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Environmental Modeling Community of Practice

Definition and Characteristics of Low Flows

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


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 setting permit discharge limits.

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.

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 or more consecutive years.

What is the difference between low flow and drought?

A low flow is typically 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 an 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.

What factors affect 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.

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, a year starting 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.

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. They 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.

States use aquatic life and other water quality (e.g., human health, microbial) criteria in developing their water quality standards. State water quality standards are used by states and EPA issuing National Pollution Discharge Elimination System (NPDES) permit limits and in establishing Total Maximum Daily Loads (TMDLs).  A TMDL ensures that the waterbody will attain and maintain water quality standards (to protect human health, aquatic life and designated uses).

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 format accounts for 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.

  • The Criterion Continuous Concentration (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 Criterion Maximum Concentration (CMC) is a one-hour average concentration in the ambient water that should not be exceeded more than once every three years. This criterion is used to protect aquatic life from acute effects.

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. More information can be found in Novak et al., 2016.

How do low flows affect the determination of wasteload allocations for TMDLs?

A wasteload allocation is the portion of a TMDL allocated to a point source of a pollutant. 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.

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.

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.

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.

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, 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.

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).

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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EPA. 1991. Technical Support Document for Water Quality-based Toxics Control. EPA/505/2-90-001.

EPA. 1986. Technical Guidance Manual for Performing Wasteload Allocation. Book IV: Design Conditions, Chapter 1.

Novak, Rachael, Kennen, J.G., Abele, R.W., Baschon, C.F., Carlisle, D.M., Dlugolecki, Laura, Eignor, D.M., Flotemersch, J.E., Ford, Peter, Fowler, Jamie, Galer, Rose, Gordon, L.P., Hansen, S.E., Herbold, Bruce, Johnson, T.E., Johnston, J.M., Konrad, C.P., Leamond, Beth, and Seelbach, P.W, 2016, Final EPA-USGS Technical Report:Protecting Aquatic Life from Effects of Hydrologic Alteration: U.S. Geological Survey Scientific Investigations Report 2016–5164, U.S. Environmental Protection Agency EPA Report 822-R-156-007, 156 p.,

Federal Register. Vol. 63, No. 129. 7 July 1998. p 36763.

Smakhtin, V.U. 2001. "Low flow hydrology: a review." Journal of Hydrology. 147-186.

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