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Hydrological Low Flow Indices and their Uses

WSC Report No. 04-2004

August 2004

Rich Pyrce, Ph.D. Fluvial Geomorphologist/Hydrologist Watershed Science Centre Trent University, Symons Campus 1600 West Bank Dr. Peterborough, Ontario K9J 7B8 Email: [email protected] Phone: 705.748.1011 x.7567 Fax: 705.755.2276

Watershed Science Centre. Trent University. Symons Campus

1600 West Bank Drive, Peterborough, Ontario K9J 7B8.

www.trentu.ca/wsc

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Pyrce, R.S., 2004. Hydrological Low Flow Indices and their Uses. WSC Report No.04-2004.

Watershed Science Centre, Peterborough, Ontario, 33 p.

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Table of Contents

Table of Contents......................................................................................................................................................... i List of Figures .............................................................................................................................................................. i List of Tables................................................................................................................................................................ i 1.0 Introduction......................................................................................................................................................1 2.0 Hydrological Flow Methods ...........................................................................................................................1 3.0 Low Flow Indices and Exceedance Percentiles..............................................................................................5

3.1 The 7Q10 Flow.............................................................................................................................5 3.2 Other 7Q Low Flows....................................................................................................................8 3.3 Flow Duration Indices ..................................................................................................................9 3.4 Other Low Flow Indices.............................................................................................................11

4.0 Instream Methods and Low Flow Indices.................................................................................................... 14 5.0 Baseflow and Low Flow .............................................................................................................................. 15 6.0 Prediction of Low Flow Indices for Ungauged Catchments ....................................................................... 17 7.0 Ontario Low Flow Regionalisation.............................................................................................................. 19 8.0 Summary....................................................................................................................................................... 23 9.0 Acknowledgements ...................................................................................................................................... 25 10.0 References .................................................................................................................................................... 26

List of Figures

Figure 1. Hydrologically based low flow estimates using: a) flow indices, and b) flow duration values..... 13 Figure 2. Regionalisation of low flows for northern Ontario (Cumming Cockburn Limited, 1995a; Belore,

1995) and southern Ontario (Cumming Cockburn Limited, 1995a) ................................................... 22

List of Tables

Table 1. Environmental flow methodologies (adapted from Karim et al., 1995; Tharme, 2003)................... 2 Table 2. The Tennant (Montana) method (1976) ........................................................................................... 3 Table 3. Magnitude of low flow events .......................................................................................................... 3 Table 4. Frequency of low flow events .......................................................................................................... 4 Table 5. Duration of low flow events ............................................................................................................. 4 Table 6. Timing of low flow events ............................................................................................................... 5 Table 7. Uses of the 7Q10 flow...................................................................................................................... 6 Table 8. Uses of the other 7Q flows ............................................................................................................... 8 Table 9. Flow duration indices used for low flow study............................................................................... 10 Table 10. Other low flow indices ................................................................................................................. 11 Table 11. Hydrologically based instream flow methods .............................................................................. 15 Table 12. Determining stream baseflows using (low) flow indices.............................................................. 16 Table 13. Prediction of the 7Q10 and 4Q3 flows for ungauged catchments in U.S. states .......................... 17 Table 14. List of parameters to predict Ontario low flows (from Belore, 1995; CCL, 1990) ...................... 19 Table 15. Low flow results from the graphical index method (Belore, 1995; CCL, 1995a) ........................ 20 Table 16. Low flow regression results for the Central Region (Chang et al., 2002) .................................... 20 Table 17. Low flow regression results for the Southeastern Region (Chang et al., 2002)............................ 20 Table 18. Low flow regression results for the Southwestern & West Central Regions (Chang et al., 2002) 21 Table 19. Low flow regression results for the Northern Regions 1, 2, and 3 (Chang et al., 2002)............... 21 Table 20. Most commonly used low flow indices from review studies ....................................................... 24


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1.0 Introduction

Low flow investigations for the Province of Ontario have traditionally used

hydrologically based flow indices and exceedance percentiles to recommend low flow and

instream conditions for Ontario rivers and streams. Flow indices have been used extensively and

are considered appropriate at the planning level of water resource development, providing a

convenient desktop method to assess flow thresholds.

An examination of low flow indices revealed that there are typically numerous uses for a

specific index, and the uses may be conflicting and cover a wide range of engineering, physical,

and biological needs. These findings prompted this low flow summary and review, using

academic and government literature to uncover the various uses of the most common low flow

indices. The low flow indices reviewed include the various 7Q flows, other flow indices (e.g.

4Q3), and the flow duration indices. Instream flow methods and baseflow are also included in

this report as both these topics are closely related to low flows. To predict low flows at ungauged

catchments, various low flow regionalisations have been developed using multiple regression

techniques, and an overview of these methods for the Province of Ontario and many U.S. states

are included in this report.

2.0 Hydrological Flow Methods

Hydrological flow indices are arguably the most straight-forward of the four basic types

of environmental flow methods (Table 1). Hydrological methods are typically desktop

techniques that primarily rely on published hydrological data in the form of historical monthly or

daily flow discharge data, for making environmental flow recommendations (Tharme, 2003).

Hydrological methods often seek a specified minimum flow, and there are many regionalisation

techniques to derive results for gauged and ungauged rivers. Hydrological methods are

considered rapid and non-resource intensive, and appropriate at the planning level of water

resource development (Tharme, 2003).

For the Province of Ontario, hydrological methods provide a sensible way to estimate low

flows or instream flows which can be regionalised for any location within the Province. The

hydraulic rating, habitat rating, and holistic methods often provide more detailed results, but also

are more time, labour, and data intensive, requiring advanced knowledge about the watershed.


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Table 1. Environmental flow methodologies (adapted from Karim et al., 1995; Tharme, 2003)

Method Description

1. Hydrological • Environmental flow recommendations are made using simple desktop methods primarily using hydrological data (daily or monthly flow records)

• Typically a rapid, non-resource intensive method, providing low-resolution environmental flow estimates

• Considered appropriate at the planning level of water resource development, or in low controversy situations where used as a primary flow target

• The most widely used hydrological method worldwide is the Tennant (or modified Tennant) method (1976)

• The second most widely used method include various flow duration exceedance percentiles (e.g. Q95, Q75), or single low flow indices (e.g. 7Q10, 7Q2)

2. Hydraulic Rating • Uses changes in hydraulic variables (such as river stage or wetted perimeter) to assess habitat factors known or assumed to be limiting to target biota, thus a threshold value of the selected hydraulic parameter will sustain biota/ecosystem integrity

3. Habitat Rating • These methods attempt to assess environmental flow requirements on the basis of detailed analyses of the suitability of instream physical habitat under different flow discharges using integrated hydrological, hydraulic and biological response data

• Flow is typically modelled using data on flow depth, channel slope, cross-section shape, etc.. collected at multiple cross-sections within a study reach

• The results usually take the form of habitat-discharge curves to predict optimum flows as environmental flow requirements

4. Holistic • The requirements of the complete ecosystem are integrated and considered (including the river channel, source areas, riparian zone, floodplain, etc.)

