U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2015–5016

Prepared in cooperation with the Colorado Water Conservation Board

Evaluation of Mean-Monthly Streamflow-Regression 

Equations for Colorado, 2014
Cover. Photograph showing Missouri Creek near Gold Park, Colorado by Jeff Foster, U.S. Geological 

Survey. 
Evaluation of Mean-Monthly 

Streamflow-Regression Equations 

for Colorado, 2014

By Michael S. Kohn, Michael R. Stevens, Andrew R. Bock, and Stephen J. Char

Prepared in cooperation with the Colorado Water Conservation Board

Scientific Investigations Report 2015–5016

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Kohn, M.S., Stevens, M.R., Bock, A.R., and Char, S.J., 2015, Evaluation of mean-monthly streamflow-regression 

equations for Colorado, 2014: U.S. Geological Survey Scientific Investigations Report 2015–5016, 53 p., http://dx.doi.

org/10.3133/sir20155016.

ISSN 2328-0328 (online)
iii

Contents

Acknowledgments.......................................................................................................................................vii

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................2

Purpose and Scope ..............................................................................................................................2

Previous Studies and Background Information...............................................................................2

Description of the Study Area ............................................................................................................3

Methods...........................................................................................................................................................4

Mean-Monthly Streamflow from Streamgage Record...................................................................4

Mean-Monthly Streamflow from Basin Characteristics and Regression Equations ................6

Quality Assurance.................................................................................................................................7

Evaluation of Mean-Monthly Streamflow-Regression Equations .........................................................7

Graphical Comparison of Observed and Predicted Mean-Monthly Streamflow................................7

Absolute Percent Difference .......................................................................................................................9

Graphical Analysis of Monthly Bias............................................................................................................9

Adjusted Coefficient of Determination.....................................................................................................16

Standard Error of Prediction......................................................................................................................17

Summary........................................................................................................................................................20

References Cited..........................................................................................................................................21

Appendix 1. Streamgage Summary ..........................................................................................................24

Appendix 2. Table of Mean-Monthly Streamflow-Regression Equations from Figures 3–6 of

Capesius and Stephens (2009) .....................................................................................................38

Appendix 3. Figures Showing Absolute Percentage Difference Between Observed and

Predicted Mean-Monthly Streamflow at all Streamgages .....................................................42
iv

Figures

1. Map showing the boundaries of the hydrologic regions in Colorado that extend

50 miles into the adjacent States included in the study area................................................3

2. Map showing the location of the streamgages used to evaluate the

mean-monthly streamflow-regression equations; old streamgages were used in

the 2009 study, and new streamgages were not used in the 2009 study.............................5

3. Graph showing the drainage area and mean-annual precipitation for the 278 old

streamgages and the 154 new streamgages that were used for analysis in this

report ..............................................................................................................................................7

4. Graph showing the drainage area and mean-basin elevation for the 278 old

streamgages and the 154 new streamgages that were used for analysis in this

report ..............................................................................................................................................8

5. Graph showing the mean-annual precipitation and mean-basin elevation for the

278 old streamgages and the 154 new streamgages that were used for analysis in

this report.......................................................................................................................................8

6. Graph showing comparison of observed and predicted mean-monthly

streamflow in the Mountain hydrologic region for all 12 months.......................................10

7 Graph showing comparison of observed and predicted mean-monthly streamflow

in the Northwest hydrologic region for all 12 months ..........................................................11

8. Graph showing comparison of observed and predicted mean-monthly

streamflow in the Rio Grande hydrologic region for all 12 months ....................................12

9. Graph showing comparison of observed and predicted mean-monthly

streamflow in the Southwest hydrologic region for all 12 months.....................................13

10. Boxplots showing residual streamflow for each month for the Mountain

hydrologic region........................................................................................................................14

11. Boxplots showing residual streamflow for each month for the Northwest

hydrologic region .......................................................................................................................15

12. Boxplots showing residual streamflow for each month for the Rio Grande

hydrologic region........................................................................................................................15

13. Boxplots showing residual streamflow for each month for the Southwest

hydrologic region........................................................................................................................16

Appendix Figures

3–1. Map showing absolute percentage difference between observed and predicted

mean-monthly streamflow at all streamgages used in the analysis for the month

of January ....................................................................................................................................42

3–2. Map showing absolute percentage difference between observed and predicted

mean-monthly streamflow at all streamgages used in the analysis for the month

of February...................................................................................................................................43

3–3. Map showing absolute percentage difference between observed and predicted

mean-monthly streamflow at all streamgages used in the analysis for the month

of March.......................................................................................................................................44

3–4. Map showing absolute percentage difference between observed and predicted

mean-monthly streamflow at all streamgages used in the analysis for the month

of April ..........................................................................................................................................45
v

3–5. Map showing absolute percentage difference between observed and predicted

mean-monthly streamflow at all streamgages used in the analysis for the month

of May...........................................................................................................................................46

3–6. Map showing absolute percentage difference between observed and predicted

mean-monthly streamflow at all streamgages used in the analysis for the month

of June..........................................................................................................................................47

3–7. Map showing absolute percentage difference between observed and predicted

mean-monthly streamflow at all streamgages used in the analysis for the month

of July ...........................................................................................................................................48

3–8. Map showing absolute percentage difference between observed and predicted

mean-monthly streamflow at all streamgages used in the analysis for the month

of August......................................................................................................................................49

3–9. Map showing absolute percentage difference between observed and predicted

mean-monthly streamflow at all streamgages used in the analysis for the month

of September...............................................................................................................................50

3–10. Map showing absolute percentage difference between observed and predicted

mean-monthly streamflow at all streamgages used in the analysis for the month

of October ....................................................................................................................................51

3–11. Map showing absolute percentage difference between observed and predicted

mean-monthly streamflow at all streamgages used in the analysis for the month

of November................................................................................................................................52

3–12. Map showing absolute percentage difference between observed and predicted

mean-monthly streamflow at all streamgages used in the analysis for the month

of December................................................................................................................................53

Tables

1. Streamgages used in analysis sorted by hydrologic region, whether the

streamgage was used in the 2009 study, and the agency that collected the data............6

2. Comparison of basin and climate characteristics of the 278 old streamgages and

the 154 new streamgages that were used for analysis in this report..................................6

3. The median absolute percent difference between the observed and predicted

mean-monthly streamflow for each of the 48 mean-monthly

streamflow-regression equations............................................................................................14

4. The adjusted coefficient of determination for the 48 mean-monthly

streamflow-regression equations............................................................................................17

5. Mean standard error of prediction for mean-monthly streamflow from the 

2009 study.....................................................................................................................................17

6. Percentages of data from streamgages that are within the 95-percent prediction

interval, based on the standard error of prediction from the 2009 study .........................19

Appendix Table

1–1. Summary of the streamgages used in the analysis..............................................................24
vi

Conversion Factors

[Inch/Pound to International System of Units]

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2

) 259.0 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2) 

Volume

cubic foot (ft3

) 28.32 cubic decimeter (dm3) 

cubic foot (ft3

) 0.02832 cubic meter (m3) 

Flow rate

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Elevation, as used in this report, refers to distance above the vertical datum.

Supplemental Information

Water year in this report is defined as the period from October 1st of one year through 

September 30th of the following year and is named for the year of the ending date.

Abbreviations

adjR2 adjusted-coefficient of determination

CDWR Colorado Division of Water Resources

NWIS National Water Information System

SEP standard error of prediction

USGS U.S. Geological Survey
vii

Acknowledgments

Jeff Baessler and Brandy Logan of the Colorado Water Conservation Board provided helpful 

guidance and thoughtful feedback throughout the study. Chris Brown and Doug Stenzel of the 

Colorado Division of Water Resources were instrumental in coordinating the compilation of 

Colorado Division of Water Resources streamgage data to be used in this study. The following 

Colorado Division of Water Resources lead hydrographers helped determine which Colorado 

Division of Water Resources streamgages were representative of natural streamflow conditions and could be used in this study: Russell Stroud (Region 1), Joseph Talbott (Region 2), Scott 

Veneman (Region 3), Jerry Thrush (Region 4), Craig Bruner (Region 5), Dan Meyer (Region 6), and 

Brian Boughton (Region 7).

William Asquith and Andrea Veilleux of the U.S. Geological Survey (USGS) offered invaluable 

technical assistance during the undertaking of the study. Julie Kiang of the USGS provided support and guidance throughout the study and as a technical reviewer. John Fulton of the USGS 

contributed constructive comments as a technical reviewer, which enhanced the quality of the 

analysis and report.

Evaluation of Mean-Monthly Streamflow-Regression 

Equations for Colorado

By Michael S. Kohn, Michael R. Stevens, Andrew R. Bock, and Stephen J. Char

Abstract

The U.S. Geological Survey, in cooperation with the 

Colorado Water Conservation Board, evaluated the predictive 

uncertainty of mean-monthly streamflow-regression equations 

representative of natural streamflow conditions in Colorado.

This study evaluates the predictive uncertainty of meanmonthly streamflow-regression equations developed in a 2009 

U.S. Geological Survey study using streamflow data collected 

over the entire period of record at each streamgage through 

calendar year 2013. The study area for this report is limited to 

the Mountain, Northwest, Rio Grande, and Southwest hydrologic regions of Colorado.

Data collected from the beginning of the period of record 

through calendar year 2013 were used to evaluate the meanmonthly streamflow equations using the same basin characteristics as in the 2009 study. U.S. Geological Survey and 

Colorado Division of Water Resources streamgages with at 

least 10 years of streamflow record and identified as representative of natural streamflow conditions were selected for 

this study. During the streamgage selection process, a total of 

432 streamgages, composed of 278 from the 2009 study and 

154 new streamgages, were identified.

The updated standard error of prediction and adjusted 

coefficient of determination values that correspond to the 

mean-monthly streamflow equations developed in the 2009 

study are in close agreement with the results of this study. 

The old streamgages performed slightly better than the new 

streamgages, with approximately 88 and 85 percent of the data 

within the prediction intervals, respectively. This result was 

expected because the streamgages used to develop the regression equations should yield a better performance than the new 

streamgages.

For all hydrologic regions, approximately 87 percent of 

the data are within the 95-percent prediction intervals. The 

explanation for why fewer than 95 percent of the data are 

within the prediction intervals is that the data do not conform 

perfectly to the regression assumptions required to accurately 

estimate performance metrics. The equations for the Rio 

Grande hydrologic region had the best fit with the parametric prediction-interval assumptions, with approximately 

91.8 percent of the data within the prediction interval (average 

12 months). The Mountain, Northwest, and Southwest hydrologic regions had 87.8, 84.9, and 83.5 percent of the data 

contained within the prediction interval, respectively.

Monthly adjusted coefficient of determination values 

were computed and have the same general pattern for all four 

hydrologic regions. The largest values usually occur in March 

or April, and the lowest values usually occur in August or September. Only the Rio Grande hydrologic region deviates from 

this seasonal pattern, exhibiting a decrease in adjusted coefficient of determination values in August and September, with 

the lowest values occurring in the winter months (December, 

January, and February). Generally, the adjusted coefficient 

of determination values for this report are just slightly less 

(0.76 compared to 0.79) than the values computed in the 2009 

study. The similarity of values, even when tested with data 

not used to originally develop the mean-monthly streamflowregression equations, provides confidence that the predictive 

uncertainty of mean-monthly regression equations in the 2009 

study are accurate. The fact that the results for the two datasets 

are very similar provides assurance that when these equations 

are applied to locations not used to develop the equations, the 

standard error of prediction and adjusted-coefficient of determination error metrics should be similar to those established in 

the 2009 study for locations with natural streamflow.

The median absolute differences between the observed 

and computed mean-monthly streamflow for Mountain, Northwest, and Southwest hydrologic regions are fairly uniform 

throughout the year, with the exception of late summer and 

early fall (July, August, and September), when each hydrologic region exhibits a substantial increase in median absolute 

percent difference. The greatest difference occurs in the Northwest hydrologic region, and the smallest difference occurs in 

the Mountain hydrologic region. The Rio Grande hydrologic 

region shows seasonal variation in median absolute percent 

difference with March, April, August, and September having a 

median absolute difference near or below 40 percent, and the 

remaining months of the year having a median absolute difference near or above 50 percent. In the Mountain, Northwest, 

and Southwest hydrologic regions, the mean-monthly streamflow equations perform the best during spring (March, April, 

and May). However, in the Rio Grande hydrologic region, the 

mean-monthly streamflow equations perform the best during 

late summer and early fall (August and September).
2 Evaluation of Mean-Monthly Streamflow-Regression Equations for Colorado, 2014

Introduction

Streamflow-regression equations are statistical relations between streamflow statistics computed from available 

streamgage records (including mean-monthly streamflow) and 

relevant basin and climatic characteristics. Streamflow-regression equations generally are developed for geographic regions 

where basin and climatic conditions are relatively consistent. 

The equations are accompanied by estimates of predictive 

uncertainty and provide useful and economic tools for calculating streamflow statistics at ungaged locations. Streamflowregression equations are commonly used to estimate streamflow statistics at ungaged sites across the Nation (Capesius and 

Stephens, 2009). Reliable estimates of streamflow statistics are 

critical for water-resource management, stream-related structural design, stream-hazard identification, and water-quality 

management.

The U.S. Geological Survey (USGS) developed a Webbased computer program called StreamStats (Ries and others, 

2004). The software facilitates the computation of streamflow 

statistics using regional regression equations or other procedures that have been published previously. StreamStats allows 

the user to compute streamflow statistics for both gaged and 

ungaged sites by selecting a specific stream location on a map 

interface. If the location of interest lacks a streamgage, the 

algorithms in StreamStats delineate the basin for the location, 

compute basin and climatic characteristics, and provide estimates of the streamflow statistics using the available regression equations.

The USGS, in cooperation with the Colorado Water 

Conservation Board, evaluated the predictive uncertainty of 

mean-monthly streamflow-regression equations representative 

of natural streamflow conditions in Colorado. Streamflowregression equations were previously developed to estimate 

natural streamflow statistics at ungaged sites in Colorado by 

Capesius and Stephens (2009), which is hereinafter referred to 

as the “2009 study.” The present study evaluates the predictive 

uncertainty of mean-monthly streamflow-regression equations 

developed in the 2009 study using streamflow data collected 

over the entire period of record at each streamgage through 

calendar year 2013. Mean-monthly streamflow data from the 

regression equations were compared to mean-monthly streamflow data from streamgage records to evaluate the predictive 

uncertainty.

Purpose and Scope

The purpose of this report is to evaluate the streamflowregression equations presented in the 2009 study by comparing the predictive uncertainty using streamflow data through 

calendar year 2013 for computation of mean-monthly streamflow for Colorado basins with hydrology that is influenced 

predominantly by natural runoff processes (fig. 1).

The 2009 study updated mean-monthly streamflow 

equations developed by Kircher and others (1985) in four 

(Mountain, Northwest, Rio Grande, and Southwest) of the 

five Colorado hydrologic regions. The 2009 study determined 

that data in the Plains hydrologic region were inadequate for 

regression-equation development for any streamflow statistics 

other than peak streamflow, so no mean-monthly streamflow 

equations exist for this hydrologic region. The study area for 

this report is therefore limited to the Mountain, Northwest, 

Rio Grande, and Southwest hydrologic regions of Colorado. 

The appropriate area for the use of the equations is limited to 

Colorado, despite the extension of the study area to include 

streamgages within a 50-mile boundary or buffer surrounding 

Colorado for the purpose of equation development (Capesius 

and Stephens, 2009).

The regression equations for mean-monthly streamflow 

estimation in Colorado were developed in the 2009 study by 

Capesius and Stephens using streamflow data collected from 

the beginning of the period of record at each streamgage 

through water year 2007 (October 1, 2006, through September 30, 2007). Data collected from the beginning of the period 

of record through calendar year 2013 were used to evaluate 

the mean-monthly streamflow equations using the same basin 

characteristics as in the 2009 study.

Regression equations computed in the 2009 study are 

used to estimate natural streamflow statistics for ungaged sites. 

