**StreamStats in Colorado**

StreamStats is implemented for all of Colorado. This application implements regression equations for estimating instantaneous peak flows with probabilities of occurring in any given year of 50, 20, 10, 4, 2, 1, 0.5, and 0.2 percent. These statistics also are referred to as the 2-, 5-, 10-, 25-, 50-, 100-, 200-, and 500-year floods. New peak-flow equations were made available in version 3 for eastern Colorado on Nov. 23, 2016, but they are not yet available in version 4. StreamStats also implements regression equations for estimating annual mean flows, mean flows for each month, the flows exceeded 10, 25, 50, 75, and 90 percent of the time, 7-day minimum flows with recurrence intervals of 2, 10, and 50 years, and 7-day maximum flows with recurrence intervals of 2, 10, and 50 years. These non-peak-flow equations are not applicable to the Plains region, which comprises approximately the eastern half of Colorado. The equations in Colorado StreamStats are documented in the reports:

- Capesius, J.P., and Stephens, V.C., 2009, Regional regression equations for estimation of natural streamflow statistics in Colorado: U.S. Geological Survey Scientific Investigations Report 2009–5136, 46 p.
- Kohn, M.S., Stevens, M.R., Harden, T.M., Godaire, J.E., Klinger, R.E., and Mommandi, Amanullah, 2016, Paleoflood investigations to improve peak-streamflow regional-regression equations for natural streamflow in eastern Colorado, 2015: U.S. Geological Survey Scientific Investigations Report 2016–5099, 58 p.

In addition to presentation of the equations, the reports also describe the errors associated with the estimates obtained from the equations, the methods used to develop the equations, and the methods used to compute the basin characteristics included as explanatory variables in the equations. StreamStats users should familiarize themselves with the reports before attempting to obtain estimates of streamflow statistics for ungaged sites. Application of the equations to ungaged sites assumes that flows at the sites occur under natural conditions. The equations in the above reports supersede previous equations that were published in 1985 and 2000 by the U.S. Geological Survey.

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

**Test for Flow Regulation by Dams**

Colorado's StreamStats application includes a 'Check for Upstream Regulation' tool that can be used to identify area(s) within a delineated basin that are regulated by dams. If dams are present within the delineated basin, the tool will compute the drainage area upstream of the dams and downstream of the dams, display locations of the regulated areas, and compute the percentage of the basin that is regulated. If a basin characteristics report is produced, the table will show the values for the entire basin as well as the regulated portion of the basin. The dams identified by the tool are only those present within the National Inventory of Dams (NID) in 2014. The NID includes only dams that meet one or more of the following criteria:

1. High hazard potential classification (loss of human life is likely if the dam fails)

2. Significant hazard potential classification (no probable loss of human life but can cause economic loss, environmental damage, disruption of lifeline facilities, or impact other concerns)

3. Equal or exceed 25 feet in height and exceed 15 acre-feet in storage

4. Equal or exceed 50 acre-feet storage and exceed 6 feet in height

5. Have a contributing drainage area of at least 1 square mile

U.S. Army Corps of Engineers, National Inventory of Dams at http://nid.usace.army.mil/

**Basin Characteristics**

Some of the basin characteristics available within Colorado StreamStats have use limitations related to the methodology of their source calculations or due to land use changes. For example, Time-of-Concentration (TOC) is based on the Natural Resources Conservation Service Hydrology National Engineering Handbook for Time of Concentration (NRCS, 2010), and the computational methods are intended for use on small basins ranging from 1.3 acres to 19 square miles (Folmar and Miller, 2008). StreamStats should not be used to compute TOC values in areas where land use has recently undergone wide-scale changes, such as in burn areas or locations with rapid urbanization. TOC estimates from StreamStats may not be applicable for ephemeral streams with rare flood events. TOC, Runoff Curve Number (RCN), and the Colorado Soil Runoff Coefficient (RUNCO_CO) values have not been calibrated to any flood events or special circumstances.

**References:**

- Colorado Water Conservation Board, 2006, Hydrologic analysis, chap. 9 of Floodplain and stormwater criteria manual: Colorado Department of Natural Resources, 631 p.
- Dhakal, N., 2012, Estimation of volumetric runoff coefficients for Texas watersheds using land-use and rainfall-runoff data, chap. 2 of Development of guidance for runoff coefficient selection and modified rational unit hydrograph method for hydrologic design: Auburn, Alabama, Auburn University, Ph.D. dissertation, 161 p.
- Folmar, N.D., and Miller, A.C., 2008, Development of an empirical lag time equation: American Society of Civil Engineering Journal of Irrigation and Drainage, v. 134, no. 4.
- Natural Resources Conservation Service [NRCS], 1986, Urban hydrology for small watersheds : U.S. Department of Agriculture Natural Resources Conservation Service, Conservation Engineering Division, Technical Release 55, 201-VI-TR-55, 95 p. plus appendixes.
- Natural Resources Conservation Service [NRCS], 2010, Time of concentration, chap. 15 of Hydrology, part 630 of National Engineering Handbook: U.S. Department of Agriculture Natural Resources Conservation Service, 210-VI-NEH, 23 p.

IMPORTANT NOTICE:Prior to June 2, 2014, the Prediction Errors (standard errors of predication, in percent) reported for mean annual and monthly flows and for flow-durations in the outputs from the Estimate Flows using Regression Equations tool in StreamStats were erroneous. The reported prediction errors generally were smaller than the actual values. The errors in StreamStats outputs were the result of errors in the formulas originally used by Capesius and Stephens, 2009, to compute the error terms. As a result, the report was revised, and a notice of errata that describes the nature of the errors and corrections was released on April 25, 2014. The notice of errata is on-line at http://pubs.usgs.gov/sir/2009/5136/versionHist.txt. Note that only the prediction errors were erroneous. All flow estimates presented by StreamStats were correct. |

StreamStats for Colorado was developed in cooperation with the Colorado Department of Transportation and the Colorado Water Conservation Board.