• The natural regime of the river is the fundamental guide, and must be incorporated into the modified flow regimes

• Critical flow criteria are identified for some or all major components of the riverine ecosystem

• The basis for most approaches is a systematic construction of a modified flow regime on a month-by-month and element-by-element basis which defines features of the flow regime to achieve particular ecological, geomorphological, water quality, social or other objectives of the modified system

• Advanced holistic methods routinely utilize several of the tools found in hydrologic, hydraulic and habitat rating methods

The Tennant (or Montana) method (1976) is the most common hydrological method

applied worldwide, and has been used by at least 25 countries in either the original or modified

form (Tharme, 2003). It’s appeal is in it’s simplicity and ease of use, as the Tennant method uses

a percentage of the mean annual flow (MAF) for two different six month periods to define

conditions of flow regarding “instream flow regimens for fish, wildlife, recreation, and related

environmental resources” (Table 2). Tennant (1976) used original headings of “Recommended


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base flow regimens Oct.-Mar. (and Apr.-Sept.)”, but it seems unlikely that he was referring to

actual baseflow, but rather a base or basic “excellent” or “good” flow condition associated with

perceived recreational and wildlife needs.

Table 2. The Tennant (Montana) method (1976)

Narrative description of general condition of flow

Recommended flow regimens (% of MAF) October to March

Recommended flow regimens (% of MAF) April to September

Flushing or maximum 200% 200%

Optimum range 60-100% 60-100%

Outstanding 40% 60%

Excellent 30% 50%

Good 20% 40%

Fair or degrading 10% 30%

Poor or minimum 10% 10%

Severe degradation <10% <10%

Olden and Poff (2003) provided a list of hydrological indices used in riverine ecological

studies related to magnitude, frequency, duration, and timing of flow events. Those specific to

low flow are listed below in Tables 3 to 6. Many of the listed indices also have corresponding co-

efficients of variation also used in analyses of low flows, but are not included here.

Table 3. Magnitude of low flow events

Index Explanation Units Sources

Minimum monthly flows Mean minimum monthly flow for all months

m3s-1 Wood et al. (2000)

Annual minimum flows Mean of the lowest annual daily flow divided by median annual daily flow averaged across all years

- Wood et al. (2000)

Median of the lowest annual daily flows divided by median annual daily flows averaged across all years

- Clausen et al. (2000)

Mean annual minimum flows divided by catchment area

m3s-1

km-2

Low flow index Mean of the lowest annual daily flow divided by mean annual daily flow averaged across all years

- Poff and Ward (1989)


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Baseflow index (1) Ratio of baseflow volume to total flow volume

- Clausen and Biggs (1997, 2000), Clausen et al. (2000)

Baseflow index (2) Seven-day minimum flow divided by mean annual daily flows averaged across all years

- Richter et al. (1998)

Baseflow index (3) Mean of the ratio of the lowest annual daily flow to the mean annual daily flow (*100) averaged across all years

- Poff (1996)

Table 4. Frequency of low flow events


Low flood pulse count Number of annual occurrences during which the magnitude of flow remains below a certain threshold

yr-1 Richter et al. (1996, 1997, 1998)

Frequency of low flow spells

Total number of low flow spells (threshold equal to 5% of mean daily flow) divided by the record length

yr-1 Hughes and James (1989)

Table 5. Duration of low flow events


Annual minima of daily discharge

Magnitude of minimum annual flow of various duration (1-, 3-, 7-, 30-, 90-day), ranging from daily to seasonal

m3 s-1 Richter et al. (1996, 1997, 1998)

Means of minima of daily discharge

Mean annual 1-day/7-day/30-day minimum, respectively, divided by median flow

- Clausen et al. (2000)

Low exceedence flows Mean magnitude of flows exceeded 75% and 90% of the time, divided by Q50 over all years (Q75/Q50, Q90/Q50)

- Clausen and Biggs (1997, 2000), Clausen et al. (2000)

Low flow pulse duration Mean duration of low flood pulse count

days Richter et al. (1996, 1997, 1998)

Number of zero-flow days

Mean annual number of days having zero daily flow

yr-1 Poff and Ward (1989), Poff (1996), Richter et al. (1997)

Percent of zero-flow months

Percentage of all months with zero flow

- Puckridge et al. (1998)


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Table 6. Timing of low flow events

Index Explanation Units Source

Julian date of annual minimum

The mean Julian date of the 1-day minimum flow over all years

- Clausen et al. (2000), Richter et al. (1996, 1997, 1998)

Seasonal predictability of low flow

Proportion of low-flow events greater than or equal to 5-year magnitude falling in a 60-day “seasonal” window

- Poff (1996)

Seasonal predictability of non-low flow

Maximum proportion of the year during which no 5-year + low flows have ever occurred over the entire period of record

- Poff (1996)

3.0 Low Flow Indices and Exceedance Percentiles

Low flow analysis in the Province of Ontario has made extensive use of single flow

indices or exceedance (flow duration) percentiles, which are the second most widely used

hydrological environmental flow method, after the Tennant method (Tharme, 2003). A flow

index, such as the 7Q10 flow can be interpreted as the 7-day low flow with a 10-year return

period, using daily discharge data. The exceedance percentile Q95 can be interpreted as the flow

discharge which can be expected to be exceeded 95% of the time. Previous analysis of Ontario

low flows focused primarily on the 7Q2, 7Q5, 7Q10, and 7Q20 flows (Cumming Cockburn

Limited 1989, 1990, 1995a, 1995b, 1995c).

An important question however is to examine the use of these indices. For example,

what is the reasoning behind choosing a 7Q10 flow, rather than a 7Q20 flow? Academic and

government literature were examined in an attempt to define flow indices and exceedance

percentiles presently in use, and the rationale for their use.

