In 1986, the EPA determined that the hydrological-based 7Q10 design flow was similar to the biologically-based 4B3 design flow and recommended the use of either design flow for water quality standards and toxic wasteload allocation studies relating to chronic effects on aquatic life. Although the 7Q10 is used by about half the states in the nation, the 7Q10 is sometimes characterized as being either overprotective or under-protected of aquatic life in various areas of the country.

States regularly propose alternative hydrologically based design flow statistics for their water quality standards (in the form of xQy where x is the duration and y is the frequency). For example, one state currently uses the 3Q2 statistic for conventional pollutants and several other states use a 7Q2 statistic. States often justify the use of a design flow other than 7Q10 on the basis of different hydrogeology. States sometimes suggest the use of a percentile flow (e.g., the 4th percentile) on the basis of ease of calculation and communication with the public.

We seek to improve the ability of EPA and state regulators to evaluate alternative flow statistics. DFLOW can be readily used to compare various design flow statistics. EPA has performed case studies on a few states, comparing the 7Q10, 4B3, and alternative statistics used by states. The output from DFLOW can quickly answer questions such as: "What hydrologically based design flow is closest to the 4B3 (or another specified biologically-based design flow)?" and "Is an alternative flow statistic (e.g., 3Q2) more or less protective of aquatic life than the 4B3 on the basis of the historical flow record?".

Case Study on Design Flows

EPA recently used DFLOW to examine national and state design flow statistics in a state of interest. The process and results are briefly summarized below as an example of how DFLOW can be used and what types of results it can provide. For additional information, download the Case Study PowerPoint Presentation.

Data Acquisition

The BASINS Download Tool was used to download stream flow data for gauges of interest. (You can also download stream gauge data manually from the USGS Daily Streamflow for the Nation Exitor another stream data repository.) Each file contained the flow record for one gauge station.

Data Filtering

All state stream records with less than 20 years (7300 days) of observations were removed from the study. (This was done in accordance with the 1980 American Society of Civil Engineering Task Committee on Low-Flow Evaluation, Methods, and the Needs of the Committee on Surface-Water Hydrology of the Hydraulics Division recommendation that calculations of hydrological design flows be made using at least 15-20 years of flow records.)

EPA contacted the USGS district office to obtain a spreadsheet of information about stream exceptions (e.g., regulation, urbanizations) that could affect the consistency of stream flow measurements. All gauging stations without 20 years of consistent data from statistical consideration (e.g., a station with 10 years of regulated flow and 15 years of unregulated flow) were excluded from the study. Out of the original 422 gauging station, 74 (18%) remained for analysis.

Analysis

The objective of the state stream study was to examine the relationship between the 4B3 and the 3Q2 (an alternative design flow statistic proposed by the state) and the relationship between the 4B3 and 7Q10. The study also sought to compare the relationships and explore the probability distribution of 4B3 percentiles. The data files from the 74 stations with at least 20 years of consistent data served as the input for DFLOW. The DLFOW user then specified that the 7Q10, 4B3, and 3Q2 be calculated for the 74 stations. DFLOW displayed the results in tabular form and the user copied the DFLOW output into a spreadsheet for further analysis and graphical display of the results.

Results

The relationship between the 3Q2 and 4B3 and the relationship between 7Q10 and 4B3 were determined for all streams, large-flow streams (4B3 > 1000 cfs), medium-flow streams (100 cfs < 4B3 < 1000 cfs) and small-flow streams (4B3 < 100 cfs). Some of the primary findings for the state stream study were:

• For all streams, there was a strong correlation between 3Q2 and 4B3, but on average, the 3Q2 flow was 22% greater than the 4B3 flow.
• The correlation between the 3Q2 and 4B3 was weaker for small- and medium-flow streams than large-flow streams.
• For all streams, there was a strong correlation between the 7Q10 and 4B3 and the 7Q10 flow was on average 1% greater than the 4B3 flow.
• The correlation between the 7Q10 and 4B3 was weaker for small-flow streams than medium- and large-flow streams.
• By definition, the 4B3 will lead to one excursion every three years. In this study, using the 3Q2, instead of the 4B3 would lead to 4-8 excursions every 3 years and using the 7Q10 would lead to 0-2 excursions every 3 years (i.e. the 3Q2 was higher than the 4B3 and the 7Q10 was lower).
• In the state analyzed, the 4B3 percentiles ranged from 0% to 1.48%, meaning an percentile limit above 1.48% would under-protect streams.