“To clarify, the equations are based on analysis of streamflow 

data representing streamflow conditions relatively unaffected 

by anthropogenic influences such as regulation and diversion 

or return flows such as from a municipality, or mining operation, or urban development in a basin” (Capesius and Stephens, 2009, p. 3). “Kircher and others (1985) defined natural 

streamflow as streamflow from drainage basins relatively unaffected by urban development or water-management activities 

such as substantial reservoir storage, streamflow diversions, 

or return flows of previously diverted streamflow. Further, 

those authors defined natural streamflow as streamflow having 

less than about 10 percent of the mean-annual streamflow 

volume at the streamgage affected by anthropogenic activity” 

(Capesius and Stephens, 2009, p. 3). This report includes only 

streamgages that have been determined to meet the Kircher 

and others (1985) criteria.

Previous Studies and Background Information

Many studies have computed regression equations for 

estimating flood-frequency streamflow statistics in Colorado—Patterson (1964, 1965), Patterson and Somers (1966), 

and Matthai (1968), Headman and others (1972), McCain 

and Jarrett (1976), Kircher and others (1985), Livingston 

and Minges (1987), Vaill (1999), and the 2009 study—but 

fewer studies have developed regression equations for meanmonthly streamflow, such as Kircher and others (1985) and the 

2009 study. The hydrologic regions used in this report were 

delineated by McCain and Jarrett (1976) and were incorporated as the regional framework in Kircher and others (1985). 

Kircher and others (1985) developed regression equations for 
Introduction 3

mean-monthly streamflow in western Colorado for data collected through 1983. The 2009 study published Statewide peak 

and non-peak (with the exception of the Plains hydrologic 

region) statistics (including mean-monthly streamflow) using 

USGS streamflow data from the beginning of the period of 

record at each streamgage through water years 2006 and 2007, 

respectively. In the 2009 study, error associated with the meanmonthly streamflow-regression equations was characterized 

using the standard error of prediction (SEP, in percent) and the 

adjusted-coefficient of determination (adjR2

, dimensionless).

Description of the Study Area

Colorado has a diverse landscape and climate and 

includes the headwaters of the major river basins of the Colorado, Rio Grande, Platte, and Arkansas Rivers. The physiographic differences in Colorado can be described by three 

major physiographic provinces, which trend north to south 

across the State (Fenneman, 1931). The Great Plains Province, in the eastern 40 percent of the State, consists mostly of 

grasslands with scattered hills, bluffs, shallow river valleys, 

Pueblo

Colorado

Springs

Denver

Fort Collins

Grand 

Junction

South Platte River

North Platte River

Arikaree River

South Fork Republican River

Republican River

Arkansas River

Purgatoire River

San Juan River

Dolores River

Gunnison R

Colorado River

Yampa River

Green River

COLORADO

NEW MEXICO OKLAHOMA

TEXAS

KANSAS

NEBRASKA

WYOMING

UTAH

ARIZONA

Plains

Region

Mountain

Region

Rio Grande

Region

Southwest

Region

Northwest

Region

Colorado River

White River

No rth Platte River

Rio Grande

Huerf ano River

0 50 100 MILES

0 50 100 KILOMETERS 

Gre en R iv er

Animas River

Base modified from ESRI ArcGIS Online, 2013

State Plane, Colorado, Central

North American Datum of 1983

Hydrologic region boundary

EXPLANATION

110° 108° 106° 104° 102°

40°

38°

Figure 1. Boundaries of the hydrologic regions in Colorado that extend 50 miles into the adjacent States included in the study area.
4 Evaluation of Mean-Monthly Streamflow-Regression Equations for Colorado, 2014

and some cultivated areas. The Southern Rocky Mountains 

Province, west of the Great Plains, includes most of central 

Colorado from north to south and is characterized by mountain ranges and intermountain valleys. The Colorado Plateaus 

Province is in western Colorado between the Utah border to 

the west and the Southern Rocky Mountains to the east. The 

landscape is distinguished by mesas, plateaus, and eroded 

canyon terrain that includes much of the western quarter of 

Colorado from north to south. More detailed descriptions of 

the major physiographic provinces can be found in Fenneman 

(1931) and the 2009 study.

For this report “...a hydrologic region is qualitatively 

defined as a region of similar hydrology and climatology. The 

five hydrologic regions of Colorado were defined on the basis 

of the physiographic and climatic characteristics that were 

used to develop best-fit regression equations. The Mountain hydrologic region is identified as that region of central 

Colorado above about 7,500 feet in elevation located between 

the Colorado-Wyoming border and the Rio Grande basin. The 

Mountain hydrologic region encompasses the headwaters of 

most major river basins in Colorado where the annual peak 

streamflow generally is produced by snowmelt runoff. The 

Northwest hydrologic region is defined as the northwestern 

part of Colorado below 7,500 feet and encompassing substantial areas of the Yampa, White, and Gunnison River basins. 

The Rio Grande hydrologic region ranges in elevation from 

about 5,000 feet near the Colorado-New Mexico border to 

more than 14,000 feet in the northern parts and encompasses 

the Rio Grande basin. The Southwest hydrologic region is 

defined as the region located south of the Gunnison River 

basin and west of the Rio Grande basin and encompasses 

the Dolores, Animas, and San Juan River basins. The Plains 

hydrologic region is east of the Rocky Mountains and below 

7,500 feet in the South Platte River basin and below 9,000 feet 

in the Arkansas River basin” (Capesius and Stephens, 2009, 

p. 4). Because hydrology is not affected by the political borders between States, the hydrologic region boundaries were 

extended 50 miles into all States surrounding Colorado (fig. 1) 

(Capesius and Stephens, 2009). As a result, the study area 

includes parts of Arizona, New Mexico, Utah, and Wyoming 

along with the four western hydrologic regions in Colorado.

Methods

This section describes the methods used in data acquisition, processing, and computations necessary to determine 

mean-monthly streamflows and evaluation statistics.

Mean-Monthly Streamflow from Streamgage 

Record

The mean-monthly streamflow-regression equations 

were evaluated by analyzing the predictive uncertainties of 

the equations presented in the 2009 study. Mean-monthly 

streamflow data from the beginning of the period of record at 

each streamgage through calendar year 2013 from USGS and 

Colorado Division of Water Resources (CDWR) streamgages 

were compared to mean-monthly streamflow determined from 

the regression equations at all suitable streamgages in the 

study area. During the streamgage selection process, a total of 

432 streamgages, composed of 278 from the 2009 study and 

154 new streamgages, were identified, and the mean-monthly 

streamflow was determined from the streamgage records. At 

the streamgage locations, basin and climate characteristics 

were used to compute mean-monthly streamflow with regression equations (fig. 2, tables 1 and 2, appendix 1). Observed 

(streamgage data) and predicted (regression equation values) 

data were compared by scatter plots, computation of median 

absolute percent difference in streamflow, graphical analysis of monthly bias by examination of boxplots of residual 

streamflow, adjR2 statistics, and SEP statistics.

Streamgages selected for the analysis were chosen on the 

basis of location, inclusion in the 2009 study, and available 

data. For the four hydrologic regions, all streamgages used 

in the 2009 study were selected. The USGS National Water 

Information System (NWIS) mapper (USGS, 2013a) was 

used to compile the USGS streamgages within 50 miles of the 

Colorado border. Only USGS and CDWR streamgages with at 

least 10 years of streamflow records and identified as representative of natural streamflow conditions were selected. In 

determining which streamgages were representative of natural 

streamflow conditions, codes from the NWIS peak streamflow 

database were followed along with professional judgment. 

In each of the CDWR regions, the lead CDWR hydrographer 

for that respective region was contacted and engaged to help 

determine which streamgages were representative of natural 

streamflow conditions as defined in a 1985 study by Kircher 

and others. A number of the selected streamgages have been 

operated at different periods of time by both the USGS and 

the CDWR. In these special cases, if 10 years of data had been 

collected between the two agencies, the streamgage was used. 

A total of 300 daily-mean values for a month are approximately equal to 10 years of record. Hereinafter in this report, 

streamgages used in the 2009 study will be referred to as “old 

streamgages” and streamgages not used in the 2009 study will 

be referred to as “new streamgages.”

Mean-monthly streamflow was computed following 

the same procedure used in the 2009 study and as described 

herein. Daily-mean streamflow data were retrieved from the 

USGS NWIS database (USGS, 2013b) with an automated 

script developed in Python 2.7 (Python Software Foundation, 

2013). Data were retrieved for each of the 432 streamgages 

operated by the USGS from the beginning of the period of 

record through the 2013 calendar year. Daily-mean streamflow 

data for each of the 47 streamgages operated by the CDWR 

were retrieved from the beginning of the period of record 

through the 2013 calendar year from the CDWR Web page 

(Colorado Division of Water Resources, 2013). The CDWR 

streamflow data included 19 streamgages operated solely 

by the CDWR and 28 streamgages that have been operated 
Methods 5

by both the USGS and CDWR. CDWR daily-mean streamflow data for the nonoverlapping period of record for the 

28 streamgages that have been operated jointly by the CDWR 

and USGS (fig. 2 and table 1) were appended to the USGS 

NWIS daily-mean streamflow data.

The statistical software package R (R Core Team, 2013) 

was used to calculate mean-monthly streamflow at each 

streamgage from the daily-mean value dataset for each of 

the 12 months. In addition, the total number of days for each 

month with no data collected at a streamgage, such as at gages 

that were operated seasonally, was summarized. The summaries of the number of days with no data did not include days 

when the streamgage was operating normally, but the streamflow was below the reportable limit of the streamgage computation. From these summaries of the number of days with 

no data, the number of daily-mean values used to compute the 

mean-monthly streamflow at each streamgage for each of the 

12 months was tabulated. Any streamgage with fewer than 

280 daily-mean values for February or fewer than 300 dailymean values for all other months for the computation of meanmonthly streamflow was omitted from analysis for February 

and all other months, similar to the criteria in the 2009 study. 

Some streamgages used in the analysis are operated seasonally 

and computations could not be made for all 12 months.

COLORADO

NEW MEXICO OKLAHOMA

TEXAS

KANSAS

NEBRASKA

WYOMING

UTAH

ARIZONA

0 50 100 MILES

0 50 100 KILOMETERS 

Base modified from ESRI ArcGIS Online, 2013

State Plane, Colorado, Central

North American Datum of 1983

Hydrologic region boundary

Major river

Old streamgage

New streamgage

EXPLANATION

110° 108° 106° 104° 102°

40°

38°

Figure 2. Location of the streamgages used to evaluate the mean-monthly streamflow-regression equations; old streamgages were 

used in the 2009 study, and new streamgages were not used in the 2009 study.
6 Evaluation of Mean-Monthly Streamflow-Regression Equations for Colorado, 2014

Mean-Monthly Streamflow from Basin 

Characteristics and Regression Equations

Environmental Systems Research Institute, Inc. (Esri), 

ArcMap 10.1 was used to determine the basin and climate 

characteristics, which were used in the regression equations to 

determine the mean-monthly streamflow at each streamgage 

(Esri, 2014). For the computation of mean-monthly streamflow in the Mountain, Northwest, and Southwest hydrologic 

regions, the drainage area (in square miles), and the meanannual precipitation (in inches), were determined for the basin 

of each streamgage. In the Rio Grande hydrologic region, the 

mean elevation of the basin (in feet) also was determined for 

the computations.

First, the location coordinates of every streamgage were 

converted into GIS data points. To perform the basin delineation, the locations of some points were moved slightly to lie 

directly on the digital stream network used in the 2009 study. 

For streamgages used in the 2009 study, the drainage area was 

determined using the National Elevation Dataset (Gesch and 

others, 2009), which is the same elevation raster dataset used 

in the 2009 study. Then, using the elevation and precipitation 

raster data that were used in the 2009 study (Parameter-elevation Regressions on Independent Slopes Model Total Precipitation, 1971–2000; Daly and others, 1994), the mean elevation 

of the basin, in feet, and the mean-annual precipitation, in 

inches, were determined for each streamgage.

For streamgages not used in the 2009 study but within 

the area covered by Colorado StreamStats, basin data were 

determined by submitting the point data to the USGS StreamStats Web site (USGS, 2013c). Results were returned in a 

vector GIS dataset. Because various methodologies are supported within the National StreamStats program, the results of 

the basin characterization were checked to ensure the results 

were reasonable by comparing the results from the different 

methodologies and confirming the different methodologies 

provided similar solutions.

For streamgages in adjacent States outside the Colorado 

StreamStats domain, for which there were no StreamStats 

data because those States do not currently have StreamStats, 

basins were generated using elevation and flow-direction raster 

data from the Elevation Derivatives for National Applications (EDNA) program (USGS, 2013d). Points representing 

streamgage locations were assigned to raster cells of maximumflow accumulation (Esri, 2014) before the basins were generated. EDNA data are coarser in spatial resolution than StreamStats data, but this did not affect computation of the specific 

basin characteristics. The basins were converted from a raster 

format into a vector format and submitted to the Geo Data Portal of the USGS Center for Integrated Data Analysis to determine the basin characteristics needed to use the mean-monthly 

streamflow-regression equations (Blodgett, 2013). The mean 

elevation of the basin, in feet, was determined from the Geo 

Data Portal using the National Elevation Dataset Digital Elevation Model Web Coverage Service (Gesch and others, 2009). 

The mean-annual precipitation, in inches, was determined from 

Table 1. Streamgages used in analysis sorted by (1) hydrologic 

region, (2) whether the streamgage was used in the 2009 study, 

and (3) the agency that collected the data.

[USGS, U.S. Geological Survey; CDWR, Colorado Division of Water 

Resources; new streamgage, a streamgage not used by Capesius and Stephens 

(2009); old streamgage, a streamgage used by Capesius and Stephens (2009)]

All 

sites

Mountain 

region

Northwest 

region

Rio 

Grande 

region

Southwest 

region

Both new and old streamgages

USGS data only 385 162 113 31 79

CDWR data only 19 6 1 10 2

Both USGS and 

CDWR data

28 11 1 14 2

Total streamgages 432 179 115 55 83

New streamgages only

USGS data only 131 44 53 8 26

CDWR data only 19 6 1 10 2

Both USGS and 

CDWR data

4 3 0 1 0

Total streamgages 154 53 54 19 28

Old streamgages only

USGS data only 254 118 60 23 53

CDWR data only 0 0 0 0 0

Both USGS and 

CDWR data

24 8 1 13 2

Total streamgages 278 126 61 36 55

Table 2. Comparison of basin and climate characteristics of the 

278 old streamgages and the 154 new streamgages that were 

used for analysis in this report.

[mi2, square miles; in, inches; ft, feet]

Drainage 

area 

(mi2

)

Mean-annual 

precipitation 

(in)

Mean-basin 

elevation 

(ft)

Number of 

daily-mean 

values in a 

month

Mean

Old streamgages 181 28.9 9,710 1,230

New streamgages 185 28.0 9,450 786

Median

Old streamgages 51.7 28.9 9,860 899

New streamgages 71.9 27.6 9,670 565
Graphical Comparison of Observed and Predicted Mean-Monthly Streamflow 7

the Geo Data Portal using Parameter-elevation Regressions on 

Independent Slopes Model monthly Climate Data for the Continental United States (Daly and others, 1994).

The mean-monthly streamflow for all 12 months of the 

year was computed by using the regression equations from the 

2009 study (appendix 2) and the following basin characteristics: drainage area, mean-annual precipitation of the basin, and 

mean elevation of the basin.

Quality Assurance

The streamgaged basins that were initially complied to 

evaluate the mean-monthly streamflow-regression equations 

were analyzed to confirm that their basin characteristics were 

within the constraints outlined in the 2009 study. This eliminated 11 new streamgages from the study resulting in 154 new 

streamgages for the analysis. The regression equations in the 

2009 study were developed for streamgages with drainage 

areas between 1 and 5,250 square miles, mean-annual precipitation between 8 and 51 inches, and mean basin elevations 

between 4,808 and 11,955 feet for the Mountain, Northwest, 

Rio Grande, and Southwest hydrologic regions. A comparison of the drainage area, precipitation, and elevation for the 

278 old streamgages and the 154 new streamgages is shown 

in figures 3–5. The comparison of drainage area, precipitation, 

and elevation for old and new gages provided assurance that 

only streamgages that fit the ranges of basin characteristics 

listed above would be used for this analysis.