3.1 The 7Q10 Flow

The 7Q10 flow is the most commonly used single flow index. By the early 1970’s, U.S.

agencies which regulated stream pollution based their stream water quality standards on the 7-day

10-year low flow condition (Singh, 1974). By the mid-1970’s, minimum low flow releases in


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Pennsylvania were required from impoundments greater than 1.3 km2 (0.5 mi2) in size (Chiang

and Johnson, 1976). Initially a low flow of 0.01 m3 s-1 km-2 (or 0.5 ft3 s-1 mi-2) was recommended,

however this single criterion was criticized as it failed to consider watershed area, the size of the

impoundment, or the natural low flow yield of the regulated stream. The water quality of any

stream was considered to be acceptable unless the streamflow was below the 7-day, 10-year low

flow (7Q10); any diversion made beyond the 7Q10 could degrade the water quality of the stream

beyond the accepted standard (Chiang and Johnson, 1976).

Table 7 summarises the many uses of the 7Q10 flow. The sources reflect a wide range of

current and past guidelines to help manage watersheds in various parts of the world.

Table 7. Uses of the 7Q10 flow

Index Uses Reference

7Q10 • one of the most widely used (design or reference) low flow indices/instream flow methods

Riggs et al. (1980), Caissie et al. (1998), Smakhtin and Toulouse (1998), Caruso (2000), Smakhtin (2001), Tharme (2003)

• to protect/regulate water quality from wastewater discharges or waste load allocations (to prevent adverse biological/ecological impacts on the receiving water)

Riggs et al. (1980), Diamond et al. (1994), Schreffler (1998), Gu and Dong (1998), Chaudhury et al. (1998), Reis and Friesz (2000), Mohamed et al. (2002), Wallace and Cox (2002), Deksissa et al. (2003), Flynn (2003), State of Massachusetts (2004)

• waste load allocation for discharges into flowing receiving waters for chronic aquatic life criteria (except for ammonia-nitrogen)

• stream design flow used to determine waste load allocations to maintain water quality criteria for NH3-N toxicity: May-November for summer acute aquatic life, December-February for winter acute aquatic life

Ohio Environmental Protection Agency Division of Surface Water (1997)

• used by the State of Georgia to regulate water withdrawals and discharges into streams

• general indicator of prevalent drought conditions which normally cover large areas

Carter and Putnam (1978)

• default design low flow for calculating steady state waste load allocations for aquatic life: chronic criteria

Virginia Department of Environmental Quality (2004)

• total maximum daily load to assess aquatic life protection

New York State Department of Environmental Conservation (1996)


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• minimum quantity of streamflow necessary to protect habitat during a drought situation

Delaware Water Supply (2004)

• waste load allocation for Great Lakes Initiative pollutants in the absence of a Total Maximum Daily Load stream design flow

Minnesota Office of the Revisor of Statutes (2004)

• continuous chronic criterion for aquatic life U.S. Environmental Protection Agency (1999)

• chronic criteria/estimate for aquatic life/habitat maintenance or protection

Flynn (2003)

• possible indicator of potential mortality of aquatic life

Imhof and Brown (2003)

• compared to whole effluent toxicity (WET) compliance (U.S. Environmental Protection Agency – National Pollutant Discharge Elimination System)

Diamond and Daley (2000)

• to compare the impacts of climate change and irrigation on low surface streamflows (related to total maximum daily loads)

Eheart and Tornil (1999), Eheart et al. (1999)

• examined as an instream flow requirement for Atlantic salmon

Caissie et al. (1998)

• annual design low flow for effluent wastewater discharge and minimum flow periods and volumes

Cusimano (1992)

• used as a local extinction flow Ontario Ministry of Natural Resources (1994)

• considered as the worst case scenario in water quality modelling

Mohamed et al. (2002)

• some use as a specific design application for stormwater holding facilities based on stormwater modelling

Odom (2004, personal communication)

Table 7 indicates there are numerous and diverse reasons applied to the use of the 7Q10 flow for

regulation purposes, ranging from: i) protection or regulation of water quality from wastewater

discharges or waste load allocations, ii) habitat protection during drought conditions, iii) chronic

criteria for aquatic life, and iv) a local extinction flow. The original use of the 7Q10 flow is

related to stream water quality standards to regulate pollution, however the uses have expanded to

include and serve many other interests. There have been concerns about the suitability of

applying the 7Q10 flow as a design or index flow. The U.S. Fish and Wildlife Service (1981)

argued that the 7Q10 flow had been misused in the past as a minimum flow for protection of the

aquatic community, however it is not an acceptable instream flow method; the 7Q10 flow is a

flow statistic used in identifying the volume for dilution to set permit limits for wastewater

discharge, which does not protect aquatic life and its use to do so is inappropriate. Caissie and

El-Jabi (1995) warned that the use of the 7Q10 flow could significantly underestimate instream

flows, and harmful biological effects could arise from application of these methods, therefore the

use of this index for this purpose was not recommended. The State of Massachusetts (2004)


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stated that the 7Q10 flow statistic is sometimes claimed to represent an adequate streamflow for

maintaining a healthy ecosystem, when in fact, much higher streamflow levels are required.

3.2 Other 7Q Low Flows

Along with the 7Q10 flow there are a variety of other 7Q flows that have been used or are

currently in use, including the annual 7-day low flow (7Q1), the 7Q2, 7Q5, 7Q20, and 7Q25

flows. Table 8 details the uses of these flows.

Table 8. Uses of the other 7Q flows

7Q Flow Uses Source

7Q1 • known as the “dry weather flow” Smakhtin (2001)

• used for abstraction licensing Smakhtin (2001), Smakhtin and Toulouse (1998)

• used to remove the effect of minor river regulation

Matalas (1963)

7Q2 • one of the most widely used design low flow indices

Smakhtin (2001), Smakhtin and Toulouse (1998)

• habitat maintenance flow (represents a period of stress on the system that causes some reduction in populations)

Ontario Ministry of Natural Resources (1994)

• criteria for developing permits for wasteload allocations

Tortorelli (2002)

• used as an instream flow Caissie and El-Jabi (2003)

• some use as a specific design application for stormwater holding facilities based on stormwater modelling

Odom (2004, personal communication)

• not defined Beran and Gustard (1977), Hayes (1991), Ries and Friesz (2000)

7Q5 • critical low flow for low quality fishery waters (a stream classified for the beneficial use of warmwater semi-permanent fish life propagation or warmwater marginal fish life propagation)

South Dakota Department of Environment and Natural Resources (1998)

7Q20 • used as a systems extinction flow (causes significant stress on the system)

Ontario Ministry of Natural Resources (1994)

