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.
- What is low flow?
- What is a drought?
- What is a hydrological drought?
- What is the difference between low flow and drought?
- What factors affect variations in flow?
- Do low flows occur at the same time each year?
- Why does a low flow year begin in April while a water year begins in October?
Low Flows and Aquatic Life Criteria
- What are water quality criteria?
- What are aquatic life criteria and how do they relate to water quality standards, TMDLs, and NPDES permits?
- How do low flows affect water quality criteria for aquatic life?
- How are water quality criteria for aquatic life determined?
- How are aquatic life criteria expressed?
- What is the difference between the two concentrations used in aquatic life criteria?
- What is a wasteload allocation and how does it relate to protecting aquatic health?
- How do low flows affect the determination of wasteload allocations?
Design Flows: Definitions and Methods
- What are the two methods used to calculate stream design flows for water quality standards?
- What is the hydrologically-based design flow method?
- What are the 1Q10 and 7Q10?
- What is the biologically-based design flow method?
- How are hydrological-based design flows different from biologically-based design flows?
- What is the difference between the 4B3 and the 4Q3?
- What methods and design flows does the EPA recommend using?
- What method is a better predictor of excursions from the aquatic life criteria?
- What is regulated flow and how does it affect water quality standards and design flows?
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.
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 year or several consecutive years.
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.
- 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)
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.
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.
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.
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.
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.
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.
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.
- 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.
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).
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.
- Hydrologically-based design flow method
- Biologically-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.
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.
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.
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.
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.
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.
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).
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.
Smakhtin, V.U. 2001. "Low flow hydrology: a review." Journal of Hydrology. 147-186.