Evaluation of Mean-Monthly 

Streamflow-Regression Equations

Evaluation of mean-monthly streamflow-regression 

equations was accomplished through the use of scatter plots of 

observed and predicted data, computation of median absolute percent difference in streamflow between observed and 

predicted streamflow, graphical analysis of monthly bias by 

examination of boxplots of residual streamflow, adjR2 statistics, and SEP statistics (Helsel and Hirsch, 2002).

Graphical Comparison of Observed and 

Predicted Mean-Monthly Streamflow

The observed (streamgage record) mean-monthly streamflows are plotted with the predicted (regression computed) 

mean-monthly streamflows for each of the four hydrologic 

regions for both the old streamgages and the new streamgages 

for all 12 months. These plots facilitated identification of variance and bias for evaluation of the regression equations.

Generally, the mean-monthly streamflow-regression 

equations in the Mountain and the Rio Grande hydrologic 

regions had the least amount of variance over the range of 

streamflows as shown graphically by more narrow clustering of data points along the line of agreement in figures 6 

0

10

20

30

40

50

60

0.10 1 10 100 1,000 10,000

Mean-annual precipitation, in inches

Drainage area, in square miles

Old streamgage

New streamgage

EXPLANATION

Figure 3. Drainage area and mean-annual precipitation for the 278 old streamgages and the 154 new 

streamgages that were used for analysis in this report.
8 Evaluation of Mean-Monthly Streamflow-Regression Equations for Colorado, 2014 Mean-basin elevation, in feet

Old streamgage

New streamgage

EXPLANATION

Drainage area, in square miles

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

0.10 1 10 100 1,000 10,000

Figure 4. Drainage area and mean-basin elevation for the 278 old streamgages and the 154 new 

streamgages that were used for analysis in this report.

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

0 10 20 30 40 50 60

Mean-basin elevation, in feet

Old streamgage

New streamgage

EXPLANATION

Mean-annual precipitation, in inches

Figure 5. Mean-annual precipitation and mean-basin elevation for the 278 old streamgages and the 154 

new streamgages that were used for analysis in this report.
Graphical Analysis of Monthly Bias 9

and 8, respectively. The regression equations for the Mountain hydrologic region have the best agreement between 

observed and predicted values (although still somewhat biased 

toward underprediction at the high end) when the predicted 

streamflows are greater than 100 cubic feet per second (ft3/s), 

evidenced by data that more closely fit the line of agreement 

when compared to the streamflows less than 100 ft3/s (fig. 6). 

At predicted streamflows of less than 3 ft3/s in the Mountain 

hydrologic region, the regression equations tend to overpredict 

mean-monthly streamflow (fig. 6).

The Northwest hydrologic region follows the same pattern as the Mountain hydrologic region showing predicted 

streamflows greater than 200 ft3/s with a closer fit (although 

still biased toward underprediction at the high end) with the 

line of agreement, and at streamflows less than 10 ft3/s in the 

Northwest hydrologic region, the regression equation bias 

tends to overpredict mean-monthly streamflow (fig. 7).

Unlike the other three hydrologic regions, the Rio Grande 

has relatively consistent variance from the line of agreement 

throughout the range of streamflows. However, similar to the 

other three hydrologic regions, predicted streamflows at the 

high end are biased low. At streamflows less than 3 ft3/s, the 

regression equations in the Rio Grande hydrologic region 

show high bias and tend to overpredict mean-monthly streamflow. The bias seems mainly to be a result of extremely low 

streamflow values from the new streamgages added for this 

report (fig. 8).

The Southwest hydrologic region exhibits the lowest 

variance and least bias at predicted streamflows greater than 

100 ft3/s and the greatest variance at predicted streamflows 

less than 20 ft3/s of any of the four hydrologic regions. At predicted streamflows less than 20 ft3

/s, the regression equations 

are imprecise and tend to overpredict mean-monthly streamflow (fig. 9).

Absolute Percent Difference

The absolute percent difference between the predicted 

(regression equations) and observed (streamgage record) 

mean-monthly streamflow, expressed as a percent, was determined as

 (1)

where

dr is absolute difference, in percent,

Qpredicted is mean-monthly streamflow from the 

regression equation, in cubic feet per 

second, and

Qobserved is mean-monthly streamflow from the 

streamgage record, in cubic feet per 

second.

The absolute percent difference between the observed 

and predicted streamflows for each of the 48 mean-monthly 

streamflow-regression equations is listed in table 3 and shown 

on maps in appendix 3 (figs. 3–1 through 3–12). This statistic 

provides a metric for assessing performance of the regression 

equation based on all currently (2013) available streamgage data.

The median absolute differences between the observed 

and predicted streamflows computed for Mountain, Northwest, 

and Southwest hydrologic regions have fairly uniform values 

throughout the year (table 3), with the exception of late summer and early fall (July, August, and September), when each 

hydrologic region exhibits a substantial increase in median 

absolute percent difference. The greatest difference occurs in 

the Northwest hydrologic region, and the smallest difference 

occurs in the Mountain hydrologic region (table 3). The Rio 

Grande hydrologic region shows seasonal variation in median 

absolute percent difference with March, April, August, and 

September having a median absolute difference near or below 

40 percent and the remaining months of the year having a 

median absolute difference near or above 50 percent. In the 

Mountain, Northwest, and Southwest hydrologic regions, the 

mean-monthly streamflow equations perform the best during 

spring (March, April, and May). However, in the Rio Grande 

hydrologic region, the mean-monthly streamflow equations 

perform the best during late summer and early fall (August 

and September).

The 30 mean-monthly streamflow equations identified as 

having “no bias” in the 2009 study (appendix 2), seem to have 

less bias than the remaining 18 mean-monthly streamflow 

equations when comparing the median absolute percent differences. The 30 equations identified as “no bias” have a median 

absolute difference of 40 percent, on average; whereas, the 

18 equations identified in the 2009 study as having a bias have 

a median absolute difference of 54 percent, on average.

Graphical Analysis of Monthly Bias

The residual streamflow (in cubic feet per second) is the 

difference between the observed (from streamgage record) and 

predicted (from regression equations) mean-monthly streamflow. Residual streamflows were determined for each of the 

48 mean-monthly streamflow-regression equations and are 

shown on boxplots, by hydrologic region, in figures 10–13, 

using all available data. These figures exhibit a tendency for 

over- or underprediction bias for the 48 equations when compared to the observed data by illustrating measures of central 

tendency (median), interquartile range (central tendency and 

symmetry of the middle 50 percent of the data), and range of 

extremes of the data (5th and 95th percentiles).

In the Mountain hydrologic region, boxplots of residuals 

indicate slight overprediction bias in the months of May, June, 

and July based on median differences (fig. 10). The residual plots 

for the September through March equations seem to indicate 

some positive bias in the upper quartile (50th to 75th percentile 

range). The April plot indicates some negative bias (underprediction in the lower quartile (25th to 50th percentile range).

d

Q Q

Q r

predicted observed

observed

=    −  *100
Old streamgageNew streamgage

EXPLANATION

Mountain hydrologic region

Overpredicting

Predicted mean-monthly streamflow, in cubic feet per second

Observed mean-monthly streamflow, in cubic feet per second

0.01

0.1

1

10

100

1,000

10,000

0.01 0.1 1 10 100 1,000 

Figure 6. Comparison of observed and predicted mean-monthly streamflow in the Mountain hydrologic region for all 12 months.
Old streamgage New streamgage EXPLANATION

Northwest hydrologic region

Overpredicting

Predicted mean-monthly streamflow, in cubic feet per second

Observed mean-monthly streamflow, in cubic feet per second

0.01

0.1

1

10

100

1,000

10,000

0.01 0.1 1 10 100 1,000 

Figure 7. Comparison of observed and predicted mean-monthly streamflow in the Northwest hydrologic region for all 12 months.
Old streamgageNew streamgage

EXPLANATION

Rio Grande hydrologic region

Overpredicting

Predicted mean-monthly streamflow, in cubic feet per second

Observed mean-monthly streamflow, in cubic feet per second

0.01

0.1

1

10

100

1,000

10,000

0.01 0.1 1 10 100 1,000 

Figure 8. Comparison of observed and predicted mean-monthly streamflow in the Rio Grande hydrologic region for all 12 months. 
Figure 9. Comparison of observed and predicted mean-monthly streamflow in the Southwest hydrologic region for all 12 months. Old streamgage New streamgage EXPLANATION

Southwest hydrologic region

Overpredicting

Predicted mean-monthly streamflow, in cubic feet per second

Observed mean-monthly streamflow, in cubic feet per second

0.01

0.1

1

10

100

1,000

10,000

0.01 0.1 1 10 100 1,000 
14 Evaluation of Mean-Monthly Streamflow-Regression Equations for Colorado, 2014

Table 3. The median absolute percent difference between the observed and predicted mean-monthly 

streamflow for each of the 48 mean-monthly streamflow-regression equations.

[Jan., January; Feb., February; Mar., March; Apr., April; Aug., August; Sep., September; Oct., October; Nov., November; Dec., 

December]

Hydrologic region

Median absolute difference, in percent

Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec.

Mountain hydrologic 

region 

35 35 28 28 28 30 42 40 40 30 31 31

Northwest hydrologic 

region

44 51 35 45 39 44 72 110 109 62 47 45

Rio Grande hydrologic 

region

49 49 43 41 53 52 48 34 37 45 49 46

Southwest hydrologic 

region

48 44 34 34 27 45 52 52 62 35 43 44

−300

−250

−200

−150

−100

−50

0

50

100

150

200

250

Mountain hydrologic region

Residual streamflow, in cubic feet per second

Month

January February March April May June July August September October November December

EXPLANATION

95th percentile

75th percentile

50th percentile (median)

25th percentile

5th percentile

Mean

Figure 10. Residual streamflow for each month for the Mountain hydrologic region.
Graphical Analysis of Monthly Bias 15

Residual streamflow, in cubic feet per second

Month

January February March April May June July August September October November December

EXPLANATION

95th percentile

75th percentile

50th percentile (median)

25th percentile

5th percentile

Mean

−700

−600

−500

−400

−300

−200

−100

0

100

200

300

400

Northwest hydrologic region

Figure 11. Residual streamflow for each month for the Northwest hydrologic region. Residual streamflow, in cubic feet per second

Month

January February March April May June July August September October November December

EXPLANATION

95th percentile

75th percentile

50th percentile (median)

25th percentile

5th percentile

Mean

Rio Grande hydrologic region

−570

−470

−370

−270

−170

−70

30

130

230

100

200

0

−100

−200

−300

−400

−500

Figure 12. Residual streamflow for each month for the Rio Grande hydrologic region.
16 Evaluation of Mean-Monthly Streamflow-Regression Equations for Colorado, 2014

In the Northwest hydrologic region, the residual plots 

(fig. 11) for April and June indicate the equation underpredicts 

the lower quartile. Equations for July, August, and September 

overpredict mean-monthly streamflow; whereas, the October 

through March residual plots indicate some positive bias in the 

upper quartile (50th to 75th percentile range).

In the Rio Grande hydrologic region, residual plots 

(fig. 12) seem to indicate small bias of the predicted values, 

with the exception of May and June, which tend to overpredict 

mean-monthly streamflow. Small under- and overpredictions 

(interquartile range bias) are evident in the July through April 

months.

In the Southwest hydrologic region, the residual plots 

for March, April, May, and September seem to indicate some 

underprediction of streamflows; whereas, the equations for 

June and July tend to overpredict streamflow. Residuals for the 

remaining months seem to indicate little substantial bias.

Adjusted Coefficient of Determination

The adjusted coefficient of determination is indicative of 

goodness-of-fit (accuracy) for the data in the regression equation (higher values usually indicate better fit) (Eng and others, 

2009). The adjusted coefficient of determination (adjR2) 

compensates for the number of independent variables used in 

the regression. The adjR2 values for the mean-monthly streamflow for each of the 48 mean-monthly streamflow-regression 

equations are shown in table 4 (dataset current through the 

2013 calendar year); adjR2

 values were computed for each 

equation using all available streamgage data. The adjR2 values 

were determined as follows (Eng and others, 2009):

 (2)

where

SSr is residual sum of squares,

n is total number of samples,

k is number of independent variables, and

SST is total sum of squares.

The residual sum of squares is determined using a logarithmic transformation (base 10) (Eng and others, 2009):

 (3)

where

ei is the residual errors,

The total sum of squares is determined using a logarithmic transformation (base 10) (Eng and others, 2009):

 (4)

Residual streamflow, in cubic feet per second

Month

January February March April May June July August September October November December

EXPLANATION

95th percentile

75th percentile

50th percentile (median)

25th percentile

5th percentile

Mean

Southwest hydrologic region

−370

−270

−170

−70

30

130

230

330

430

530

−300

−200

−100

0

100

200

300

400

500

630

600

Figure 13. Residual streamflow for each month for the Southwest hydrologic region.

adjR

SS

n k

SS

n

r

T

2 1

1

1

= − ( ) − −

( ) −

 

SS

n

e

n r i log Qpredicted o log Q bs    = =        ( )− 1 1 2 Σ Σ ( ) erved





 





2

SST p = =  S l    

 og ( ) Q l redicted   − og ( ) Q 

 

 Σ Σ 

2
Standard Error of Prediction 17

where

S are squares that are equal to the sum of the 

amount of variability in the observations, 

and

Q is average of the mean-monthly streamflow 

from the regression equation, in cubic feet 

per second.

The monthly adjR2 values were computed and have the 

same general pattern for all four hydrologic regions (table 4). 

The largest values usually occur in March or April, and the 

lowest values usually occur in August or September. Only the 

Rio Grande hydrologic region deviates from this seasonal pattern, exhibiting a decrease in adjR2

 values in August and September, with the lowest values occurring in the winter months 

(December, January, and February). Generally, the adjR2

values for this report are just slightly less (0.76 compared to 

0.79) than the values computed in the 2009 study. The similarity of values, even when tested with data not used to originally 

develop the mean-monthly streamflow-regression equations, 

provides confidence that the predictive uncertainty of meanmonthly regression equations in the 2009 study are accurate.

Standard Error of Prediction

The SEP, in percent, in the 2009 study was used as a 

measure of the precision of values predicted from the regression equation. Standard error of prediction as a percentage 

for each hydrologic region and each month at all streamgages 

from the 2009 study is shown in table 5. The average of the 

12 monthly SEPs for the Mountain hydrologic region was 

smallest at 53 percent, followed in increasing order by Rio 

Grande (64 percent), Northwest (83 percent), and Southwest 

(91 percent). Generally, many of the largest mean SEPs among 

all hydrologic regions tended to be associated with the April 

through October open-water season, but there is variation 

among the hydrologic regions. In the Mountain hydrologic

region, the largest SEPs were in July and August (76 and 

80 percent, respectively). In the Northwest hydrologic region, 

the largest SEPs were in August, September, and October (90, 

104, and 94 percent, respectively). In the Rio Grande hydrologic region, the largest SEPs were in May and June (84 percent for both months), and in the Southwest hydrologic region, 

Table 4. The adjusted coefficient of determination for the 48 mean-monthly streamflow-regression 

equations.

[Jan., January; Feb., February; Mar., March; Apr., April; Aug., August; Sep., September; Oct., October; Nov., November; Dec., December]

Hydrologic region

Adjusted coefficient of determination (adjR2

)

Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec.

Mountain hydrologic 

region 

0.81 0.82 0.81 0.89 0.86 0.80 0.68 0.67 0.72 0.82 0.84 0.83

Northwest hydrologic 

region

0.69 0.83 0.86 0.76 0.78 0.84 0.77 0.65 0.67 0.75 0.74 0.78

Rio Grande hydrologic 

region

0.72 0.76 0.85 0.86 0.82 0.83 0.80 0.76 0.76 0.81 0.81 0.74

Southwest hydrologic 

region

0.64 0.70 0.76 0.78 0.77 0.77 0.66 0.61 0.44 0.70 0.70 0.69

Table 5. Mean standard error of prediction for mean-monthly streamflow from the 2009 study.

[Ann., annual; Jan., January; Feb., February; Mar., March; Apr., April; Aug., August; Sep., September; Oct., October; Nov., 

November; Dec., December]

Ann. Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec.