• used as an indicator of the minimum flow needed to maintain the ecosystem

Ontario Ministry of Natural Resources et al. (2002)

• limiting condition for sewage treatment and wastewater disposal for a receiving water body

Ontario Ministry of the Environment (2000)

• indicator of potential mortality of aquatic life for larger streams

Imhof and Brown (2003)


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• summer design low flow for effluent wastewater discharge and drought flow periods and volumes

Cusimano (1992)

• flow for sustainable yield/carrying capacity for eco-tourism

Shrivastava (2003)

7Q25 • critical low flow for high quality fishery waters (surface waters designated for the beneficial use of coldwater permanent fish life propagation, coldwater marginal fish life propagation, or warmwater permanent fish life propagation)

South Dakota Department of Environment and Natural Resources (1998)

From Table 8 it is apparent that the Ontario Ministry of Natural Resources (MNR)

(1994), the Ontario Ministry of Natural Resources et al. (2002) and Ontario Ministry of

Environment (MOE) (2000; Odom, 2004) both make use of 7Q20 and 7Q2 flows. The MNR

(1994, 2002) use the 7Q20 and 7Q2 flows as a measure of habitat or ecosystem maintenance or

systems extinction. The MOE (2000) use the indices as a design or limiting condition for

stormwater or wastewater discharges and water taking. For continuous and non-continuous point

source discharges, the 7Q20 is used as the basic design flow for the receiving stream (Ministry of

Environment and Energy, 1994). The 7Q20 is essentially a conservative approach to ensure that

sufficient streamflow is available to assimilate/dilute point source discharges (Stainton 2004,

personal communication). Odom (2004, personal communication), Hammond (2004, personal

communication), and Yang (2004, personal communication) confirmed that the 7Q20 flow is

used for most of the Ontario Ministry of the Environment’s assessments. Yang (2004)

specifically discussed the use of the 7Q20 flow with regard to: i) the Ontario Ministry of the

Environment’s Application for Certificate of Approval (C of A) for point source effluent to be

discharged into receiving waters, and ii) the Ontario Ministry of the Environment’s Application

for Permit to Take Water (PTTW). The 7Q5 and 7Q25 flows are used exclusively by the South

Dakota Department of Environment and Natural Resources (1998) with regard to critical low

flows for low and high quality fishery waters, respectively. The annual 7-day low flow (7Q1; or

MAM7, the mean annual 7-day average minimum flow) is used as an alternative index in the

United Kingdom for water abstraction licensing (Smakhtin and Toulouse, 1998).

3.3 Flow Duration Indices

A flow duration curve is one of the most informative means of displaying the complete

range of river discharges, from low flows to flood events (Smakhtin, 2001). Using average daily

discharge data, flow duration curves are cumulative frequency distributions that show the percent


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of time that a specified discharge is equaled or exceeded during a period of interest (daily,

monthly, annual, or entire period of record). Smakhtin (2001) indicated that the “design” low

flow range of a flow duration curve is the 70%-99% range, or the Q70 to Q99 range. The Q95 and

Q90 flows are most often used as low flow indices in the government literature and academic

sources, and the numerous uses are listed in Table 9. Q75, Q84, Q96, Q97, Q98, and Q99 flows are

occasionally noticed in the literature as well. Monthly median flows during summer months is

another common flow duration index and is included in Table 9.

Table 9. Flow duration indices used for low flow study

Flow Index

Use Study

Q95 • commonly used low flow index or indicator of extreme low flow conditions

Riggs (1980), Brilly et al. (1997) Smakhtin (2001), Wallace and Cox (2002), Tharme (2003)

• minimum flow to protect the river Petts et al. (1997)

• minimum monthly condition for point discharges Michigan Department of Environmental Quality (2002)

• licensing of surface water extractions and effluent discharge limits assessment

Higgs and Petts (1988), Smakhtin and Toulouse (1998)

• biological index for mean monthly flow Dakova et al. (2000)

• used to maintain the natural monthly seasonal variation

• used to optimize environmental flow rules

Stewardson and Gippel (2003)

Q90 • commonly used low flow index Smakhtin et al. (1995), Smakhtin (2001)

• monthly value provides stable and average flow conditions

Caissie and El-Jabi (1995)

• monthly value gives minimum flow for aquatic habitat

Yulanti and Burn (1998)

• used to examine discharge-duration patterns of small streams

Ogunkoya (1989)

• threshold for warning water managers of critical streamflow levels

Rivera-Ramirez et al. (2002)

• describes limiting streamflow conditions, and is used as a conservative estimator of mean baseflow

Wallace and Cox (2002)

Monthly Q50

• aquatic baseflow policy for water resources planning and management

Ries and Friesz (2000), Ries (1997)

• used to protect aquatic biota U.S. Fish and Wildlife Service (1981)

• used to recommend seasonal minimum discharges for waterpower rivers

Metcalfe et al. (2003)


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The uses of the Q95 and Q90 flows are varied and are similar to the 7Q low flow indices. Similar

to the 7Q10 flow, the Q95 flow has been used as a biological index, for the licensing of water

takings, and for effluent discharge limits.

3.4 Other Low Flow Indices

Numerous other low flow indices are found in the literature, and only the most frequent

are listed below in Table 10. Many of these indices are specific to certain U.S. states or regions,

assessing the chronic continuous criteria or chronic maximum criteria for aquatic life or human

health.

Table 10. Other low flow indices

Flow Index

Use Source

30Q10 • stream design flow used to determine waste load allocations to maintain water quality criteria for NH3-N toxicity: May-November for summer chronic aquatic life, December-February for winter chronic aquatic life

Ohio Environmental Protection Agency Division of Surface Water (1997)

• default design low flow for calculating steady state waste load allocations for aquatic life: chronic criteria (ammonia)


• chronic criteria for aquatic life regarding ammonia or ammonia-nitrogen loadings

U.S. Environmental Protection Agency (1999)

• total maximum daily load to assess human health protection of drinking water resources

New York State Department of Environmental Conservation (1996)

• the basis for monitoring the attainment of in-stream water quality flow targets

Tri-State Water Quality Council (2004)

4Q3 • waste load allocations for point sources • chronic criteria for aquatic life


• design for total maximum daily loads for various water quality constituents

Waltemeyer (2002)

90Q10 • design flow for wildlife values such as mercury Michigan Department of Environmental Quality (2002),

• waste load allocation for discharges into flowing receiving waters for wildlife criteria

Ohio EPA Division of Surface Water (1997)

