method,
wasteload allocations are calculated to meet each numeric water quality
criterion at a selected low flow (i.e., when the available dilution
is low). The critical-low-flow method insures that wasteload allocations
will maintain water quality criteria throughout the rest of the year
when flows are higher than the critical flow, except in cases of significant
non-point source pollution. Although criteria may be exceeded when flows
fall below the critical low flow, properly selecting the flow will minimize
the frequency of excursions to the level specified by a state's water
quality standards.
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Design Flows: Definitions and Methods
What are the two methods used to calculate stream
design flows for water quality standards?
- Hydrologically-based design flow method
- Biologically-based design flow method
What is the hydrologically-based design
flow method?
The hydrologically-based design flow method was developed by the U.S.
Geological Survey to answer questions relating to water supply and high
flows. Most states currently use hydrologically-based design flow method.
A hydrologically-based design flow is computed using the single lowest
flow event from each year of record and then examining these flows for
a series of years. This statistical method is based on selecting and
identifying an extreme value, such as the lowest 7-day average flow
in a ten year period (i.e., 7Q10). The advantage of this method is that
it utilizes extreme value analytical techniques (e.g., log-Pearson Type
III flow estimating technique) supported by past engineering and statistical
practice. The disadvantages of this method are that it is independent
of biological considerations and it cannot easily utilize site-specific
durations and frequencies that are sometimes specified in aquatic life
criteria.
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What are the 1Q10 and 7Q10?
The 1Q10 and 7Q10 are both hydrologically-based design flows. The 1Q10
is the lowest 1-day average flow that occurs (on average) once every
10 years. The 7Q10 is the lowest 7-day average flow that occurs (on
average) once every 10 years.
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What is the biologically-based design
flow method?
The biologically-based design flow method was developed by the U.S. EPA
Office of Research and Development. The biological method examines all
low flow events within a period of record, even if several occur in one
year. The biologically-based design flow is intended to examine the actual
frequency of biological exposure. The method directly uses site-specific
durations (i.e., averaging periods) and frequencies specified in the aquatic
life criteria (e.g., 1 day and 3 years for CMC and 4 days and 3 years
for CCC).
Since biologically-based design flows are based on durations and frequencies
specified in water quality criteria for individual pollutants and whole
effluents, they can be based on the available biological, ecological,
and toxicological information concerning the stresses that aquatic organisms,
ecosystems, and their uses can tolerate. The biologically-based calculation
method is flexible enough to make full use of special averaging periods
and frequencies that might be selected for specific pollutants (e.g.,
ammonia) or site-specific criteria. This method is empirical, not statistical,
because it deals with the actual flow record itself, not with a statistical
distribution that is intended to describe the flow record.
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How are hydrological-based design flows different
from biologically-based design flows?
Hydrologically-based design flows are determined by performing extreme
value statistical analysis of the single lowest flow event in each of
the X years of record. Biologically-based design flows are determined
by analyzing the absolute X lowest flow events in the combined X years
of record. The biologically-based flow event calculation may therefore
include multiple low flow events in a single year and no events from other
years.
The rationale for the two methods is also different. The hydrologically-based
design flow method was initially developed to answer questions relating
to water supply, such as "On average, in how many years out of ten will
the flow be below a certain level?". The biologically-based method was
developed to facilitate the use of two averaging periods specified in
the two concentrations (i.e., the CCC and CMC) used to express aquatic
life criteria in calculating design flows. Biologically-based design
flows are intended to measure the actual occurrence of low flow events
with respect to both the duration and frequency (i.e., the number of
days aquatic life is subjected to flows below a certain level within
a period of several years). Although the extreme value analytical techniques
used to calculate hydrologically-based design flows have been used extensively
in the field of hydrology and in state water quality standards, these
methods do not capture the cumulative nature of effects of low flow
events because they only consider the most extreme low flow in any given
year. By considering all low flow events with a year, the biologically-based
design flow method accounts for the cumulative nature of the biological
effects related to low flow events.
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What is the difference between the 4B3 and the
4Q3?
The 4B3 is a biologically-based 4-day average flow event which occurs
(on average) once every 3 years. The 4B3 is often used as a basis for
U.S. EPA chronic aquatic life criteria. The 4Q3 is a hydrologically-based
design flow and does not equate to the 4B3.
What methods and design flows does the EPA
recommend using?
In the document,
Technical Guidance Manual for Performing Wasteload Allocation. Book
IV: Design Conditions, Chapter 1 (PDF, 3MB), EPA discusses and
recommends two methods for determining design flows, the hydrologically-based
method and the biologically-based method, and the flows that should
be used for both the CCC and CMC. For toxic wasteload allocation studies
in which the hydrologically-based method is used, EPA recommends the
use of the 1Q10 flow as the design flow for the CMC and the 7Q10 as
the design flow for the CCC. The biologically-based method makes exact
use of whatever duration and frequency are specified in the CMC and
CCC. This might be 1B3 for CMC and 4B3 for the CCC or site-specific
durations and frequencies.
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What method is a better predictor of excursions
from the aquatic life criteria?
EPA used both the hydrologically-based design flow method and the biologically-based
design flow method on approximately 60 rivers to compare the 1Q10 with
the 1B3 and the 7Q10 with the 4B3. For most of the rivers, the hydrologically-based
design flows (i.e., 1Q10, 7Q10) resulted in more than the allowed excursions.
For some of the rivers the 1Q10 and 7Q10 allowed substantially more
or fewer excursions than the intended number of excursions. Since the
biologically-based method calculates the design flows directly from
the national or site-specific duration and frequency, it always provides
the maximum allowed number of excursions (and never provides more or
fewer excursions than allowed).
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What is regulated flow and how does it affect
water quality standards and design flows?
Flow is regulated when it is managed to achieve various goals, such
as maintaining a minimum flow downstream of a reservoir or maintaining
a minimum depth for shipping. Since human regulation of flow masks natural
fluctuations, alternative design flows, such as the minimum guaranteed
release flow for a reservoir, are often used instead of traditional
design flows on regulated rivers because they better reflect the actual
flow regimes.
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References
EPA. 1991. Technical Support Document for Water Quality-based Toxics
Control. EPA/505/2-90-001.
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