Mountain hydrologic 

region

33 50 51 49 44 46 46 76 80 59 45 46 47

Northwest hydrologic 

region

55 85 77 68 84 71 80 75 90 104 94 83 79

Rio Grande hydrologic 

region

73 64 56 49 79 84 84 74 53 57 55 53 63

Southwest hydrologic 

region

60 77 58 47 50 62 121 180 119 120 106 80 75
18 Evaluation of Mean-Monthly Streamflow-Regression Equations for Colorado, 2014

the largest SEPs were in June, July, August, September, and 

October (121, 180, 119, 120, and 106 percent, respectively). 

Generally, SEPs tended to be smallest during November 

through March (table 5).

In this study, the data were split into subsets based 

on hydrologic region, period of record, and old and new 

streamgages (table 6). To evaluate the performance of the 

48 mean-monthly streamflow equations, the upper and lower 

prediction intervals (calculated for the 95-percent confidence 

level) were used to determine the number of data points from 

observed streamgage data inside those limits. The SEP was 

converted from percent to logarithmic (base 10) units by 

(Tasker, 1978):

 (5)

where

SEPpercent is standard error of prediction, in percent.

Then, the upper and lower prediction intervals (PI) were 

determined as follows:

 (6)

Each streamgage for every month was categorized as 

outside or within the 95-percent prediction intervals, established by the SEP values in the 2009 study. For all hydrologic 

regions, approximately 87 percent of the data are within the 

95-percent prediction intervals (average of the 12 months in 

table 6). The explanation for why fewer than 95 percent of 

the data are within the prediction intervals is that the data do 

not conform perfectly to the regression assumptions required 

to accurately estimate performance metrics. For example, 

if the regression residuals are not normally distributed and 

homoscedastic, then the predictions intervals are inexact. 

In addition, the regression equations were developed using 

weighted least squares giving greater weight to streamgages 

with longer periods of record. As a result, the regression 

line is pulled slightly toward the longer record stations. In 

contrast, the prediction intervals are evaluated without regard 

to record length, and this difference may somewhat confound 

the analysis.

The equations for the Rio Grande hydrologic region had 

the best fit with the parametric (Helsel and Hirsch, 2002) prediction-interval assumptions, with approximately 91.8 percent 

of the data within the prediction interval (mean for 12 months, 

table 6). The Mountain, Northwest, and Southwest hydrologic 

regions had 87.8, 84.9, and 83.5 percent contained within the 

prediction interval, respectively (mean for 12 months, table 6). 

The performance of the equations did not change when 

analyzing different the periods of record of the streamgage. 

Streamflow data from 1971 through 2000 were used to analyze 

the equations because the precipitation data from Daly and 

others (1994) that were used to generate the regression equations in the 2009 study only included data from 1971 through 

2000. When compared to the dataset for the entire period of 

record, the results from the 1971 through 2000 dataset (average of the mean in all four hydrologic regions) were equivalent with both datasets having 87 percent of the data within the 

prediction interval. The old streamgages performed slightly 

better than the new streamgages, with approximately 88 and 

85 percent of the data within the prediction intervals, respectively. This result was expected as the streamgages used to 

develop the regression equations should yield a better performance than the new streamgages. The fact that the results for 

the two datasets are very similar provides assurance that when 

these equations are applied to locations not used to develop the 

equations, the SEP and adjR2

 error metrics should be similar to 

those established in the 2009 study for locations with natural 

streamflow. The 30 mean-monthly streamflow equations identified as having no bias in the 2009 study (appendix 2) did not 

have substantially lower SEPs than the remaining 18 meanmonthly streamflow equations. The 30 equations identified 

in the 2009 study as having no bias were determined to have 

86 percent of their data within the prediction intervals, and the 

18 equations identified in the 2009 study as having a bias were 

determined to have 83 percent of their data within the prediction intervals. In April 2014, a miscalculation of the SEP in 

Capesius and Stephens (2009) was uncovered. As a result, in 

April 2014, the SEP was updated. The updated SEP and adjR2

values that correspond to the mean-monthly streamflow equations developed in a study by Capesius and Stephens in 2009 

are in close agreement with the results of this study. Based on 

the results presented in this report, the updated standard error 

of prediction and adjusted coefficient of determination values 

for the mean-monthly streamflow equations developed in the 

2009 study are consistent with the findings of this study.

percent SEP

Ln

2

2

100 1

10

ln ( ) / +

( )

SEPlog10 units =

[ ]

PI log Q upper l, 1 ower in log u0 nits predicted log 2 ( ) = ( ) ± EP 10 units
Table 6. Percentages of data from streamgages that are within the 95-percent prediction interval, based on the standard error of predic2009 study. [Jan., January; Feb., February; Mar., March; Apr., April; Aug., August; Sep., September; Oct., October; Nov., November; Dec., December; POR, period of record

Percentage of data within prediction interval

Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. D

Mountain hydrologic region, all streamgages, all POR 86.1 87.3 94.1 89.7 92.0 86.4 88.3 87.2 85.5 84.7 88.2 

Mountain hydrologic region, old streamgages, all POR 88.6 89.4 95.1 91.3 92.1 87.3 88.9 88.1 85.7 86.5 90.3 

Mountain hydrologic region, new streamgages, all POR 79.1 81.4 91.3 85.7 92.0 84.3 86.8 84.9 84.9 80.0 82.6 

Northwest hydrologic region, all streamgages, all POR 88.8 89.9 88.2 88.4 89.5 89.5 72.2 72.2 75.7 88.3 88.1 

Northwest hydrologic region, old streamgages, all POR 85.0 86.7 86.7 88.1 86.9 88.5 83.6 82.0 78.7 90.0 86.7 

Northwest hydrologic region, new streamgages, all POR 93.6 93.9 90.0 88.7 92.5 90.6 59.3 61.1 72.2 86.3 89.8 

Rio Grande hydrologic region, all streamgages, all POR 83.7 87.8 89.8 98.1 96.3 96.4 96.4 94.5 90.9 94.3 87.8 

Rio Grande hydrologic region, old streamgages, all POR 84.4 90.6 93.8 97.1 94.4 94.4 94.4 91.7 86.1 91.4 90.6 

Rio Grande hydrologic region, new streamgages, all POR 82.4 82.4 82.4 100 100 100 100 100 100 100 82.4 

Southwest hydrologic region, all streamgages, all POR 81.5 76.5 79.0 78.0 94.0 90.4 90.4 85.5 73.5 86.4 85.0 

Southwest hydrologic region, old streamgages, all POR 83.3 79.6 81.5 75.9 94.5 92.7 89.1 87.3 81.8 86.8 86.8 

Southwest hydrologic region, new streamgages, all POR 77.8 70.4 74.1 82.1 92.9 85.7 92.9 82.1 57.1 85.7 81.5 

All hydrologic regions, all streamgages, all POR 85.6 85.9 89.0 88.1 92.3 89.3 85.4 83.8 81.3 87.2 87.5 

Mountain hydrologic region, all streamgages, 1971–2000 87.1 88.2 91.5 87.2 88.3 91.6 92.6 88.4 86.3 84.1 89.5 

Northwest hydrologic region, all streamgages, 1971–2000 88.3 88.7 87.1 87.3 89.1 89.1 73.8 72.3 73.8 85.5 90.0 

Rio Grande hydrologic region, all streamgages, 1971–2000 86.7 96.7 90.3 96.9 94.1 94.1 94.1 91.2 88.2 89.7 89.7 

Southwest hydrologic region, all streamgages, 1971–2000 84.4 75.6 68.9 84.4 95.6 93.3 95.6 80.0 64.4 81.8 84.1 
20 Evaluation of Mean-Monthly Streamflow-Regression Equations for Colorado, 2014

Summary

The U.S. Geological Survey, in cooperation with the 

Colorado Water Conservation Board, evaluated the predictive 

uncertainty of mean-monthly streamflow-regression equations 

representative of natural streamflow conditions in Colorado. 

The purpose of this report is to evaluate the streamflow-regression equations presented in a 2009 U.S. Geological Survey 

study by comparing the predictive uncertainty using streamflow data through calendar year 2013 for computation of 

mean-monthly streamflow for Colorado basins with hydrology 

that is influenced predominantly by natural runoff processes. 

The study area for this report is limited to the Mountain, 

Northwest, Rio Grande, and Southwest hydrologic regions of 

Colorado.

Data collected from the beginning of the period of record 

through calendar year 2013 were used to evaluate the meanmonthly streamflow equations using the same basin characteristics as in the 2009 study. U.S. Geological Survey and Colorado Division of Water Resources streamgages with at least 

10 years of streamflow record and identified as representative 

of natural streamflow conditions were selected. During the 

streamgage selection process, a total of 432 streamgages, composed of 278 from the 2009 study and 154 new streamgages,

were identified.

Generally, the mean-monthly streamflow-regression 

equations in the Mountain and the Rio Grande hydrologic 

regions had the least amount of variance over the range of 

streamflows as shown graphically by more narrow clustering of data points. The regression equations for the Mountain 

hydrologic region have the best agreement between observed 

and predicted values (although still somewhat biased toward 

underprediction at the high end) when the predicted streamflows are greater than 100 cubic feet per second (ft3

/s), 

evidenced by data that more closely fit the line of agreement when compared to the streamflows less than 100 ft3

/s. 

At predicted streamflows of less than 3 ft3/s in the Mountain 

hydrologic region, the regression equations tend to overpredict 

mean-monthly streamflow.

The Northwest hydrologic region follows the same pattern as the Mountain hydrologic region showing predicted 

streamflows greater than 200 ft3

/s with a better fit (although 

still biased toward underprediction at the high end) with the 

line of agreement, and at streamflows less than 10 ft3/s in the 

Northwest hydrologic region, the regression equation bias 

tends to overpredict mean-monthly streamflow.

Unlike the other three hydrologic regions, the Rio Grande 

has relatively consistent variance from the line of agreement 

throughout the range of streamflows. However, similar to the 

other three hydrologic regions, predicted streamflows at the 

high end are biased low. At streamflows less than 3 ft3/s, the 

regression equations in the Rio Grande hydrologic region 

show high bias and tend to overpredict mean-monthly streamflow. The bias seems mainly to be a result of extremely low 

streamflow values from the new streamgages added for this 

report.

The Southwest hydrologic region exhibits the lowest 

variance and least bias at predicted streamflows greater than 

100 ft3

/s and the greatest variance at predicted streamflows 

less than 20 ft3/s of any of the four hydrologic regions. At 

predicted streamflows less than 20 ft3/s, the regression equations are imprecise and tend to overpredict mean-monthly 

streamflow.

The median absolute differences between the observed 

and predicted streamflow computed for Mountain, Northwest, 

and Southwest hydrologic regions have fairly uniform values 

throughout the year, with the exception of late summer and 

early fall (July, August, and September) when each hydrologic 

region exhibits a substantial increase in median absolute percent difference. The greatest difference occurs in the Northwest hydrologic region, and the smallest difference occurs in 

the Mountain hydrologic region. The Rio Grande hydrologic 

region shows seasonal variation in median absolute percent 

difference with March, April, August, and September having 

a median absolute difference near or below 40 percent and the 

remaining months of the year having a median absolute difference near or above 50 percent. In the Mountain, Northwest, 

and Southwest hydrologic regions, the mean-monthly streamflow equations perform the best during spring (March, April, 

and May). However, in the Rio Grande hydrologic region, the 

mean-monthly streamflow equations perform the best during 

late summer and early fall (August and September).

In the Mountain hydrologic region, boxplots of residuals 

indicate slight overprediction bias in the months of May, June, 

and July based on median differences. The residual plots for 

the September through March equations seem to indicate some 

positive bias in the upper quartile (50th to 75th percentile 

range). The April plot indicates some negative bias (underprediction) in the lower quartile (25th to 50th percentile range).

In the Northwest hydrologic region, the residual plots for 

April and June indicate the equation underpredicts the lower 

quartile. Equations for July, August, and September overpredict mean-monthly streamflow; whereas, the October through 

March residual plots indicate some positive bias in the upper 

quartile (50th to 75th percentile range).

In the Rio Grande hydrologic region, residual plots seem 

to indicate small bias of the predicted values, with the exception of May and June, which tend to overpredict mean-monthly 

streamflow. Small under- and overpredictions (interquartile 

range bias) are evident in the July through April months.

In the Southwest hydrologic region, the residual plots 

for March, April, May, and September seem to indicate some 

underprediction of streamflows; whereas, the equations for 

June and July tend to overpredict streamflow. Residuals for the 

remaining months seem to indicate little substantial bias.

The adjusted coefficient of determination (adjR2

) is 

indicative of goodness-of-fit (accuracy) for the data in the 

regression equation (higher values usually indicate better fit). 

The monthly adjR2 values were computed and have the same 

general pattern for all four hydrologic regions. The largest 

values usually occur in March or April, and the lowest values 

usually occur in August or September. Only the Rio Grande 
References Cited 21

hydrologic region deviates from this seasonal pattern, exhibiting a decrease in adjR2

 values in August and September, with 

the lowest values occurring in the winter months (December, 

January, and February). Generally, the adjR2 values for this 

report are just slightly less (0.76 compared to 0.79) than the 

values computed in the 2009 study. The similarity of values, 

even when tested with data not used to originally develop 

the mean-monthly streamflow-regression equations, provides 

confidence that the predictive uncertainty of mean-monthly 

regression equations in the 2009 study are accurate.

Each streamgage for every month was categorized as 

outside or within the 95-percent prediction intervals established by the SEP values in the 2009 study. For all hydrologic 

regions, approximately 87 percent of the data are within the 

prediction intervals. The explanation for why fewer than 95 

percent of the data are within the prediction intervals is that 

the data do not conform perfectly to the regression assumptions required to accurately estimate performance metrics. 

For example, if the regression residuals are not normally 

distributed and homoscedastic, then the predictions intervals 

are inexact. In addition, the regression equations were developed using weighted least squares giving greater weight to 

streamgages with longer periods of record. As a result, the 

regression line is pulled slightly toward the longer record stations. In contrast, the prediction intervals are evaluated without regard to record length, and this difference may somewhat 

confound the analysis.

The Rio Grande hydrologic region had the best fit with 

the parametric prediction-interval assumptions, with approximately 91.8 percent of the data within the prediction. The 

Mountain, Northwest, and Southwest hydrologic regions 

had 87.8, 84.9, and 83.5 percent of the data contained within 

the prediction interval, respectively. The performance of the 

equations did not change when analyzing different periods of 

record of the streamgage. Streamflow data from 1971 through 

2000 were used to analyze the equations because the precipitation data used to generate the regression equations in the 2009 

study only included data from 1971 through 2000. When compared to the dataset for the entire period of record, the results 

from the 1971 through 2000 dataset (average of the mean in 

all four hydrologic regions) were equivalent with both datasets 

having 87 percent of the data within the prediction interval. 

The old streamgages performed slightly better than the new 

streamgages, with approximately 88 and 85 percent of the data 

within the prediction intervals, respectively. This result was 

expected because the streamgages used to develop the regression equations should yield a better performance than the new 

streamgages. The fact that the results for the two datasets are 

very similar provides assurance that when these equations are 

applied to locations not used to develop the equations, the SEP 

and adjR2

 error metrics should be similar to those established 

in the 2009 study for locations with natural streamflow. Based 

on the results presented in this report, the updated standard 

error of prediction and adjusted coefficient of determination 

values of the mean-monthly streamflow equations developed 

in the 2009 study are consistent with the findings of this study.