• wildlife chronic standard or criterion for waste load allocations


30Q2 • a reasonable estimate of annual average baseflow for any given year



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• general indicator of initial drought conditions which may cover large areas

• may be used by State regulators to determine water use restrictions

Carter and Putnam (1978)

1Q10 • the criterion maximum concentration (ammonia) for aquatic life


• default design low flow for calculating steady state waste load allocations for aquatic life: acute criteria


• waste load allocation for discharges into flowing receiving waters for acute aquatic life criteria (except for ammonia-nitrogen)


• maximum standard or criterion for waste load allocations


30Q5 • design flow for the continuous chronic criterion for ammonia


• default design low flow for calculating steady state waste load allocations for human health: non-carcinogens


Harmonic mean flow

• waste load allocation for discharges into flowing receiving waters for agricultural water supply, human health, and aesthetic criteria


• human health chronic standard or criterion for waste load allocations


• to evaluate the effects from contaminated groundwater

Schreffler (1998)

• flow for implementing the human health criteria for carcinogens


3Q20 • permissible rate of waste disposal into Tennessee streams

Bingham (1986)

The harmonic mean flow is a streamflow characteristic that describes an average daily discharge

for a stream (Rifai et al., 2000). The U.S. Environmental Protection Agency (1999) uses a

number of the non-7Q indices, recommending the 1Q10 flow as the design flow for the

continuous maximum criterion for aquatic life, and the 30Q10 or the 30Q5 flow for continuous

chronic criteria for ammonia.

Globally, the most widely used indices are the 7Q10 and Q95 flows. Figure 1 displays a

graph of the frequently used low flow and flow duration indices. Commonly used indices include

the 7Q2, 7Q20, 1Q10, 30Q10, Q90, and Q75 flows. However, for each of these eight indices there

are a number of uses found in literature (see Tables 8-10).


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Figure 1. Hydrologically based low flow estimates using: a) flow indices, and b) flow duration values


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4.0 Instream Methods and Low Flow Indices

Instream flows have been defined as the minimum flows required to protect and maintain

aquatic resources in streams and rivers (Tennant, 1976; Reiser et al., 1989). Sactena (2004)

surveyed the current status of instream flow practices in the Caribbean Basin, and respondents

identified i) effluent discharges, ii) downstream water quality, and iii) existing extraction permits

as the most common sources of instream flow needs. The increasing demand for river water

conflicts with the environmental needs for sustaining flows during drought and low flow periods,

leads to competition between water taking and instream flow needs (Caissie and El-Jabi, 2003).

As instream flows are related to minimum river flows, low flow indices are often used as

instream methods. Table 11 lists four studies that reviewed and analysed flow indices for the

purpose of instream methods.

Reiser et al. (1989) detailed the instream flow methods most often used in North America

as: i) IFIM (Instream Flow Incremental Methodology), which is a component of the Physical

Habitat Simulation (PHABSIM) system, based on a modification of incremental assessments of

effects of flow reduction on fish habitat, through the collection of physical and biological data, ii)

the Tennant method (1976), iii) the Wetted Perimeter method (Gippel and Stewardson, 1998), iv)

the Aquatic Base Flow method, and v) the 7Q10 flow. The Wetted Perimeter method assumes

that there is a direct relation between the wetted perimeter in a riffle and fish habitat in streams

(Parker and Armstrong, 2001). Karim et al., (1995) detailed instream hydrological flow methods

used in Australia, including: i) the Tennant method, ii) flow duration (e.g. Q95, Q90), and iii) the

Constant Yield method, which uses a combination of median flow and constant yield statistics

(runoff per watershed area) to represent watershed hydrology. Caissie and El-Jabi (1995)

examined and compared five instream flow methods for 70 rivers in Atlantic Canada, which

included: i) the Tennant method, ii) 25% of the MAF (a derivation of the Tennant method), used

commonly throughout Atlantic Canada as a low minimum flow required to maintain aquatic life,

regardless of season or species, iii) the median monthly flow (Q50), developed for the New

England region by the U.S. Fish and Wildlife Service, which is considered sufficient to protect

aquatic biota, iv) the Aquatic Base Flow method (ABF), which is an August median monthly flow

of 0.0367 m3 s-1 km-2 (0.5 ft3 s-1 mi-2), recommended by the U.S. Fish and Wildlife Service for

small ungauged basins, v) the Q90 flow, based on the Northern Great Plains Resource Program

(1974), where it is assumed that the Q90 will provide more stable and average hydrologic

conditions, and vi) the 7Q10 flow. Caissie and El-Jabi (1995) recommended the use of the Q50

flow for gauged basins, and the Tennant method, 25% of the MAF, and ABF were recommended


15

Table 11. Hydrologically based instream flow methods

Study Purpose Indices Used

Reiser et al. (1989) Instream flow methods most often used in North America

1. IFIM 2. Tennant method (1976) 3. Wetted Perimeter 4. Aquatic Base Flow Method 5. 7Q10 flow

Karim et al. (1995) Instream flow methods used in Australia

1. Tennant method 2. Flow duration (Q95, Q90) 3. Constant yield

Caissie and El-Jabi (1995) To compare hydrologically based instream flow methods in Atlantic Canada

1. Tennant method 2. 25% of the MAF 3. Monthly Q50 4. Aquatic Base Flow method 5. Q90 flow 6. 7Q10 flow

Yulianti and Burn (1998) To examine links between climate warming and low streamflow in the Canadian Prairies

1. Seasonal 7-day low flow 2. Seasonal 25% of mean flow 3. Seasonal Q80 4. Monthly Q50 5. Monthly Q90

for ungauged basins. Investigating the Prairie region of Canada, Yulianti and Burn (1998)

examined links between climate warming and low streamflow, using the following seasonal (May

to August) and monthly methods: i) the seasonal 7-day low flow, ii) the seasonal 25% of the

mean flow, iii) the seasonal Q80, iv) monthly Q50, and v) monthly Q90. From a comparison of

methods, it is evident that the most frequently used instream flows are: i) the Tennant method (%

of MAF), ii) the seasonal or annual 7Q10, iii) the monthly Q50, and iv) the monthly Q90. The

7Q10 flow is a widely used index and Q90 flow is a commonly used index (Figure 1). The

monthly median flow (Q50) is also a common index (Table 9).

