A Division of the Georgia Department of Natural Resources

Water Quality Modeling

The Georgia Environmental Protection (GA EPD) is responsible for protecting, maintaining, and enhancing water quality throughout the State.  To that end, GA EPD develops wasteload allocations (WLAs) and Total Maximum Daily Loads (TMDLs) for facilities that discharge treated wastewater (for example, domestic sewage and industrial waste) into waters of the state to assure that the permitted effluent limits meet applicable state Water Quality Standards.  A WLA is the portion of receiving water’s assimilative capacity that is allocated to point sources of pollution. The calculation of a WLA is based on conservative assumptions, to protect the water quality under worst case conditions such as low flow (7Q10) and high temperature.  Facilities submit site-specific information on the receiving waterbody and the discharge flow for consideration.

Annual and monthly 7Q10 flows were determined by USGS for 197 long term gaged streams.  Using this information USGS developed regional regression equations for estimating selected low-flow frequency (7Q10 and 1Q10) and mean annual flow statistics for ungaged streams in north Georgia that are not substantially affected by regulation, diversions, or urbanization.  The equations can be found in Methods for estimating selected low-flow frequency statistics and mean annual flow for ungaged locations on streams in North Georgia, Scientific Investigations Report 2017-5001. These equations can be used to provide estimates of the natural flows for selected ungaged stream locations north of the Fall Line and an interactive map can be found at the USGS StreamStats website.  StreamStats is a Web-based geographic information system that provides streamflow statistics and basin characteristics for USGS streamgage locations and ungaged sites of interest.  The users will be able to can select an ungaged location along any streams in North Georgia and get the low flow statistics. 

To determine critical stream-temperature, temperature measurements were analyzed using a harmonic curve-fitting procedure to compute a statewide stream-temperature harmonic equation, derived using latitude, drainage area, and altitude for natural streams having drainage areas greater than about 40 square miles. The equation can be used to compute long-term natural harmonic stream-temperature coefficients to within an on average of about 0.4 ° C.  In addition, harmonic-function coefficients for each station were computed from the least-squares sinusoidal fit of the measured stream-temperature data.  These results can be found in the USGS Water-Resources Investigations Report 96-4203, Stream-Temperature Characteristics in Georgia.

National Pollutant Discharge Elimination System (NPDES) permits typically have permit limits for oxygen demanding substances: 5-day biochemical oxygen demand (BOD5) and ammonia (NH3).  These constituents affect the dissolved oxygen (DO) levels in streams as they naturally decay.  Water quality models are used to predict the instream DO levels; however, they model ultimate biochemical oxygen demand (BODu) rather than BOD5.  Many NPDES permits require long-term BOD tests be performed to determine the f-ratio (ratio of the BODu to BOD5) and the carbonaceous and nitrogenous biochemical oxygen decay rates.  References on the long-term BOD tests can be found at Long-Term BOD References.

Water quality models can be used to predict water quality under varying present and future conditions. These tools are to conducting the analyses to determine the available assimilative capacity and maximum daily load which can be placed on the waterbodies wastewater dischargers and non-point sources. The model results form a basis for the issuance of permits limiting wastewater discharges into the surface waters of Georgia. A brief description of the water quality models GA EPD uses most often is given below.  These models can be downloaded at http://epdsoftware.wileng.com/



  • Mathematical simulation using modified Streeter-Phelps
  • Steady-state
  • One-dimensional
  • Can model branching river systems


  • Most widely used tool for the NPDES permit limits
  • Determines oxygen demanding permit limits (BOD5, ammonia, and DO)
  • Management tool that can model various conditions
  • Used to develop  DO TMDLs
  • Can express water intakes and impoundment dams
  • Flexible spreadsheet-style entry of model inputs
  • Not suitable for modeling the estuaries, wetlands, lakes, and very slow moving streams



  • Mathematical simulation using Thomann’s method
  • Advection-dispersion equation for mass transport
  • Steady-state
  • One-dimensional
  • Tidally-averaged (mid-tide)


  • Determines DO sag in coastal rivers and estuaries
  • Determines oxygen demanding permit limits (BOD5, ammonia, and DO)
  • Determines DO TMDLs



  • Core of the modeling system is U.S. Army Corps of Engineers’ CE-QUAL-Riv1
  • One-dimensional
  • Hydrodynamic
  • Simulates rivers with highly transient flow conditions commonly experienced downstream from reservoirs to predict water quality response


  • Defensible tool for determining the current and future wasteload allocations
  • Determine minimum flow targets
  • Examine the effect of minimum flow variances
  • Look at drought impacts
  • Assist in the development of water temperature criteria
  • Used to determine Temperature and  DO TMDLs