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72 p.
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United States—Part 6B, Missouri River Basin below Sioux 

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Appendixes 1–3
Table 1–1. Summary of the streamgages used in the analysis. [USGS, U.S. Geological Survey; CDWR, Colorado Division of Water Resources; A, drainage area; mi2, square miles; P, mean-annual precipitation; in, inches; E, mean basin ele

Colorado; Wyo., Wyoming; N. Mex., New Mexico; Ariz., Arizona] USGS streamgage number Streamgage name Latitude (decimal degrees) Longitude (decimal degrees) Hydrologic region USGS data CDWR data A (mi2) 

06614800 Michigan River near Cameron Pass, Colo. 40.496094 –105.865012 Mountain X -- 1.53 

06615500 Michigan River near Lindland, Colo. 40.553592 –106.041685 Mountain X -- 60.2 

06616000 North Fork Michigan River near Gould, Colo. 40.549426 –106.021129 Mountain X -- 20.5 

06617100 Michigan River at Walden, Colo. 40.741088 –106.279470 Mountain X X 182 

06617500 Illinois Creek near Rand, Colo. 40.462478 –106.176972 Mountain -- X 70.5 

06618500 Illinois Creek at Walden, Colo. 40.726366 –106.290582 Mountain X -- 255 

06619000 Michigan River near Cowdrey, Colo. 40.861641 –106.337249 Mountain X -- 476 

06619500 Canadian River at Cowdrey, Colo. 40.863030 –106.311415 Mountain X -- 180 

06620000 North Platte River near Northgate, Colo. 40.936639 –106.339194 Mountain X -- 1,430 

06620400 Douglas Creek above Keystone, Wyo. 41.183306 –106.270018 Mountain X -- 22.3 

06621000 Douglas Creek near Foxpark, Wyo. 41.081084 –106.307522 Mountain X -- 117 

06622500 French Creek near French, Wyo. 41.213889 –106.511944 Mountain X -- 59.9 

06622700 North Brush Creek near Saratoga, Wyo. 41.370245 –106.520582 Mountain X -- 38.1 

06623800 Encampment River above Hog Park Creek, near Encampment, Wyo. 41.023579 –106.824766 Mountain X -- 72.5 

06624500 Encampment River at Encampment, Wyo. 41.210521 –106.778928 Mountain X -- 208 

06627500 Jack Creek at Matheson Ranch, near Saratoga, Wyo. 41.401111 –107.016389 Northwest X -- 33.2 

06631000 Medicine Bow River near Medicine Bow, Wyo. 41.724722 –106.318611 Northwest X -- 193 

06632400 Rock Creek Above King Canyon Canal, near Arlington, Wyo. 41.585245 –106.222791 Mountain X -- 62.8 

06696980 Tarryall Creek at Upper Station near Como, Colo. 39.339433 –105.911681 Mountain X -- 23.7 

06698500 Tarryall Creek near Jefferson, Colo. 39.294989 –105.718620 Mountain X -- 181 

06705500 Geneva Creek at Grant, Colo. 39.472210 –105.682228 Mountain X -- 74.7 

06706000 North Fork South Platte River below Geneva Creek, at Grant, Colo. 39.457210 –105.658616 Mountain X -- 127 

06714800 Leavenworth Creek at Mouth near Georgetown, Colo. 39.687210 –105.700282 Mountain X -- 12.0 

06715000 Clear Creek above West Fork Clear Creek near Empire, Colo. 39.751932 –105.661947 Mountain X -- 86.1 

06716100 West Fork Clear Creek above Mouth near Empire, Colo. 39.758876 –105.660003 Mountain X -- 57.4 

06716500 Clear Creek near Lawson, Colo. 39.765821 –105.626112 Mountain X -- 147 

06717400 Chicago Creek below Devils Canyon near Idaho Springs, Colo. 39.716377 –105.571388 Mountain X -- 43.7 

06721500 North Saint Vrain Creek near Allens Park, Colo. 40.218874 –105.528333 Mountain X -- 32.5 

06722500 South Saint Vrain Creek near Ward, Colo. 40.090819 –105.514443 Mountain X X 13.4 

06725500 Middle Boulder Creek at Nederland, Colo. 39.961654 –105.504442 Mountain X X 36.5 

06729000 South Boulder Creek near Rollinsville, Colo. 39.914709 –105.501942 Mountain X -- 43.6 
USGS Latitude Longitude Hydrologic USGS CDWR A 

streamgage Streamgage name (decimal (decimal region data data (mi2) 

number degrees) degrees)

06732000 Glacier Creek near Estes Park, Colo. 40.345262 –105.585557 Mountain X -- 24.6 06733000 Big Thompson River at Estes Park, Colo. 40.378317 –105.513887 Mountain X X 137 

06734500 Fish Creek near Estes Park, Colo. 40.368595 –105.493886 Mountain X X 15.8 06746095 Joe Wright Creek above Joe Wright Reservoir, Colo. 40.539982 –105.882790 Mountain X -- 3.33 06748200 Fall Creek near Rustic, Colo. 40.551649 –105.626946 Mountain X -- 3.60 06748510 Little Beaver Creek near Idylwilde, Colo. 40.638593 –105.661668 Mountain X -- 0.89 06748530 Little Beaver Creek near Rustic, Colo. 40.623037 –105.564998 Mountain X -- 12.4 06748600 South Fork Cache La Poudre River near Rustic, Colo. 40.646926 –105.493605 Mountain X -- 92.6 07079500 East Fork Arkansas River near Leadville, Colo. 39.259711 –106.340581 Mountain X -- 51.2 07081000 Tennessee Creek near Leadville, Colo. 39.264156 –106.340859 Mountain X -- 45.7 07082000 Lake Fork above Sugar Loaf Reservoir, Colo. 39.269711 –106.395027 Mountain X -- 24.3 07083000 Halfmoon Creek near Malta, Colo. 39.172213 –106.389193 Mountain X -- 23.5 07086500 Clear Creek above Clear Creek Reservoir, Colo. 39.018049 –106.277800 Mountain X X 67.0 07091015 Chalk Creek at Nathrop, Colo. 38.733607 –106.160017 Mountain -- X 83.0 08216500 Willow Creek at Creede, Colo. 37.856110 –106.927546 Rio Grande X -- 34.3 08218000 Goose Creek near Wagonwheel Gap, Colo. 37.687889 –106.844722 Rio Grande X -- 53.6 08218500 Goose Creek at Wagonwheel Gap, Colo. 37.751947 –106.830044 Rio Grande X X 91.1 08219500 South Fork Rio Grande River at South Fork, Colo. 37.656949 –106.649208 Rio Grande X X 211 

08220500 Pinos Creek near Del Norte, Colo. 37.591671 –106.450038 Rio Grande X X 69.2 08223500 Rock Creek near Monte Vista, Colo. 37.490282 –106.259477 Rio Grande X -- 32.9 08224500 Kerber Creek above Little Kerber Creek near Villa Grove, Colo. 38.220277 –106.089741 Rio Grande X X 45.4 08226700 Cotton Creek near Mineral Hot Springs, Colo. 38.131941 –105.788620 Rio Grande -- X 13.7 08227000 Saguache Creek near Saguache, Colo. 38.163333 –106.290583 Rio Grande X X 517 08227500 North Crestone Creek near Crestone, Colo. 38.013610 –105.692788 Rio Grande X X 12.9 08229500 Cottonwood Creek near Crestone, Colo. 37.933300 –105.645560 Rio Grande -- X 6.74 08230500 Carnero Creek near La Garita, Colo. 37.859723 –106.319477 Rio Grande X X 106 08231000 La Garita Creek near La Garita, Colo. 37.813335 –106.318644 Rio Grande X X 62.2 08235250 Alamosa River above Wightman Fork near Jasper, Colo. 37.402505 –106.521982 Rio Grande X X 37.8 08236000 Alamosa River above Terrace Reservoir, Colo. 37.374728 –106.334756 Rio Grande X X 106 08240500 Trinchera Creek above Turners Ranch, near Fort Garland, Colo. 37.374733 –105.295010 Rio Grande X X 52.6 08241500 Sangre De Cristo Creek near Fort Garland, Colo. 37.425009 –105.415011 Rio Grande X X 183 

Table 1–1. Summary of the streamgages used in the analysis.—Continued[USGS, U.S. Geological Survey; CDWR, Colorado Division of Water Resources; A, drainage area; mi2, square miles; P, mean-annual precipitation; in, inches; E, mean basin ele

Colorado; Wyo., Wyoming; N. Mex., New Mexico; Ariz., Arizona]
USGS Latitude Longitude Hydrologic USGS CDWR A 

streamgage Streamgage name (decimal (decimal region data data (mi2) 

number degrees) degrees) 08242500 Ute Creek near Fort Garland, Colo. 37.447222 –105.425833 Rio Grande X X 40.3 

08245500 Conejos River at Platoro, Colo. 37.353895 –106.525037 Rio Grande X -- 41.0 08246500 Conejos River near Mogote, Colo. 37.053900 –106.187527 Rio Grande X -- 281 08247500 San Antonio River at Ortiz, Colo. 36.993069 –106.038633 Rio Grande X -- 116 08248000 Los Pinos River near Ortiz, Colo. 36.982236 –106.073633 Rio Grande X -- 153 08248500 San Antonio River at Mouth, near Manassa, Colo. 37.176956 –105.878076 Rio Grande X X 372 08249000 Conejos River near Lasauses, Colo. 37.300288 –105.746963 Rio Grande X -- 766 08252500 Costilla Creek above Costilla Dam, N. Mex. 36.898361 –105.254667 Rio Grande X -- 25.2 08253000 Casias Creek near Costilla, N. Mex. 36.896856 –105.260458 Rio Grande X -- 18.8 08253500 Santistevan Creek near Costilla, N. Mex. 36.884167 –105.281111 Rio Grande X -- 2.47 08255000 Ute Creek near Amalia, N. Mex. 36.952802 –105.410285 Rio Grande X -- 11.2 08263000 Latir Creek near Cerro, N. Mex. 36.829192 –105.547785 Rio Grande X -- 10.6 08264000 Red River near Red River, N. Mex. 36.622252 –105.389451 Rio Grande X -- 19.3 08264500 Red River below Zwergle Damsite near Red River, N. Mex. 36.673641 –105.381116 Rio Grande X -- 29.1 08265000 Red River near Questa, N. Mex. 36.703311 –105.568431 Rio Grande X -- 112 08266820 Red River below Fish Hatchery, near Questa, N. Mex. 36.682839 –105.654122 Rio Grande X -- 183 08267000 Red River at Mouth, near Questa, N. Mex. 36.648078 –105.693342 Rio Grande X -- 189 08267500 Rio Hondo near Valdez, N. Mex. 36.541797 –105.556522 Rio Grande X -- 37.1 08268500 Arroyo Hondo at Arroyo Hondo, N. Mex. 36.532245 –105.685567 Rio Grande X -- 66.7 08269000 Rio Pueblo De Taos near Taos, N. Mex. 36.439444 –105.503611 Rio Grande X -- 58.0 08271000 Rio Lucero near Arroyo Seco, N. Mex. 36.508289 –105.530964 Rio Grande X -- 16.7 08275000 Rio Fernando De Taos near Taos, N. Mex. 36.375583 –105.549178 Rio Grande X -- 60.8 08275500 Rio Grande Del Rancho near Talpa, N. Mex. 36.303103 –105.581003 Rio Grande X -- 80.4 08275600 Rio Chiquito near Talpa, N. Mex. 36.331971 –105.578901 Rio Grande X -- 37.9 08283500 Rio Chama at Park View, N. Mex. 36.737514 –106.578369 Rio Grande X -- 396 08284100 Rio Chama near La Puente, N. Mex. 36.662658 –106.633367 Rio Grande X -- 472 08284300 Horse Lake Creek above Heron Reservoir near Los Ojos, N. Mex. 36.706681 –106.745594 Rio Grande X -- 43.1 08284500 Willow Creek near Park View, N. Mex. 36.668070 –106.704760 Rio Grande X -- 141 08288000 El Rito near El Rito, N. Mex. 36.391684 –106.239470 Rio Grande X -- 50.1 08289000 Rio Ojo Caliente at La Madera, N. Mex. 36.349742 –106.044186 Rio Grande X -- 412 09016500 Arapaho Creek at Monarch Lake Outlet, Colo. 40.112486 –105.749730 Mountain X -- 47.8 

Table 1–1. Summary of the streamgages used in the analysis.—Continued[USGS, U.S. Geological Survey; CDWR, Colorado Division of Water Resources; A, drainage area; mi2, square miles; P, mean-annual precipitation; in, inches; E, mean basin ele

Colorado; Wyo., Wyoming; N. Mex., New Mexico; Ariz., Arizona]
USGS Latitude Longitude Hydrologic USGS CDWR A 

streamgage Streamgage name (decimal (decimal region data data (mi2) 

number degrees) degrees)

09020000 Willow Creek near Granby, Colo. 40.180541 –106.009185 Mountain X -- 109 09022000 Fraser River at Upper Station, near Winter Park, Colo. 39.845820 –105.751951 Mountain X -- 10.5 09024000 Fraser River at Winter Park, Colo. 39.899987 –105.776674 Mountain X -- 27.6 09026500 Saint Louis Creek near Fraser, Colo. 39.909987 –105.878345 Mountain X -- 32.8 09032000 Ranch Creek near Fraser, Colo. 39.949987 –105.765563 Mountain X -- 19.9 09032500 Ranch Creek near Tabernash, Colo. 39.997487 –105.823621 Mountain X -- 51.1 09033000 Meadow Creek near Tabernash, Colo. 40.050820 –105.777508 Mountain X -- 8.15 09033300 Fraser River below Crooked Creek at Tabernash, Colo. 40.006892 –105.848272 Mountain X -- 224 09034000 Fraser River at Granby, Colo. 40.085264 –105.955294 Mountain X -- 296 09034500 Colorado River at Hot Sulphur Springs, Colo. 40.083319 –106.088077 Mountain X -- 824 09034900 Bobtail Creek near Jones Pass, Colo. 39.760264 –105.906401 Mountain X -- 5.53 09035500 Williams Fork below Steelman Creek, Colo. 39.778875 –105.928347 Mountain X -- 16.4 09035700 Williams Fork above Darling Creek, near Leal, Colo. 39.797194 –106.025639 Mountain X -- 35.2 09035800 Darling Creek near Leal, Colo. 39.800542 –106.026407 Mountain X -- 8.30 09035900 South Fork of Williams Fork near Leal, Colo. 39.795820 –106.030573 Mountain X -- 27.4 09036500 Keyser Creek near Leal, Colo. 39.907486 –106.017240 Mountain X -- 13.9 09039000 Troublesome Creek near Pearmont, Colo. 40.217484 –106.313085 Mountain X -- 44.6 09040000 East Fork Troublesome Creek near Troublesome, Colo. 40.157485 –106.283362 Mountain X -- 75.7 09040500 Troublesome Creek near Troublesome, Colo. 40.059152 –106.305584 Northwest X -- 168 09041000 Muddy Creek near Kremmling, Colo. 40.293595 –106.483647 Mountain X -- 87.3 09041090 Muddy Creek above Antelope Creek near Kremmling, Colo. 40.202484 –106.422534 Mountain X -- 144 09041100 Antelope Creek near Kremmling, Colo. 40.240540 –106.373643 Mountain X -- 11.5 09046530 French Gulch at Breckenridge, Colo. 39.493042 –106.044741 Mountain X -- 11.1 09047000 Blue River at Dillon, Colo. 39.613875 –106.051963 Mountain X -- 128 09047500 Snake River near Montezuma, Colo. 39.605361 –105.943131 Mountain X -- 57.8 09047700 Keystone Gulch near Dillon, Colo. 39.594431 –105.972516 Mountain X -- 9.13 09048000 Snake River at Dillon, Colo. 39.612487 –106.042241 Mountain X -- 90.8 09050100 Tenmile Creek below North Tenmile Creek, at Frisco, Colo. 39.575264 –106.110577 Mountain X -- 92.2 09050500 Tenmile Creek at Dillon, Colo. 39.612487 –106.054741 Mountain X -- 111 09051050 Straight Creek below Laskey Gulch near Dillon, Colo. 39.639709 –106.040296 Mountain X -- 18.4 09052000 Rock Creek near Dillon, Colo. 39.723042 –106.128633 Mountain X -- 15.6 

Table 1–1. Summary of the streamgages used in the analysis.—Continued[USGS, U.S. Geological Survey; CDWR, Colorado Division of Water Resources; A, drainage area; mi2, square miles; P, mean-annual precipitation; in, inches; E, mean basin ele

Colorado; Wyo., Wyoming; N. Mex., New Mexico; Ariz., Arizona]
USGS Latitude Longitude Hydrologic USGS CDWR A 

streamgage Streamgage name (decimal (decimal region data data (mi2) 

number degrees) degrees) 09052400 Boulder Creek at Upper Station, near Dillon, Colo. 39.728041 –106.173357 Mountain X -- 8.32 