5.0 Baseflow and Low Flow

River baseflow is defined as water which enters a stream or river from persistent, slowly

varying sources, maintaining streamflow between water-input (precipitation, snowmelt) events

(Dingman, 1994). This contrasts with water that enters a stream or river promptly in response to

individual water-input events, called storm flow or event flow. Numerous studies have associated

low flow with base flow, including various ways to estimate base flow using flow indices (Table

12). Methods to estimate base flow include the Tennant method (1976), flow duration values


16

(Wallace and Cox, 2002; Hayes and Nelms, 2002; Petts et al., 1997), the August median flow

(Ries, 1997), and flow indices (U.S. Environmental Protection Agency, 2003).

Table 12. Determining stream baseflows using (low) flow indices

Study Flow Index Definition

Tennant (1976) 0.30*(ADF) 30% of the average daily flow (ADF)

U.S. Fish and Wildlife Service (1981)

0.50 ft3s-2mi-2 New England Aquatic Base Flow method (based on the August median monthly flow)

Ries (1997) August median flow Considered as a uniform aquatic baseflow policy for water resources

Wallace and Cox (2002)

Q90 Used as a conservative estimator of mean baseflow

Hayes and Nelms (2001)

Q50

Q50/Q90

Estimator of mean baseflow in some regions of Virginia Baseflow variability index

Petts et al. (1997) Q10/Q95

Groundwater (baseflow) dominated streams have values ranging from 2.5 to 6.5


30Q2 The minimum 30-day flow with a two year return period

Petts et al. (1997) described the flows available for the environmental needs of the

channel as: i) a minimum flow to protect the river, ii) the unused part of the reliable baseflow, iii)

additional baseflow due to recharge in wetter years, iv) artificial baseflow due to sewage

discharges and recharge due to leakage, and v) all surface runoff. Over the past two decades,

increasing pressure on water resources and the increasing use of ‘stacking’ abstraction licenses

has led to greater exploitation of the reliable baseflow. Consequently, the flows available for the

environment have declined and the need has arisen to define environmental flow requirements

more precisely. Wilson (2000) estimated low flow frequencies from baseflow measurements

using data for streams in Indiana. Low flow frequencies were estimated by relating baseflow

measurements to concurrent daily flows at nearby streamflow gauging stations for which low

flow frequency curves had been developed, focusing on 7Q2 and 7Q10 flows. Furey and Gupta

(2000) derived an equation for low flow from baseflow, as a function of saturated hydraulic

conductivity, drainage density squared, and basin drainage area. Furey and Gupta’s low flow

equation indicated that QB/AB0.5 (where QB is average baseflow and AB is drainage basin area)

should be regressed onto AB0.5 for monthly single day minimum daily discharges (for the months

of August, September, and October) for unnested basins.


17

6.0 Prediction of Low Flow Indices for Ungauged Catchments

Sivapalan (2003) indicated that the prediction of surface water flows in ungauged basins

is an urgent problem, of immediate relevance to society, dealing with questions such as the

impacts of land use and climatic change, biodiversity and sustainable development. There have

been numerous attempts to predict low flows using regression equations in the United States;

Table 13 lists some predictive equations from the literature that estimate the 7Q10 and 4Q3 low

flows for ungauged catchments within specific U.S. states. Most of the equations come from U.S.

Geological Survey Water-Resources Investigations Reports or Scientific Investigations Report

series. All actual constants and exponents have been replaced by letters (a, b, c, etc..) to focus on

the various parameters used in the predictions.

Table 13. Prediction of the 7Q10 and 4Q3 flows for ungauged catchments in U.S. states

Prediction Equation Source and Location 7Q10 = a *exp [b + c(CO) + d(CL) + e(WF) + f(LA) + g(EL)] Chang and Boyer (1977; West Virginia)

7Q10 = a (DA)b (G)c Bingham (1986; Tennessee)

log 7Q10 = C + a(log DA) + b(log PI) + c(GI) + d(log S) Ehlke and Reed (1999; Pennsylvania)

7Q10 = a (DA)b (SL)c (DR/ST + 0.1)d (10 d (REG)) Ries and Friesz (2000; Massachusetts)

log (7Q10 + a) = b + c(2y24) + d(DA) + e(S) + f(SO) Rifai et al. (2000; Texas)

4Q3 = a (DA)b (PW)c (SL)d Waltemeyer (2002; New Mexico)

7Q10 = a (DA)b (ABT)c (SGP)d Flynn (2003; New Hampshire)

7Q10 = a (DA)b 10c(SG) Dudley (2004; Maine)

where: 7Q10 is the seven-day average low flow with 10-year recurrence, 4Q3 is the four-day average low flow with 3-year recurrence, ABT is average mean annual basinwide temperature, C is a regression constant, CL is main channel length, CO is basin perimeter, DA is drainage area, DR/ST is area of stratified drift per unit of total stream length, EL is mean elevation, G is streamflow recession index, GI is geological index, LA is mean latitude, PI is annual precipitation index, PW is average basin mean winter precipitation, REG is region (0 for eastern, 1 for western), S is channel slope, SG is the fraction of the basin underlain by significant sand and gravel aquifers, SGP is average summer precipitation at the gauging station, SL is mean basin slope, SO is the predominant hydrologic soil group, 2y24 is the 2-year, 24-h precipitation, WF is a watershed form factor.

Chang and Boyer (1977) examined the hydrology of 12 unregulated tributaries of the

Monongahela River (West Virginia) and found that watershed perimeter alone accounted for 88%

of the spatial variability of the 7Q10 flows in the multiple regression analysis. The inclusion of

the four other parameters raised the R2 of the regression to 0.999. The standard error of estimate

was nearly 30% of the observed mean. Bingham (1986) regionalised low flow characteristics for

west and east Tennessee streams, resulting in a regression that included basin area and an index


18

of streamflow recession. Standard error of estimate was 24%. Ehlke and Reed (1999) produced

regionalised 7Q10 values for Pennsylvania streams, modified from Flippo (1982) that used

drainage area, an annual precipitation index, a geologic index, and channel slope in the regression

analysis. For Massachusetts streams, Ries and Friesz (2000) determined the basin characteristics

that were statistically significant were drainage area, the area of stratified drift deposit per unit

stream length, mean basin slope, and an indicator variable. The standard error of prediction for

the 7Q10 flow was 70.7% (Ries and Friesz, 2000). Rifai et al. (2000) created regression

equations for the 7Q10 flow for Texas based on meteorological and physiographic data from 63

gauged streams. The regression parameters included drainage area, channel slope, predominant

hydrologic soil group, and the 2-year 24-hr precipitation; 7Q10 values ranged from 0 m3s-1 to

0.425 m3s-1. Waltemeyer (2002) estimated the 4Q3 low flow in mountainous regions of New

Mexico from the data for 40 gauging stations located above 7500 ft (2286 m) using drainage area,

average winter precipitation and mean basin slope; for this regression the standard error of

estimate was 94%. Using data from 60 gauging stations, Flynn (2003) used total drainage area,

mean summer precipitation, and average mean annual basinwide temperature to predict 7Q10

flows for New Hampshire streams. Finally, for ungauged rivers in Maine, Dudley (2004) used 26

gauging stations on unregulated rural rivers with >10 years of data to develop regression

equations. Sixty-two basin and climatic characteristics were reduced to 5 final explanatory

variables (drainage area, fraction of the drainage basin underlain by sand and gravel aquifers,

distance from the coast to the drainage basin centroid, mean annual precipitation, and mean

winter precipitation); the final 7Q10 regression equation used only the first two variables.

