StreamStats in Indiana

StreamStats for Indiana incorporates regression equations for estimating (1) peak-flow frequency statistics at the 10-, 25-, 50-, 100-, 200-, and 500-year recurrence intervals (equivalent to the 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent annual exceedance probabilities, respectively), (2) bankfull-channel dimensions of width, mean depth, and cross-sectional area, (3) harmonic mean flows, (4) 1-, 7-, and 30-day mean low flows that occur, on average, once in 10 years, and (5) the probabilities that the 1-day, 7-day, and 30-day mean low flows will be zero. The regression equations are documented in reports cited below. The documentation describes the methods used to develop the equations and to measure the basin characteristics used in the equations, references to GIS data layers used in the analysis, the errors associated with the estimates obtained from the equations, and the areas where the equations are applicable. Users should familiarize themselves with these reports before using StreamStats to obtain estimates of streamflow statistics for ungaged sites.

This application includes custom functionality that provides flood-frequency estimates for many stream reaches in Indiana that have been coordinated by the Indiana Department of Natural Resources, the Natural Resources Conservation Service, the U.S. Army Corps of Engineers, and the USGS. That is, these agencies have agreed on flood-frequency values for selected stream reaches for use in water-resources investigations and planning activities. Users of the interactive map for estimating peak-flow statistics who select points along the coordinated stream reaches will be provided with the coordinated discharges instead of flow estimates obtained from regression equations. More information about coordinated discharges can be found at https://www.in.gov/dnr/water/4898.htm.

The regression equations used in StreamStats to estimate streamflow statistics were developed using streamflow statistics and basin characteristics computed for USGS-operated streamgages. The equations are applicable with known accuracy when they are applied at locations with basin characteristics that are within the ranges of those used to develop the equations. Estimates for user-selected sites with basin characteristics that are outside of those ranges are extrapolated. These extrapolated estimates should be used with caution, as their associated errors are unknown and may be large. StreamStats outputs provide the ranges of applicability for each basin characteristic that is used as an explanatory variable in the regression equations, and warnings when those ranges are exceeded.

## Peak-flow equations

The peak-flow equations, which are applicable for unregulated streams throughout Indiana, are documented in:

Stream slope is used as an explanatory variable in the peak-flow regression equations for all hydrologic regions of Indiana except for region 8. The method used by StreamStats to compute stream slope is somewhat different than the method used to develop the dataset used by Rao (2005) for his regression analyses. The average difference between the StreamStats values of stream slope and the values used by Rao is about 4 percent, but differences for some individual stations exceeded 100 percent. As a result, users should consider that the errors associated with the flow estimates provided by StreamStats are probably somewhat greater than the published standard errors of prediction.

The Indiana Department of Natural Resources cooperated in development of the StreamStats application and the equations for estimating peak-flow statistics.

## Bankfull-channel-dimension equations

The equations for estimating bankfull-channel-dimensions are documented in:

The bankfull-channel-dimension equations were developed for non-urban streams. Impervious surfaces and storm sewers can significantly alter runoff characteristics and the dimensions of channels that drain urbanized watersheds.

The Indiana Office of Community and Rural Affairs cooperated in the development of the StreamStats application and the equations for estimating bankfull-channel dimensions.

## Harmonic mean, low-flow frequency, and probability of zero flow equations

The equations for estimating harmonic mean flow, low-flow frequencies, and the probability of zero flow are documented in:

The Indiana Department of Environmental Management cooperated in the development of the StreamStats application and the equations for estimating low-flow statistics.