09052800 Slate Creek at Upper Station, near Dillon, Colo. 39.763041 –106.192524 Mountain X -- 14.0 09053000 Slate Creek near Dillon, Colo. 39.781652 –106.167801 Mountain X -- 16.4 09054000 Black Creek below Black Lake, near Dillon, Colo. 39.799152 –106.268360 Mountain X -- 15.0 09055300 Cataract Creek near Kremmling, Colo. 39.835263 –106.316417 Mountain X -- 11.9 09058500 Piney River below Piney Lake, near Minturn, Colo. 39.708042 –106.426697 Mountain X -- 12.8 09058610 Dickson Creek near Vail, Colo. 39.704111 –106.457250 Mountain X -- 3.28 09058700 Freeman Creek near Minturn, Colo. 39.698319 –106.445586 Mountain X -- 2.99 09058800 East Meadow Creek near Minturn, Colo. 39.731653 –106.426697 Mountain X -- 3.58 09059500 Piney River near State Bridge, Colo. 39.795722 –106.574347 Northwest X -- 84.7 09060500 Rock Creek near Toponas, Colo. 40.041098 –106.655872 Mountain X -- 47.8 09060550 Rock Creek at Crater, Colo. 39.978321 –106.710039 Northwest X -- 72.5 09060770 Rock Creek at McCoy, Colo. 39.912209 –106.725594 Northwest X -- 198 09061600 East Fork Eagle River near Climax, Colo. 39.410265 –106.249747 Mountain X -- 7.77 09063200 Wearyman Creek near Red Cliff, Colo. 39.522208 –106.323638 Mountain X -- 8.75 09063400 Turkey Creek near Red Cliff, Colo. 39.522764 –106.336139 Mountain X -- 23.8 09063500 Turkey Creek at Red Cliff, Colo. 39.513875 –106.367250 Mountain X -- 29.5 09063900 Missouri Creek near Gold Park, Colo. 39.390265 –106.470029 Mountain X -- 6.42 09064500 Homestake Creek near Red Cliff, Colo. 39.473320 –106.367806 Mountain X -- 58.2 09065100 Cross Creek near Minturn, Colo. 39.568292 –106.412431 Mountain X -- 34.2 09065500 Gore Creek at Upper Station, near Minturn, Colo. 39.625819 –106.278082 Mountain X -- 14.5 09066000 Black Gore Creek near Minturn, Colo. 39.596375 –106.265026 Mountain X -- 12.5 09066100 Bighorn Creek near Minturn, Colo. 39.639986 –106.293361 Mountain X -- 4.55 09066150 Pitkin Creek near Minturn, Colo. 39.643597 –106.302527 Mountain X -- 5.32 09066200 Booth Creek near Minturn, Colo. 39.648319 –106.323083 Mountain X -- 6.08 09066300 Middle Creek near Minturn, Colo. 39.645819 –106.382251 Mountain X -- 5.90 09066310 Gore Creek, Lower Station, at Vail, Colo. 39.641097 –106.394196 Mountain X -- 77.0 09066325 Gore Creek above Red Sandstone Creek at Vail, Colo. 39.641097 –106.394752 Mountain X -- 77.1 09066400 Red Sandstone Creek near Minturn, Colo. 39.682764 –106.401418 Mountain X -- 7.39 09066500 Gore Creek near Minturn, Colo. 39.614708 –106.440030 Mountain X -- 101 09066510 Gore Creek at Mouth near Minturn, Colo. 39.609430 –106.447808 Mountain X -- 102 

Table 1–1. Summary of the streamgages used in the analysis.—Continued[USGS, U.S. Geological Survey; CDWR, Colorado Division of Water Resources; A, drainage area; mi2, square miles; P, mean-annual precipitation; in, inches; E, mean basin ele

Colorado; Wyo., Wyoming; N. Mex., New Mexico; Ariz., Arizona]
USGS Latitude Longitude Hydrologic USGS CDWR A 

streamgage Streamgage name (decimal (decimal region data data (mi2) 

number degrees) degrees)

09067000 Beaver Creek at Avon, Colo. 39.629708 –106.522810 Northwest X -- 14.7 09067005 Eagle River at Avon, Colo. 39.631653 –106.522532 Mountain X -- 395 09067020 Eagle River below Wastewater Treatment Plant at Avon, Colo. 39.634861 –106.531944 Northwest X -- 402 09067200 Lake Creek near Edwards, Colo. 39.647486 –106.609200 Northwest X -- 49.0 09067500 Eagle River at Eagle, Colo. 39.656652 –106.825317 Northwest X -- 629 09068000 Brush Creek near Eagle, Colo. 39.557207 –106.763093 Mountain X -- 71.5 09069500 Gypsum Creek near Gypsum, Colo. 39.545540 –106.934766 Mountain X -- 62.6 09070000 Eagle River below Gypsum, Colo. 39.649430 –106.953655 Mountain X -- 945 09071300 Grizzly Creek near Glenwood Springs, Colo. 39.716649 –107.310333 Mountain X -- 6.47 09073500 Roaring Fork River at Aspen, Colo. 39.189432 –106.814483 Mountain X -- 107 09073700 Hunter Creek above Midway Creek, near Aspen, Colo. 39.213877 –106.655867 Mountain X -- 6.15 09073800 Midway Creek near Aspen, Colo. 39.196377 –106.690035 Mountain X -- 8.65 09073900 No Name Creek near Aspen, Colo. 39.188877 –106.718369 Mountain X -- 6.62 09074000 Hunter Creek near Aspen, Colo. 39.205820 –106.797538 Mountain X -- 42.1 09074800 Castle Creek above Aspen, Colo. 39.087489 –106.812261 Mountain X -- 32.2 09075700 Maroon Creek above Aspen, Colo. 39.123599 –106.905320 Mountain X -- 35.3 09076520 Owl Creek near Aspen, Colo. 39.223597 –106.879763 Mountain X -- 6.54 09077200 Fryingpan River near Ivanhoe Lake, Colo. 39.245544 –106.531419 Mountain X X 18.9 09077800 South Fork Fryingpan River at Upper Station, near Norrie, Colo. 39.241654 –106.593087 Mountain X X 11.5 09078000 Fryingpan River at Norrie, Colo. 39.330820 –106.658090 Mountain X -- 88.9 09078100 North Fork Fryingpan River above Cunningham Creek, near Norrie, Colo. 39.358876 –106.568365 Mountain X -- 10.7 09078200 Cunningham Creek near Norrie, Colo. 39.334154 –106.575310 Mountain X -- 7.20 09078500 North Fork Fryingpan River near Norrie, Colo. 39.342764 –106.665868 Mountain X X 42.3 09080300 Rocky Fork Creek near Meredith, Colo. 39.361652 –106.820595 Mountain X X 12.4 09081550 Crystal River at Placita, Colo. 39.142764 –107.257828 Northwest X -- 107 09081600 Crystal River above Avalanche Creek, near Redstone, Colo. 39.232639 –107.227500 Northwest X -- 167 09082500 Crystal River near Redstone, Colo. 39.298595 –107.214217 Northwest X -- 229 09082800 North Thompson Creek near Carbondale, Colo. 39.329705 –107.333386 Mountain X -- 27.9 09083000 Thompson Creek near Carbondale, Colo. 39.330539 –107.224496 Northwest X -- 75.4 09084000 Cattle Creek near Carbondale, Colo. 39.466650 –107.052270 Mountain X -- 30.3 09085200 Canyon Creek above New Castle, Colo. 39.605259 –107.448388 Northwest X -- 23.8 

Table 1–1. Summary of the streamgages used in the analysis.—Continued[USGS, U.S. Geological Survey; CDWR, Colorado Division of Water Resources; A, drainage area; mi2, square miles; P, mean-annual precipitation; in, inches; E, mean basin ele

Colorado; Wyo., Wyoming; N. Mex., New Mexico; Ariz., Arizona]
USGS Latitude Longitude Hydrologic USGS CDWR A 

streamgage Streamgage name (decimal (decimal region data data (mi2) 

number degrees) degrees) 09085300 East Canyon Creek near New Castle, Colo. 39.609148 –107.434777 Northwest X -- 15.8 

09085400 Possum Creek near New Castle, Colo. 39.597759 –107.423943 Northwest X -- 6.22 09089000 West Divide Creek below Willow Creek, near Raven, Colo. 39.275537 –107.520055 Mountain X -- 35.0 09089500 West Divide Creek near Raven, Colo. 39.331092 –107.580056 Mountain X -- 64.2 09091500 East Rifle Creek near Rifle, Colo. 39.677758 –107.698396 Northwest X -- 34.9 09092000 Rifle Creek near Rifle, Colo. 39.619980 –107.763398 Northwest X -- 137 09092500 Beaver Creek near Rifle, Colo. 39.471924 –107.832566 Northwest X -- 7.90 09093000 Parachute Creek near Parachute, Colo. 39.566922 –108.110912 Northwest X -- 141 09093500 Parachute Creek at Parachute, Colo. 39.453034 –108.059798 Northwest X -- 198 09095000 Roan Creek near De Beque, Colo. 39.453311 –108.317031 Northwest X -- 323 09096000 Plateau Creek at Upper Station, near Collbran, Colo. 39.223591 –107.802007 Mountain X -- 22.8 09096500 Plateau Creek near Collbran, Colo. 39.250535 –107.840620 Mountain X -- 80.4 09096800 Buzzard Creek below Owens Creek, near Heiberger, Colo. 39.236092 –107.633946 Mountain X -- 50.3 09097500 Buzzard Creek near Collbran, Colo. 39.272202 –107.850621 Mountain X -- 143 09097600 Brush Creek near Collbran, Colo. 39.324979 –107.842287 Mountain X -- 9.30 09099500 Big Creek at Upper Station, near Collbran, Colo. 39.131924 –107.918678 Mountain X -- 19.9 09100500 Cottonwood Creek at Upper Station, near Molina, Colo. 39.127757 –107.996459 Mountain X -- 13.7 09104500 Mesa Creek near Mesa, Colo. 39.086369 –108.126743 Northwest X -- 5.33 09105000 Plateau Creek near Cameo, Colo. 39.183611 –108.268333 Northwest X -- 592 09107000 Taylor River at Taylor Park, Colo. 38.860271 –106.566697 Mountain X -- 128 09107500 Texas Creek at Taylor Park, Colo. 38.846944 –106.554639 Mountain X -- 40.5 09110000 Taylor River at Almont, Colo. 38.664437 –106.845317 Mountain X -- 477 09110500 East River near Crested Butte, Colo. 38.864437 –106.909764 Mountain X -- 89.3 09111500 Slate River near Crested Butte, Colo. 38.869715 –106.969489 Mountain X -- 68.9 09112000 Cement Creek near Crested Butte, Colo. 38.824437 –106.852817 Mountain X -- 32.9 09112200 East River below Cement Creek near Crested Butte, Colo. 38.784160 –106.870874 Mountain X -- 239 09112500 East River at Almont, Colo. 38.664437 –106.848095 Mountain X -- 289 09113300 Ohio Creek at Baldwin, Colo. 38.765550 –107.058381 Mountain X -- 47.7 09113500 Ohio Creek near Baldwin, Colo. 38.702216 –106.998379 Mountain X -- 119 09113980 Ohio Creek above Mouth near Gunnison, Colo. 38.587770 –106.931432 Mountain X -- 158 09114500 Gunnison River near Gunnison, Colo. 38.541936 –106.949766 Mountain X -- 1,010 

Table 1–1. Summary of the streamgages used in the analysis.—Continued[USGS, U.S. Geological Survey; CDWR, Colorado Division of Water Resources; A, drainage area; mi2, square miles; P, mean-annual precipitation; in, inches; E, mean basin ele

Colorado; Wyo., Wyoming; N. Mex., New Mexico; Ariz., Arizona]
USGS Latitude Longitude Hydrologic USGS CDWR A 

streamgage Streamgage name (decimal (decimal region data data (mi2) 

number degrees) degrees)

09115500 Tomichi Creek at Sargents, Colo. 38.395028 –106.422625 Mountain X -- 148 09117000 Tomichi Creek at Parlin, Colo. 38.497214 –106.726147 Mountain X -- 426 09118000 Quartz Creek near Ohio City, Colo. 38.559715 –106.636422 Mountain X -- 105 09119000 Tomichi Creek at Gunnison, Colo. 38.521111 –106.940958 Mountain X -- 1,060 09122000 Cebolla Creek at Powderhorn, Colo. 38.291383 –107.114496 Mountain X -- 340 09122500 Soap Creek near Sapinero, Colo. 38.560833 –107.325000 Mountain X -- 58.2 09123000 Soap Creek at Sapinero, Colo. 38.474992 –107.298944 Mountain X -- 86.1 09123500 Lake Fork at Lake City, Colo. 38.018889 –107.314444 Mountain X -- 120 09124500 Lake Fork at Gateview, Colo. 38.298883 –107.230056 Mountain X -- 339 09125000 Curecanti Creek near Sapinero, Colo. 38.487767 –107.415057 Mountain X -- 35.0 09126000 Cimarron River near Cimarron, Colo. 38.258194 –107.546111 Mountain X -- 67.7 09127500 Crystal Creek near Maher, Colo. 38.551933 –107.506169 Mountain X -- 42.3 09128500 Smith Fork near Crawford, Colo. 38.727768 –107.506723 Northwest X -- 43.4 09129600 Smith Fork near Lazear, Colo. 38.707444 –107.710139 Northwest X -- 167 09130500 East Muddy Creek near Bardine, Colo. 39.013322 –107.358385 Northwest X -- 133 09130600 West Muddy Creek near Ragged Mountain, Colo. 39.130817 –107.575334 Mountain X -- 7.33 09131490 Muddy Creek above Paonia Reservoir, Colo. 38.954150 –107.347831 Northwest -- X 238 09132500 North Fork Gunnison River near Somerset, Colo. 38.925823 –107.434221 Northwest X -- 526 09132900 West Hubbard Creek near Paonia, Colo. 39.032207 –107.613668 Mountain X -- 2.32 09132960 Hubbard Creek at Highway 133 at Mouth near Bowie, Colo. 38.925544 –107.518389 Northwest X -- 58.0 09132995 Terror Creek at Mouth near Bowie, Colo. 38.903876 –107.562001 Northwest X -- 29.5 09133000 North Fork Gunnison River near Paonia, Colo. 38.899154 –107.563668 Northwest X -- 653 09134100 North Fork Gunnison River below Paonia, Colo. 38.857500 –107.621944 Northwest X -- 742 09134500 Leroux Creek near Cedaredge, Colo. 38.926371 –107.793672 Northwest X -- 34.7 09135900 Leroux Creek at Hotchkiss, Colo. 38.798040 –107.732003 Northwest X -- 66.8 09136200 Gunnison River near Lazear, Colo. 38.783040 –107.837840 Northwest X -- 5,247 09137050 Currant Creek near Read, Colo. 38.784706 –107.938954 Northwest X -- 56.9 09137800 Dirty George Creek near Grand Mesa, Colo. 38.944704 –108.028125 Northwest X -- 12.0 09139200 Ward Creek near Grand Mesa, Colo. 38.983592 –107.972012 Mountain X -- 12.2 09140200 Kiser Creek near Grand Mesa, Colo. 38.986647 –107.943677 Mountain X -- 5.55 09140700 Cottonwood Creek near Grand Mesa, Colo. 38.977361 –107.950306 Mountain X -- 2.14 09141200 Youngs Creek near Grand Mesa, Colo. 38.958037 –107.918676 Northwest X -- 10.5 

Table 1–1. Summary of the streamgages used in the analysis.—Continued[USGS, U.S. Geological Survey; CDWR, Colorado Division of Water Resources; A, drainage area; mi2, square miles; P, mean-annual precipitation; in, inches; E, mean basin ele

Colorado; Wyo., Wyoming; N. Mex., New Mexico; Ariz., Arizona]
USGS Latitude Longitude Hydrologic USGS CDWR A 

streamgage Streamgage name (decimal (decimal region data data (mi2) 

number degrees) degrees) 09143000 Surface Creek near Cedaredge, Colo. 38.984703 –107.854508 Mountain X -- 27.4 