The 17 parameters used in the 8 regression equations (Table 13) can be grouped into four

general categories: i) physical character of the watershed (drainage area, channel length, basin

perimeter, mean elevation, mean latitude, channel slope, mean basin slope, watershed

morphology, region), ii) meteorological character of the watershed or region (annual precipitation

index, average basin winter precipitation, average summer precipitation, average basin

temperature), iii) geologic character of the watershed or region (area of stratified drift per total

stream length, geological index, fraction of the basin underlain by significant sand and gravel

aquifers), and iv) hydrology of the stream (streamflow recession index). Drainage area is the

most important variable, as it is used in seven of the eight regression equations.


19

7.0 Ontario Low Flow Regionalisation

The Province of Ontario has also regionalised low flow data and results for ungauged

catchments. Chang et al. (2002) summarised the low flow results of Cumming Cockburn Limited

(CCL) (1995a, 1995b, 1995c) (further explained by Belore, 1995) to estimate low flows in

ungauged Ontario streams or rivers. The results from the regression analysis of physiographic

and hydrometeorological parameters (Cumming Cockburn Limited 1995a, 1995b, 1995c) were

integrated into the Ontario Flow Assessment Techniques (OFAT) software (Chang et al., 2002)

for six provincial sub-regions (Figure 2). Four types of regional models developed were: i)

multiple regression, ii) index method, iii) mapped isoline method, and iv) station proration

(Cumming Cockburn Limited 1995a), however only the multiple regression and index method

results will be covered here. The analysis produced results for the 7Q2 and 7Q20 low flows, and

also the 3Q2, 3Q20, 3Q50, 30Q2, 30Q20, and 30Q50 flows. Table 14 includes the parameters

considered for the analysis.

Table 14. List of parameters to predict Ontario low flows (from Belore, 1995; CCL, 1990)

Physiographic Description

Physiographic • Drainage area (DA, km2) • Base flow index (BFI, dimensionless) • Maximum groundwater fluctuation (m) • Slope (m km-1) • Stream length (LNTH, mm) • Degree of regulation (0 for natural, 1 for regulated) • Drainage area controlled by lakes and swamps

Hydrometeorological • Mean annual precipitation (mm) • Mean annual snowfall (MAS, mm) • Mean annual runoff (MAR, mm) • Mean annual evaporation (mm)

Other parameters • Watershed location • Soil index • Quality of data (station density, record length,

measurement accuracy)

Cumming Cockburn Limited (1995a) concluded that drainage area (DA) is a good predictor of

low flows. Belore (1995) and Cumming Cockburn Limited (1995a) produced regionalised low

flow estimates using a graphical index method where the 7Q2 and 7Q20 flows are functions of

drainage area (Table 15):


20

Table 15. Low flow results from the graphical index method (Belore, 1995; CCL, 1995a)

Flow (m3/s) = a0 + a1(DA)

Flow Region a0 a1

7Q2 North Region 1 8.681 0.002080

North Region 2 -2.494 0.003250

North Region 3 -1.341 0.003530

Central 0.383 0.001610

Southeastern -1.60 0.002510

7Q20 Central 0.209 0.000589

Southeastern -1.008 0.001460

Further work to regionalise Ontario low flows (specifically the 7Q10 and Q95 flows) used natural

flow gauges with greater than 20 years of record for ecoregions of Ontario to produce power

functions relating low flow to drainage area (7Q10 = a(DA)b) (Pyrce, unpublished work).

To estimate low flows, the regression method used four parameters: drainage area (DA,

km2), base flow index (BFI, dimensionless), length of the main channel (LNTH, m), and mean

annual runoff (MAR, mm). Varying combinations of parameters depending on the region

produced the regression results (Tables 16 to 19). Refer to Figure 2 for region location.

Table 16. Low flow regression results for the Central Region (Chang et al., 2002)

Flow (m3/s) = a0 + a1(DA) + a2(BFI)

Flow Region a0 a1 a2

7Q2 Central -0.7216 0.00180600 1.7386

7Q20 Central -0.2134 0.00066184 0.7022

3Q2 Central -0.5398 0.00162600 1.2856

3Q20 Central -0.1841 0.00058893 0.6295

30Q2 Central -0.7119 0.00223800 1.6806

Table 17. Low flow regression results for the Southeastern Region (Chang et al., 2002)

Flow (m3/s) = a0 + a1(DA)3 + a2(BFI)

Flow Region a0 a1 a2

7Q2 Southeastern -0.9018 1.3049E-10 2.2728

7Q20 Southeastern -0.5084 7.6323E-11 1.1460


21

3Q2 Southeastern -1.0351 1.2409E-10 2.3828

3Q20 Southeastern -0.6133 7.0980E-11 1.2527

30Q2 Southeastern -1.0195 1.4637E-10 2.6144

Table 18. Low flow regression results for the Southwestern & West Central Regions (Chang et al., 2002)

Flow (m3/s) = a0 + a1(DA)3 + a2(BFI)2 + a3(LNTH)2

Flow Region a0 a1 a2 a3

7Q2 Southwestern & West Central -0.190 1.24E-10 1.67 8.35E-5





Table 19. Low flow regression results for the Northern Regions 1, 2, and 3 (Chang et al., 2002)

Flow (m3/s) = a0 + a1(DA) + a2(DA)½ + a3(DA)2 + a4(LNTH) + a5(LNTH)½ + a6(MAR) + a7(MAR)2

Flow Region a0 a1 a2 a3 a4 a5 a6 a7 7Q2 1 -35.766 - 0.8628 - - -4.130 - 0.000353

2 21.65 0.00337 - - - -4.791 0.1088 -

3 7.506 - - 1.581E-7 - 0.5491 -0.0156 -

7Q20 1 -25.718 - 0.5587 - - -2.89 - 0.000272

2 8.124 0.00125 - - - -0.796 -0.0104 -

3 0.4185 - - 9.777E-8 - 0.3403 -0.0055 -


22

Figure 2. Regionalisation of low flows for northern Ontario (Cumming Cockburn Limited, 1995a; Belore, 1995) and southern Ontario (Cumming Cockburn Limited, 1995a)


23

8.0 Summary

Hydrologically based flow methods are desktop analysis techniques that have been

widely used internationally, and include low flow (e.g. 7Q10) and flow duration (e.g. Q95)

indices. Low flows diminish the assimilative capacity of rivers, adversely impacting water

quality downstream of point source discharges. Instream flow methods often rely on low flow

indices to assist in maintaining and protecting aquatic resources.

