New Jersey StreamStats

**Statement to Users-2022**

StreamStats for New Jersey can be used to estimate the magnitude of flow for annual exceedance probabilities of 0.5, 0.2, 0.1, 0.04, 0.02, 0.01, and 0.002 for ungaged, unregulated or slightly regulated, non-tidal streams in New Jersey. These exceedance probabilities correspond to recurrence intervals of 2-, 5-, 10-, 25-, 50-, 100-, and 500-years, respectively. StreamStats for New Jersey also can be used to estimate monthly flow-duration and monthly low-flow frequency statistics for ungaged streams in Coastal Plain and non-coastal regions of New Jersey for baseline and current land- and water-use conditions. The equations were developed to estimate 87 different streamflow statistics, which include the monthly 99-, 90-, 85-, 75-, 50-, and 25-percentile flow-durations of the minimum 1-day daily flow; the August–September 99-, 90-, and 75-percentile minimum 1-day daily flow; and the monthly 7-day, 10-year (M7D10Y) low-flow frequency. Equations for estimating annual low flow statistics at ungaged location are not included in this application. The reports below present the regression equations used to estimate the noted streamflow statistics, describe the errors associated with the estimates, and describe the methods used to develop the equations and to measure the basin characteristics used in the equations. Users should familiarize themselves with the reports before using StreamStats to obtain estimates of streamflow statistics for gaged and ungaged stream locations.

The New Jersey StreamStats application was updated on February 18, 2022, with a LiDAR derived elevation dataset provided by the New Jersey Department of Environmental Protection (NJDEP). The updated application grid cell size was reduced from a 10-meter grid to a 10-foot grid. Because all peak and low flow equations were derived with the previous grid cell size and have remained unchanged, the streamflow statistics and basin characteristics produced by this version may deviate slightly from those generated previously. Comparisons made at gaged sites were not found to differ significantly among state regions and were distributed normally. Differences at ungaged locations were not evaluated. Additional information on testing is provided below. Further information regarding this update can be found in at the following project data release and metadata pages: New Jersey StreamStats Digital Elevation, Flow Direction, and Flow Accumulation GIS data, 2021 - ScienceBase-Catalog and Basin Characteristics Rasters for New Jersey StreamStats 2021 - ScienceBase-Catalog. Users are encouraged to follow the guidelines established in Watson and Schopp (2009) and Watson and McHugh (2014) when weighting regression estimates against observed data and derived statistics.

• Watson, K.M., and Schopp, R.D., 2009, Methodology for estimation of flood magnitude and frequency for New Jersey streams: U.S. Geological Survey Scientific Investigations Report 2009-5167
• Watson, K.M., and McHugh, A.R., 2014, Regional regression equations for the estimation of selected monthly low-flow duration and frequency statistics at ungaged sites on streams in New Jersey: U.S. Geological Survey Scientific Investigations Report 2014–5004, 59 p.

Click on this link to obtain general information on the New Jersey application, as well as specific sources and computation methods for basin characteristics. 

NOTE 1: Although 1990 population density was used for the development of the regression equations in SIR 2009-5167, the New Jersey StreamStats application uses the more current 2000 population-density data.

NOTE 2: Watson and McHugh (2014) provide two sets of regression equations for flow duration and low-flow frequency statistics, intended to reflect conditions for a “base” and a “current” period. The “base” period is defined as the years a streamgage had little to no change in development and water use. The “current” period is defined as the 1989–2008 water years (Oct. 1, 1988 to Sept. 30, 2008). The base period of record is representative of a period when the basin was unaffected by change in development. The current period is representative of more recent increased development.

NOTE 3:  The accuracies of flow estimates for New Jersey from regional regression equations for flood and low-flow conditions were quantified within the publications mentioned above. These accuracy results and flow estimates are directly tied to the quality of the base datasets and basin characteristic datasets used. The introduction of higher quality base information within the new system allows for more accurate calculation of watershed boundaries, drainage area, slope and other elevation derived information but these more accurate results will often be different than those used to develop the flow estimate equations. Use of the higher quality base data in calculations produces results that are disconnected from the equation coefficients, the standard error of estimate and standard error of prediction defined in the original report (Watson, 2009). Thus, no precise expression of standard error can be provided to StreamStats users. It is impractical at this point to consider recomputation of these standard errors because that would result in revisions to the published regional regression equations as well.

NOTE 4: Functionality was added to compute average basin slope using a method described in the USDA NRCS National Engineering Handbook, Part 630 Hydrology, Chapter 15 - Time of Concentration [OpenNonWebContent.aspx (usda.gov)]. A contour dataset was processed using the NJDEP statewide 10-foot digital elevation model and elevation data sources to account for drainage from Pennsylvania and New York. The calculation of average basin slope (SLOPLAGPER) is working successfully for basins as large as 140 square miles. Basins larger than 140 square miles appear to be too data intensive and do not report an average basin slope result. The contour dataset is available on ScienceBase on the Basin Characteristics data and metadata resource page discussed above.
 

Summary of QA/QC Testing and Findings

Comparisons were conducted at over 180 sites across the state between the original New Jersey StreamStats application and the revised version released on February 18, 2022, to determine the impact and significance of improved base elevation dataset on flow estimates and basin characteristic results. The overall findings from the QA/QC process suggest that although some differences exist between the original and the revised version, these differences were either negligible or a direct result of the new elevation dataset.

• In creating summary tables (found below) to describe QA/QC findings, the regions included were the five major geological provinces of New Jersey used in the creation of the flood frequency equations. Drainage area ranges (in square miles) were selected to highlight key divisions within percent difference data while maintaining relevant sample sizes.

• None of the various regions (Valley/Ridge, Unglaciated Piedmont, Glaciated Piedmont, Inner Coastal Plain, Outer Coastal Plain) had significantly higher percent differences for any category of data (basin characteristics, peak flows, low flows).

• Considering peak flow data, differences were often between 2 and 10 percent. Differences greater than 10 percent were uncommon and were evaluated on a case-by-case basis; large percent differences were often explained by changes in the new dataset.

  • The largest percent differences across all data occurred in basins with the smallest drainage areas. In these cases, relatively small changes in basin characteristics resulted in large percent errors.

  • By the same logic, larger basins tended to have the lowest percent differences

• The majority of outlier sites with large differences in basin characteristics were driven by changes to their drainage area and slope. Oftentimes, the most extreme differences observed were a direct result of implementing the new elevation dataset (drainage area, slope).

• All statistics are relatively evenly distributed around the mean. All previously existing skews in the basin characteristics were resolved.

• The results of the new application closely resemble both the original application as well as values published in the 2009 and 2014 SIRs.

• The low flow statistic QA/QC process evaluated 7Q10 flows and flow duration results across all months. Because the equations for these statistics generate very small flow values, changes due to drainage area amplify the percent change. Therefore, summary tables of findings are not presented below.

• Both the NJWSC and National USGS StreamStats teams determined that the results from the new application were sufficient to support migration to the publicly available website.