09143500 Surface Creek at Cedaredge, Colo. 38.901649 –107.921176 Northwest X -- 39.1 09144000 Surface Creek at Eckert, Colo. 38.833594 –107.971733 Northwest X -- 43.9 09144200 Tongue Creek at Cory, Colo. 38.787761 –107.995344 Northwest X -- 197 09146000 Uncompahgre River below Ouray, Colo. 38.031105 –107.675060 Southwest X -- 75.2 09146020 Uncompahgre River near Ouray, Colo. 38.043327 –107.683115 Southwest X -- 77.0 09146200 Uncompahgre River near Ridgway, Colo. 38.183879 –107.745892 Northwest X -- 149 09146400 West Fork Dallas Creek near Ridgway, Colo. 38.073603 –107.851174 Southwest X -- 14.1 09146500 East Fork Dallas Creek near Ridgway, Colo. 38.093325 –107.813673 Southwest X -- 16.5 09146600 Pleasant Valley Creek near Noel, Colo. 38.145546 –107.919787 Southwest X -- 8.19 09147000 Dallas Creek near Ridgway, Colo. 38.177768 –107.758393 Southwest X -- 97.2 09147100 Cow Creek near Ridgway, Colo. 38.149436 –107.644782 Mountain X -- 45.6 09147500 Uncompahgre River at Colona, Colo. 38.331377 –107.779504 Northwest X -- 449 09149500 Uncompahgre River at Delta, Colo. 38.741944 –108.080417 Northwest X -- 1,115 09150500 Roubideau Creek at Mouth, near Delta, Colo. 38.734984 –108.161740 Northwest X -- 249 09151500 Escalante Creek near Delta, Colo. 38.756651 –108.260078 Northwest X -- 209 09153400 West Salt Creek near Mack, Colo. 39.308590 –108.983720 Northwest X -- 168 09163310 East Salt Creek near Mack, Colo. 39.297201 –108.866771 Northwest X -- 197 09163490 Salt Creek near Mack, Colo. 39.221647 –108.892883 Northwest X -- 437 09165000 Dolores River below Rico, Colo. 37.638884 –108.060352 Southwest X -- 106 09166500 Dolores River at Dolores, Colo. 37.472493 –108.497591 Southwest X -- 505 09166950 Lost Canyon Creek near Dolores, Colo. 37.446105 –108.469256 Southwest X -- 71.3 09168100 Disappointment Creek near Dove Creek, Colo. 37.876658 –108.583150 Southwest X -- 147 09169500 Dolores River at Bedrock, Colo. 38.310268 –108.885381 Southwest X -- 2,029 09172000 Fall Creek near Fall Creek, Colo. 37.958326 –108.005901 Southwest X -- 33.4 09172500 San Miguel River near Placerville, Colo. 38.042500 –108.133333 Southwest X -- 310 09174500 Cottonwood Creek near Nucla, Colo. 38.273601 –108.362862 Southwest X -- 38.3 09174600 San Miguel River at Brooks Bridge near Nucla, Colo. 38.244157 –108.502034 Southwest X -- 743 09175000 West Naturita Creek near Norwood, Colo. 37.975825 –108.327861 Southwest X -- 53.3 09175500 San Miguel River at Naturita, Colo. 38.217768 –108.566481 Southwest X -- 1,070 09175900 Dry Creek near Naturita, Colo. 38.092214 –108.622039 Southwest X -- 78.5 

Table 1–1. Summary of the streamgages used in the analysis.—Continued[USGS, U.S. Geological Survey; CDWR, Colorado Division of Water Resources; A, drainage area; mi2, square miles; P, mean-annual precipitation; in, inches; E, mean basin ele

Colorado; Wyo., Wyoming; N. Mex., New Mexico; Ariz., Arizona]
USGS Latitude Longitude Hydrologic USGS CDWR A 

streamgage Streamgage name (decimal (decimal region data data (mi2) 

number degrees) degrees)

09177000 San Miguel River at Uravan, Colo. 38.357212 –108.712875 Southwest X -- 1,502 09182000 Castle Creek above Diversions, near Moab, Utah 38.592764 –109.265671 Southwest X -- 8.64 09182400 Castle Creek below Castle Valley near Moab, Utah 38.673871 –109.450117 Southwest X -- 53.1 09183000 Courthouse Wash near Moab, Utah 38.612759 –109.579840 Southwest X -- 162 09183500 Mill Creek at Sheley Tunnel, near Moab, Utah 38.483040 –109.404004 Southwest X -- 27.5 09184000 Mill Creek near Moab, Utah 38.562205 –109.514006 Southwest X -- 74.4 09185500 Hatch Wash near La Sal, Utah 38.243320 –109.440114 Southwest X -- 371 09235600 Pot Creek above Diversions, near Vernal, Utah 40.768015 –109.319022 Northwest X -- 24.8 09238500 Walton Creek near Steamboat Springs, Colo. 40.408035 –106.786992 Northwest X X 42.3 09239500 Yampa River at Steamboat Springs, Colo. 40.482986 –106.832431 Northwest X -- 567 09240900 Elk River above Clark, Colo. 40.743306 –106.855325 Mountain X -- 123 09241000 Elk River at Clark, Colo. 40.717473 –106.915883 Mountain X -- 216 09242500 Elk River near Milner, Colo. 40.514698 –106.953941 Northwest X -- 460 09243700 Middle Creek near Oak Creek, Colo. 40.385533 –106.993107 Northwest X -- 23.5 09243800 Foidel Creek near Oak Creek, Colo. 40.345811 –107.085053 Northwest X -- 8.67 09243900 Foidel Creek at Mouth near Oak Creek, Colo. 40.390255 –106.994774 Northwest X -- 17.5 09244100 Fish Creek near Milner, Colo. 40.334145 –107.139221 Northwest X -- 34.5 09244500 Elkhead Creek near Clark, Colo. 40.732194 –107.169501 Mountain X -- 44.3 09245000 Elkhead Creek near Elkhead, Colo. 40.669694 –107.285059 Northwest X -- 67.7 09245500 North Fork Elkhead Creek near Elkhead, Colo. 40.680527 –107.287281 Northwest X -- 21.4 09246200 Elkhead Creek above Long Gulch, near Hayden, Colo. 40.591639 –107.320893 Northwest X -- 171 09246920 Fortification Creek near Fortification, Colo. 40.743858 –107.540900 Northwest X -- 40.0 09247000 Fortification Creek at Craig, Colo. 40.514138 –107.541454 Northwest X -- 258 09248600 East Fork of Williams Fork above Willow Creek, Colo. 40.261090 –107.295057 Northwest X -- 107 09249000 East Fork of Williams Fork near Pagoda, Colo. 40.312477 –107.320058 Northwest X -- 144 09249200 South Fork of Williams Fork near Pagoda, Colo. 40.212199 –107.442838 Northwest X -- 47.5 09249500 Williams Fork at Hamilton, Colo. 40.369973 –107.609233 Northwest X -- 380 09249750 Williams Fork at Mouth, near Hamilton, Colo. 40.437194 –107.647846 Northwest X -- 458 09250507 Wilson Creek above Taylor Creek near Axial, Colo. 40.314695 –107.800072 Northwest X -- 20.1 09250510 Taylor Creek at Mouth near Axial, Colo. 40.313306 –107.799794 Northwest X -- 7.24 09251500 Middle Fork Little Snake River near Battle Creek, Colo. 40.990522 –107.044219 Northwest X -- 115 

Table 1–1. Summary of the streamgages used in the analysis.—Continued[USGS, U.S. Geological Survey; CDWR, Colorado Division of Water Resources; A, drainage area; mi2, square miles; P, mean-annual precipitation; in, inches; E, mean basin ele

Colorado; Wyo., Wyoming; N. Mex., New Mexico; Ariz., Arizona]
USGS Latitude Longitude Hydrologic USGS CDWR A 

streamgage Streamgage name (decimal (decimal region data data (mi2) 

number degrees) degrees) 09253000 Little Snake River near Slater, Colo. 40.999410 –107.143388 Northwest X -- 252 

09253400 Battle Creek near Encampment, Wyo. 41.132222 –107.069167 Mountain X -- 13.0 09255000 Slater Fork near Slater, Colo. 40.982466 –107.382839 Northwest X -- 151 09255500 Savery Creek at Upper Station, near Savery, Wyo. 41.218018 –107.372283 Northwest X -- 200 09256000 Savery Creek near Savery, Wyo. 41.097778 –107.381944 Northwest X -- 330 09257000 Little Snake River near Dixon, Wyo. 41.028298 –107.549233 Northwest X -- 1,020 09258000 Willow Creek near Dixon, Wyo. 40.915522 –107.521732 Northwest X -- 24.8 09260000 Little Snake River near Lily, Colo. 40.549017 –108.424322 Northwest X -- 4,040 09264000 Ashley Creek below Trout Creek near Vernal, Utah 40.733290 –109.678479 Northwest X -- 26.3 09264500 South Fork Ashley Creek near Vernal, Utah 40.733290 –109.703480 Northwest X -- 19.9 09265300 Ashley Creek above Red Pine Creek near Vernal, Utah 40.679679 –109.660978 Northwest X -- 55.9 09266500 Ashley Creek near Vernal, Utah 40.577458 –109.622086 Northwest X -- 101 09268000 Dry Fork above Sinks, near Dry Fork, Utah 40.626345 –109.820149 Northwest X -- 44.5 09268500 North Fork of Dry Fork near Dry Fork, Utah 40.642733 –109.810982 Northwest X -- 8.72 09268900 Brownie Canyon above Sinks, near Dry Fork, Utah 40.659401 –109.750981 Northwest X -- 7.44 09269000 East Fork of Dry Fork near Dry Fork, Utah 40.649956 –109.761814 Northwest X -- 10.4 09270000 Dry Fork below Springs near Dry Fork, Utah 40.569402 –109.697644 Northwest X -- 97.3 09270500 Dry Fork at Mouth near Dry Fork, Utah 40.526348 –109.605696 Northwest X -- 116 09271000 Ashley Creek, Sign of the Maine, near Vernal, Utah 40.517181 –109.596529 Northwest X -- 238 09298000 Farm Creek near Whiterocks, Utah 40.567454 –109.961541 Northwest X -- 14.8 09298500 Whiterocks River above Paradise Creek near Whiterocks, Utah 40.636064 –109.967375 Northwest X -- 89.4 09299500 Whiterocks River near Whiterocks, Utah 40.593565 –109.932374 Northwest X -- 110 09300500 Uintah River at Fort Duchesne, Utah 40.301904 –109.853201 Northwest X -- 542 09302450 Lost Creek near Buford, Colo. 40.050256 –107.468948 Mountain X -- 21.6 09302500 Marvine Creek near Buford, Colo. 40.038312 –107.488115 Mountain X -- 59.9 09302800 North Fork White River near Buford, Colo. 40.035534 –107.520894 Northwest X -- 220 09303000 North Fork White River at Buford, Colo. 39.987477 –107.614508 Northwest X -- 259 09303300 South Fork White River at Budges Resort, Colo. 39.843315 –107.334778 Mountain X -- 52.2 09303320 Wagonwheel Creek at Budges Resort, Colo. 39.842760 –107.336722 Mountain X -- 7.40 09303400 South Fork White River near Budges Resort, Colo. 39.864146 –107.533948 Mountain X -- 128 09303500 South Fork White River near Buford, Colo. 39.921646 –107.551727 Mountain X -- 152 

Table 1–1. Summary of the streamgages used in the analysis.—Continued[USGS, U.S. Geological Survey; CDWR, Colorado Division of Water Resources; A, drainage area; mi2, square miles; P, mean-annual precipitation; in, inches; E, mean basin ele

Colorado; Wyo., Wyoming; N. Mex., New Mexico; Ariz., Arizona]
USGS Latitude Longitude Hydrologic USGS CDWR A 

streamgage Streamgage name (decimal (decimal region data data (mi2) 

number degrees) degrees)

09304000 South Fork White River at Buford, Colo. 39.974422 –107.625341 Northwest X -- 176 09304300 Coal Creek near Meeker, Colo. 40.091363 –107.770069 Northwest X -- 24.6 09304500 White River near Meeker, Colo. 40.033585 –107.862295 Northwest X -- 760 09306007 Piceance Creek below Rio Blanco, Colo. 39.826085 –108.183138 Northwest X -- 177 09306036 Sorghum Gulch at Mouth, near Rio Blanco, Colo. 39.824973 –108.199250 Northwest X -- 3.64 09306039 Cottonwood Gulch near Rio Blanco, Colo. 39.826640 –108.207584 Northwest X -- 1.18 09306042 Piceance Creek tributary near Rio Blanco, Colo. 39.833584 –108.220640 Northwest X -- 1.07 09306052 Scandard Gulch at Mouth, near Rio Blanco, Colo. 39.814140 –108.243696 Northwest X -- 7.90 09306058 Willow Creek near Rio Blanco, Colo. 39.837195 –108.244252 Northwest X -- 48.3 09306061 Piceance Creek above Hunter Creek, near Rio Blanco, Colo. 39.850528 –108.258975 Northwest X -- 309 09306200 Piceance Creek below Ryan Gulch, near Rio Blanco, Colo. 39.921083 –108.297588 Northwest X -- 506 09306222 Piceance Creek at White River, Colo. 40.078026 –108.236475 Northwest X -- 652 09306235 Corral Gulch below Water Gulch, near Rangely, Colo. 39.906085 –108.532874 Northwest X -- 8.68 09306240 Box Elder Gulch near Rangely, Colo. 39.888307 –108.528429 Northwest X -- 9.19 09306242 Corral Gulch near Rangely, Colo. 39.920250 –108.472872 Northwest X -- 31.7 09306255 Yellow Creek near White River, Colo. 40.168581 –108.401205 Northwest X -- 263 09306800 Bitter Creek near Bonanza, Utah 39.753301 –109.354842 Northwest X -- 322 09307500 Willow Creek above Diversions near Ouray, Utah 39.566354 –109.587353 Northwest X -- 299 09339900 East Fork San Juan River above Sand Creek, near Pagosa Springs, Colo. 37.389730 –106.841151 Southwest X -- 65.5 09340000 East Fork San Juan River near Pagosa Springs, Colo. 37.368896 –106.892540 Southwest X -- 90.8 09340500 West Fork San Juan River above Borns Lake, near Pagosa Springs, Colo. 37.485562 –106.930321 Southwest X -- 39.9 09341500 West Fork San Juan River near Pagosa Springs, Colo. 37.392472 –106.906889 Southwest X -- 85.4 09342000 Turkey Creek near Pagosa Springs, Colo. 37.369451 –106.940318 Southwest X -- 24.1 09342500 San Juan River at Pagosa Springs, Colo. 37.265528 –107.011000 Southwest X -- 281 09343000 Rio Blanco near Pagosa Springs, Colo. 37.212786 –106.794480 Southwest X -- 57.8 09343500 Rio Blanco near Pagosa Springs, Colo. 37.193618 –106.905315 Southwest X -- 23.1 09344000 Navajo River at Banded Peak Ranch, near Chromo, Colo. 37.085288 –106.689480 Southwest X X 68.9 09344300 Navajo River above Chromo, Colo. 37.031956 –106.732814 Southwest X -- 97.3 09345200 Little Navajo River below Lake Oso Dibersion Dam, near Chromo, Colo. 37.077306 –106.811222 Southwest X -- 13.4 09345500 Little Navajo River at Chromo, Colo. 37.045566 –106.843092 Southwest X -- 23.3 09346000 Navajo River at Edith, Colo. 37.002788 –106.907537 Southwest X -- 176 

Table 1–1. Summary of the streamgages used in the analysis.—Continued[USGS, U.S. Geological Survey; CDWR, Colorado Division of Water Resources; A, drainage area; mi2, square miles; P, mean-annual precipitation; in, inches; E, mean basin ele

Colorado; Wyo., Wyoming; N. Mex., New Mexico; Ariz., Arizona]
USGS Latitude Longitude Hydrologic USGS CDWR A 

streamgage Streamgage name (decimal (decimal region data data (mi2) 

number degrees) degrees) 09346400 San Juan River near Carracas, Colo. 37.013617 –107.312267 Southwest X -- 1,250 