Low flow indices indicate when riverine water quality or aquatic habitat may be below an

accepted standard, however low flows are not entirely detrimental to rivers and streams. Poff et

al. (1997) detailed some ecological benefits related to low stream flows, which include: i)

recruitment opportunities for riparian plant species in regions where floodplains are frequently

inundated (Wharton et al., 1981), ii) invertebrate and fish species persistence in locations from

which they might be displaced by more dominant, but less tolerant species (Closs and Lake,

1996), and iii) low flow timing can provide cues for initiating life cycle transitions including

spawning, egg hatching, rearing, or migration (Sparks, 1995; Trepanier et al., 1996). Harris et al.

(2000) list ecological significance of low flows as: i) partitioning of habitat patches increases with

declining flows, and ii) habitat creation by species that can exploit exposed river margin

sediments. However Poff et al. (1997) stated that prolonged low flows can cause: i) the

concentration of aquatic organisms (Cushman, 1985; Petts, 1984), ii) diminished plant species

diversity (Taylor, 1982), iii) desertification of riparian species composition (Busch and Smith,

1995; Stromberg et al., 1996), and iv) physiological stress leading to reduced plant growth rate,

morphologic change, or mortality (Kondolf and Curry, 1986; Stromberg et al., 1996; Rood et al.,

1995).

The main findings of this report are listed below:

1. Previous reviews of low flow hydrology and environmental flow methods have identified the

most prominent low flow indices. The most common hydrological low flow indices based on

reviews by Riggs et al. (1980), Smakhtin (2001), and Tharme (2003) are the 7Q10, 7Q2, Q95, and

Q90 flows. The most widely used low flow indices based on this review are the 7Q10 and Q95

flows; commonly used indices include the 1Q10, 7Q2, 7Q20, 30Q10, Q90, and Q75 flows. In the

Province of Ontario, the use of the 7Q20 flow is predominant. The most popular instream flow

methods (Reiser et al., 1989; Karim et al., 1995; Caissie and El-Jabi, 1995, and Yulianti and

Burn, 1998) are the Tennant method (1976), the 7Q10 flow, and the monthly Q50 and Q90 flows


24

(Table 20). Flows within the range Q70-Q99 are most widely used as design low flows (Smakhtin,

2001). There are numerous suggested uses for the most common low flow indices.

Table 20. Most commonly used low flow indices from review studies

Study Most Common Low Flow Indices

Riggs et al. (1980) 7Q10, 7Q2

Smakhtin (2001) 7Q10, 7Q2 Q75, Q90, Q95

Tharme (2003) 7Q10, 1Q1 (Q364) Q95, Q90

This study 7Q10 Q95

In Ontario 7Q20

Instream flows (Reiser et al., 1989) (Karim et al., 1995) (Caissie and El-Jabi, 1995) (Yulianti and Burn, 1998)

Tennant method, 7Q10, monthly Q50, monthly Q90

2. The 7Q10 flow was originally used to indicate a threshold for wastewater discharge (e.g. Carter

and Putnam, 1978; Riggs et al., 1980; Diamond et al., 1994), however the 7Q10 now protects

aquatic life and stream habitats (Ontario Ministry of Natural Resources, 1994; New York State

Department of Environmental Conservation, 1996; U.S. Environmental Protection Agency, 1999;

Imhof and Brown, 2003). There is a necessary connection between point source discharges and

their impact on the stream; since the 1970's the use of the 7Q10 flow reflects these linkages

between flow and habitat, perhaps to the detriment of riverine habitat (U.S. Fish and Wildlife

Service, 1981; Caissie and El-Jabi, 1995; State of Massachusetts, 2004).

3. The 7Q indices (7Q2, 7Q10, and 7Q20) are used by the Ontario Ministry of Natural Resources

and Ontario Ministry of the Environment. Generally, the MNR uses the indices to indicate

habitat maintenance and extinction flows, whereas the MOE generally uses the 7Q indices as

limiting or design conditions related to wastewater discharge or water taking.

4. Numerous studies (e.g. Chang and Boyer, 1977; Cumming Cockburn Limited, 1995a; Ehlke

and Reed, 1999; Rifai et al., 2000; Waltemeyer, 2002; Flynn, 2003) regionalise low flows for

prediction at ungauged locations using multiple regression methods that make use of physical,

meteorological, geological, and hydrological parameters. Regression relationships for Ontario

have been integrated into software to facilitate prediction of low flows for any location within the

Province (Chang et al., 2002). Drainage area is a prominent parameter used in almost all

regression relationships.


25

9.0 Acknowledgements

Ian Cameron (Ontario Ministry of Natural Resources, Water Resources Section, Peterborough, Ontario) and Bob Metcalfe (MNR, Waterpower Project, Peterborough, Ontario) provided guidance regarding this report. Discussions with Rebecca Tharme (International Water Management Institute, Sri Lanka), Rob Fox (MNR, Peterborough, Ontario), Nick Jones (MNR, Peterborough, Ontario), Aaron Todd (MOE, Etobicoke, Ontario), and Bastian Schmidt (WSC, Peterborough, Ontario) assisted in considering flow needs and low flows in general. Paul Odom (MOE, Hamilton, Ontario), Clyde Hammond (MOE, Kingston, Ontario), Zhiping Yang (MOE, London, Ontario), and Ryan Stainton (WSC, Peterborough, Ontario) provided information regarding the Ontario Ministry of the Environment’s use of low flow indices. Carrie Hoskins (MNR, Peterborough, Ontario) and Valerie Von Zuben (MNR, Peterborough, Ontario) provided information and support from the Ontario Ministry of Natural Resources. Reviews by Sarah Crabbe and Ryan Stainton (WSC, Peterborough, Ontario) improved the final version of this report.


26

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