Low-Flow Statistics for Indiana Streams

Science Center Objects

Indiana and Kentucky scientists have produced new low-flow statistics for Indiana streams, specifically for 7-day, 10-year low flow (7Q10); 30-day, 10-year low flow (30Q10); 1-day, 10-year low flow (1Q10); and harmonic mean streamflow. The low-flow statistics are critical for the state of Indiana to administer the National Pollutant Discharge Elimination System (NPDES) permit program, watershed planning, development of Total Maximum Daily Loads, water use, discharge permitting, drought determination, and aquatic ecosystems. 

We have developed low-flow statistics for streams in Indiana with continuous or partial streamflow records. For streams without streamflow records, we used regression analyses to develop equations to estimate low-flow-frequency statistics and the harmonic-mean flows based on streamgage drainage-basin characteristics. We will be incorporating the low-flow statistics and equation into StreamStats.

Water pollution degrades surface waters making them unsafe for drinking, fishing, swimming, and other activities. As authorized by the Clean Water Act, the National Pollutant Discharge Elimination System (NPDES) permit program controls water pollution by regulating point sources that discharge pollutants into waters of the United States. Industrial, municipal, and other facilities must obtain NPDES permits if their discharges go directly to surface waters. The Indiana Department of Environmental Management (IDEM) requires low-flow statistics in order to administer the NPDES permit program, including the 7-day, 10-year low flow (7Q10); 30-day, 10-year low flow (30Q10); 1-day, 10-year low flow (1Q10); and harmonic mean streamflow1. Low-flow statistics also are needed for 

  • Watershed planning
  • Development of Total Maximum Daily Loads (TMDLs)
  • Water Use
  • Discharge permitting
  • Drought determination
  • Human health design flow (harmonic mean)
  • Aquatic ecosystems – Ecological flows

We are developing the low-flow statistics needed and leveraging developments in Geographic Information System (GIS) and web technologies to provide a single web-based portal to the streamflow statistics.

Calculating Low-Flow Statistics

Screenshot of Streamstats for Indiana
Screenshot of the StreamStats program for Indiana.

Low-flow-frequency characteristics were computed for 272 continuous-record stations including low-flow-frequency analysis, flow-duration analysis, and harmonic mean flows.  Standard regression techniques then were applied to develop equations for computing low-flow-frequency values and harmonic mean streamflow1 for ungaged areas. The low-flow statistics from the 272 gaged sites and the regression equations have been added into the USGS StreamStats program. StreamStats can estimate low-flow information for ungaged or partial-record sites.
To create low-flow estimates:

  • We select a point on stream.
  • StreamStats draws basin boundary and determines drainage area.
  • We determine needed basin characteristics: drainage area, average hydraulic conductivity, average transmissivity, permeability index, slope, percent forested land, and latitude.
  • StreamStats solves the appropriate regression equations to estimate streamflow statistics for the site.

Using this process, low-flow-frequency values and harmonic mean flow (if sufficient data were available) can be estimated for the 166 partial-record stations and for ungaged sites on unregulated stream reaches statewide.

1Harmonic means are used in the “design” flow for setting waste-load-discharge limits. It is the reciprocal of the arithmetic mean of the daily mean streamflow.