09347500 Piedra River at Bridge Ranger Station, near Pagosa Springs, Colo. 37.428614 –107.193381 Southwest X -- 82.4 09349500 Piedra River near Piedra, Colo. 37.222226 –107.342826 Southwest X -- 369 09349800 Piedra River near Arboles, Colo. 37.088338 –107.397826 Southwest X -- 653 09352500 Los Pinos River below Snowslide Canyon, near Weminuche Pass, Colo. 37.638889 –107.333943 Southwest X -- 24.7 09352900 Vallecito Creek near Bayfield, Colo. 37.477501 –107.543669 Southwest X -- 72.6 09353800 Los Pinos River near Ignacio, Colo. 37.166111 –107.582500 Southwest X -- 340 09354500 Los Pinos River at La Boca, Colo. 37.009448 –107.599500 Southwest X -- 520 09355000 Spring Creek at La Boca, Colo. 37.015278 –107.595333 Southwest X -- 58.3 09357500 Animas River at Howardsville, Colo. 37.833052 –107.599505 Southwest X X 57.6 09358000 Animas River at Silverton, Colo. 37.811108 –107.659228 Southwest X -- 70.6 09358550 Cement Creek at Silverton, Colo. 37.819719 –107.663672 Southwest X -- 20.1 09359000 MineraL Creek near Silverton, Colo. 37.814750 –107.695889 Southwest X -- 44.4 09359010 Mineral Creek at Silverton, Colo. 37.802774 –107.672839 Southwest X -- 52.7 09359020 Animas River below Silverton, Colo. 37.790275 –107.667561 Southwest X -- 146 09359500 Animas River at Tall Timber Resort above Tacoma, Colo. 37.570277 –107.780620 Southwest X -- 350 09361000 Hermosa Creek near Hermosa, Colo. 37.421945 –107.845067 Southwest X -- 168 09361500 Animas River at Durango, Colo. 37.279169 –107.880345 Southwest X -- 701 09362000 Lightner Creek near Durango, Colo. 37.270558 –107.893678 Southwest X -- 63.0 09362750 Florida River above Lemon Reservoir near Durango, Colo. 37.426670 –107.674445 Southwest -- X 52.6 09363000 Florida River near Durango, Colo. 37.325280 –107.748952 Southwest X -- 97.2 09363100 Salt Creek near Oxford, Colo. 37.139725 –107.753396 Southwest X -- 17.8 09365500 La Plata River at Hesperus, Colo. 37.289722 –108.040628 Southwest X -- 34.5 09366000 Cherry Creek near Red Mesa, Colo. 37.118888 –108.198689 Southwest X -- 75.3 09366500 La Plata River at Colorado-New Mexico State line 36.999722 –108.188688 Southwest X -- 309 09367561 Shumway Arroyo near Waterflow, N. Mex. 36.773334 –108.441193 Southwest X -- 136 09367950 Chaco River near Waterflow, N. Mex. 36.724445 –108.591474 Southwest X -- 4,400 09368500 West Mancos River near Mancos, Colo. 37.381663 –108.258137 Southwest X -- 39.5 09369000 East Mancos River near Mancos, Colo. 37.370274 –108.231469 Southwest X -- 12.0 09369500 Middle Mancos River near Mancos, Colo. 37.373885 –108.230636 Southwest X -- 12.1 09371000 Mancos River near Towaoc, Colo. 37.027494 –108.741484 Southwest X -- 527 

Table 1–1. Summary of the streamgages used in the analysis.—Continued[USGS, U.S. Geological Survey; CDWR, Colorado Division of Water Resources; A, drainage area; mi2, square miles; P, mean-annual precipitation; in, inches; E, mean basin ele

Colorado; Wyo., Wyoming; N. Mex., New Mexico; Ariz., Arizona]
USGS Latitude Longitude Hydrologic USGS CDWR A 

streamgage Streamgage name (decimal (decimal region data data (mi2) 

number degrees) degrees)

09371420 McElmo Creek above Alkali Canyon, near Cortez, Colo. 37.327216 –108.649262 Southwest X -- 147 09371492 Mud Creek at State Highway 32, near Cortez, Colo. 37.312772 –108.661207 Southwest X -- 34.5 09371500 McElmo Creek near Cortez, Colo. 37.322771 –108.673152 Southwest X -- 230 09371520 McElmo Creek above Trail Canyon near Cortez, Colo. 37.326660 –108.700653 Southwest X -- 234 09371700 McElmo Creek below Cortez, Colo. 37.340548 –108.805935 Southwest X -- 283 09372000 McElmo Creek near Colorado-Utah State line 37.324160 –109.015666 Southwest X -- 346 09378170 South Creek above Reservoir near Monticello, Utah 37.846661 –109.369563 Southwest X -- 8.37 09378200 Montezuma Creek at Golf Course at Monticello, Utah 37.860550 –109.342339 Southwest X -- 17.6 09378630 Recapture Creek near Blanding, Utah 37.755550 –109.476511 Southwest X -- 3.83 09378650 Recapture Creek below Johnson Creek near Blanding, Utah 37.680830 –109.462621 Southwest X -- 37.2 09378700 Cottonwood Wash near Blanding, Utah 37.560554 –109.578735 Southwest X -- 204 09379000 Comb Wash near Bluff, Utah 37.266112 –109.675678 Southwest X -- 278 09379200 Chinle Creek near Mexican Water, Ariz. 36.943891 –109.710668 Southwest X -- 3,611 394308105413800 Clear Creek above Georgetown Lake near Georgetown, Colo. 39.718876 –105.694448 Mountain X -- 80.1 DWR603 Beaver Creek above Beaver Creek Reservoir, Colo. 40.116649 –105.522223 Mountain -- X 5.88 DWR609 Middle Saint Vrain Creek at Peaceful Valley, Colo. 40.131925 –105.517223 Mountain -- X 17.7 DWR616 Wind River near Estes Park, Colo. 40.326923 –105.581956 Mountain -- X 4.19 DWR620 Michigan River near Meadow Creek Reservoir, Colo. 40.616645 –106.083916 Mountain -- X 101 DWR628 Alamosa River below Ranger Creek near Jasper, Colo. 37.389690 –106.378615 Rio Grande -- X 98.6 DWR630 Deadman Creek near Crestone, Colo. 37.884725 –105.646386 Rio Grande -- X 9.82 DWR633 Rito Alto Creek near Crestone, Colo. 38.077970 –105.759045 Rio Grande -- X 12.0 DWR634 San Isabel Creek near Crestone, Colo. 38.034435 –105.718064 Rio Grande -- X 6.81 DWR636 South Crestone Creek near Crestone, Colo. 37.983204 –105.702123 Rio Grande -- X 4.42 DWR637 Spanish Creek near Crestone, Colo. 37.952777 –105.661665 Rio Grande -- X 3.48 DWR638 Wild Cherry Creek near Crestone, Colo. 38.100252 –105.768309 Rio Grande -- X 5.90 DWR639 Willow Creek near Crestone, Colo. 37.967528 –105.676461 Rio Grande -- X 6.11 DWR641 Long Hollow at the Mouth near Red Mesa, Colo. 37.056380 –108.177853 Southwest -- X 43.5 

Table 1–1. Summary of the streamgages used in the analysis.—Continued[USGS, U.S. Geological Survey; CDWR, Colorado Division of Water Resources; A, drainage area; mi2, square miles; P, mean-annual precipitation; in, inches; E, mean basin ele

Colorado; Wyo., Wyoming; N. Mex., New Mexico; Ariz., Arizona]
38 Evaluation of Mean-Monthly Streamflow-Regression Equations for Colorado, 2014

Appendix 2. Table of Mean-Monthly Streamflow-Regression Equations from 

Figures 3–6 of Capesius and Stephens (2009)

[A, drainage area in square miles; P, mean-annual precipitation in inches; , denotes equations for which no bias was identified; SEP, standard error of prediction in percent; adjR2

, adjusted coefficient of determination]

Mean-Monthly Streamflow for Mountain Hydrologic Region
Appendixes 1–3 39

Mean-Monthly Streamflow for Northwest Hydrologic Region
40 Evaluation of Mean-Monthly Streamflow-Regression Equations for Colorado, 2014

Mean-Monthly Streamflow for Rio Grande Hydrologic Region
Appendixes 1–3 41

Mean-Monthly Streamflow for Southwest Hydrologic Region
42 Evaluation of Mean-Monthly Streamflow-Regression Equations for Colorado, 2014

Appendix 3. Figures Showing Absolute Percentage Difference Between 

Observed and Predicted Mean-Monthly Streamflow at all Streamgages

Base modified from ESRI ArcGIS Online, 2013

State Plane, Colorado, Central

North American Datum of 1983

0 50 100 MILES

0 50 100 KILOMETERS

COLORADO

UTAH

WYOMING

ARIZONA NEW MEXICO

TEXAS

OKLAHOMA

KANSAS

NEBRASKA

EXPLANATION

Hydrologic region boundary

January absolute percentage 

difference

0 to 25.0

25.1 to 50.0

50.1 to 100.0

100.1 to 200.0

200.1 to 500.0

Greater than 500.0

110° 108° 106° 104° 102°

40°

38°

Figure 3–1. Absolute percentage difference between observed and predicted mean-monthly streamflow at all streamgages used in 

the analysis for the month of January.
Appendixes 1–3 43

Base modified from ESRI ArcGIS Online, 2013

State Plane, Colorado, Central

North American Datum of 1983

0 50 100 MILES

0 50 100 KILOMETERS

COLORADO

UTAH

WYOMING

ARIZONA NEW MEXICO

TEXAS

OKLAHOMA

KANSAS

NEBRASKA

EXPLANATION

Hydrologic region boundary

February absolute percentage 

difference

0 to 25.0

25.1 to 50.0

50.1 to 100.0

100.1 to 200.0

200.1 to 500.0

Greater than 500.0

110° 108° 106° 104° 102°

40°

38°

Figure 3–2. Absolute percentage difference between observed and predicted mean-monthly streamflow at all streamgages used in 

the analysis for the month of February.
44 Evaluation of Mean-Monthly Streamflow-Regression Equations for Colorado, 2014

Base modified from ESRI ArcGIS Online, 2013

State Plane, Colorado, Central

North American Datum of 1983

0 50 100 MILES

0 50 100 KILOMETERS

COLORADO

UTAH

WYOMING

ARIZONA NEW MEXICO

TEXAS

OKLAHOMA

KANSAS

NEBRASKA

EXPLANATION

Hydrologic region boundary

March absolute percentage 

difference

0 to 25.0

25.1 to 50.0

50.1 to 100.0

100.1 to 200.0

200.1 to 500.0

Greater than 500.0

110° 108° 106° 104° 102°

40°

38°

Figure 3–3. Absolute percentage difference between observed and predicted mean-monthly streamflow at all streamgages used in 

the analysis for the month of March.
Appendixes 1–3 45

Base modified from ESRI ArcGIS Online, 2013

State Plane, Colorado, Central

North American Datum of 1983

0 50 100 MILES

0 50 100 KILOMETERS

COLORADO

UTAH

WYOMING

ARIZONA NEW MEXICO

TEXAS

OKLAHOMA

KANSAS

NEBRASKA

EXPLANATION

Hydrologic region boundary

April absolute percentage 

difference

0 to 25.0

25.1 to 50.0

50.1 to 100.0

100.1 to 200.0

200.1 to 500.0

Greater than 500.0

110° 108° 106° 104° 102°

40°

38°

Figure 3–4. Absolute percentage difference between observed and predicted mean-monthly streamflow at all streamgages used in 

the analysis for the month of April.
46 Evaluation of Mean-Monthly Streamflow-Regression Equations for Colorado, 2014

Base modified from ESRI ArcGIS Online, 2013

State Plane, Colorado, Central

North American Datum of 1983

0 50 100 MILES

0 50 100 KILOMETERS

COLORADO

UTAH

WYOMING

ARIZONA NEW MEXICO

TEXAS

OKLAHOMA

KANSAS

NEBRASKA

EXPLANATION

Hydrologic region boundary

May absolute percentage 

difference

0 to 25.0

25.1 to 50.0

50.1 to 100.0

100.1 to 200.0

200.1 to 500.0

Greater than 500.0

110° 108° 106° 104° 102°

40°

38°

Figure 3–5. Absolute percentage difference between observed and predicted mean-monthly streamflow at all streamgages used in 

the analysis for the month of May.
Appendixes 1–3 47

Base modified from ESRI ArcGIS Online, 2013

State Plane, Colorado, Central

North American Datum of 1983

0 50 100 MILES

0 50 100 KILOMETERS

COLORADO

UTAH

WYOMING

ARIZONA NEW MEXICO

TEXAS

OKLAHOMA

KANSAS

NEBRASKA

EXPLANATION

Hydrologic region boundary

June absolute percentage 

difference

0 to 25.0

25.1 to 50.0

50.1 to 100.0

100.1 to 200.0

200.1 to 500.0

Greater than 500.0

110° 108° 106° 104° 102°

40°

38°

Figure 3–6. Absolute percentage difference between observed and predicted mean-monthly streamflow at all streamgages used in 

the analysis for the month of June.
48 Evaluation of Mean-Monthly Streamflow-Regression Equations for Colorado, 2014

Base modified from ESRI ArcGIS Online, 2013

State Plane, Colorado, Central

North American Datum of 1983

0 50 100 MILES

0 50 100 KILOMETERS

COLORADO

UTAH

WYOMING

ARIZONA NEW MEXICO

TEXAS

OKLAHOMA

KANSAS

NEBRASKA

EXPLANATION

Hydrologic region boundary

July absolute percentage 

difference

0 to 25.0

25.1 to 50.0

50.1 to 100.0

100.1 to 200.0

200.1 to 500.0

Greater than 500.0

110° 108° 106° 104° 102°

40°

38°

Figure 3–7. Absolute percentage difference between observed and predicted mean-monthly streamflow at all streamgages used in 

the analysis for the month of July.
Appendixes 1–3 49

Base modified from ESRI ArcGIS Online, 2013

State Plane, Colorado, Central

North American Datum of 1983

0 50 100 MILES

0 50 100 KILOMETERS

COLORADO

UTAH

WYOMING

ARIZONA NEW MEXICO

TEXAS

OKLAHOMA

KANSAS

NEBRASKA

EXPLANATION

Hydrologic region boundary

August absolute percentage 

difference

0 to 25.0

25.1 to 50.0

50.1 to 100.0

100.1 to 200.0

200.1 to 500.0

Greater than 500.0

110° 108° 106° 104° 102°

40°

38°

Figure 3–8. Absolute percentage difference between observed and predicted mean-monthly streamflow at all streamgages used in 

the analysis for the month of August.
50 Evaluation of Mean-Monthly Streamflow-Regression Equations for Colorado, 2014

Base modified from ESRI ArcGIS Online, 2013

State Plane, Colorado, Central

North American Datum of 1983

0 50 100 MILES

0 50 100 KILOMETERS

COLORADO

UTAH

WYOMING

ARIZONA NEW MEXICO

TEXAS

OKLAHOMA

KANSAS

NEBRASKA

EXPLANATION

Hydrologic region boundary

September absolute percentage 

difference

0 to 25.0

25.1 to 50.0

50.1 to 100.0

100.1 to 200.0

200.1 to 500.0

Greater than 500.0

110° 108° 106° 104° 102°

40°

38°

Figure 3–9. Absolute percentage difference between observed and predicted mean-monthly streamflow at all streamgages used in 

the analysis for the month of September.
Appendixes 1–3 51

Base modified from ESRI ArcGIS Online, 2013

State Plane, Colorado, Central

North American Datum of 1983

0 50 100 MILES

0 50 100 KILOMETERS

COLORADO

UTAH

WYOMING

ARIZONA NEW MEXICO

TEXAS

OKLAHOMA

KANSAS

NEBRASKA

EXPLANATION

Hydrologic region boundary

October absolute percentage 

difference

0 to 25.0

25.1 to 50.0

50.1 to 100.0

100.1 to 200.0

200.1 to 500.0

Greater than 500.0

110° 108° 106° 104° 102°

40°

38°

Figure 3–10. Absolute percentage difference between observed and predicted mean-monthly streamflow at all streamgages used in 

the analysis for the month of October.
52 Evaluation of Mean-Monthly Streamflow-Regression Equations for Colorado, 2014

Base modified from ESRI ArcGIS Online, 2013

State Plane, Colorado, Central

North American Datum of 1983

0 50 100 MILES

0 50 100 KILOMETERS

COLORADO

UTAH

WYOMING

ARIZONA NEW MEXICO

TEXAS

OKLAHOMA

KANSAS

NEBRASKA

EXPLANATION

Hydrologic region boundary

November absolute percentage 

difference

0 to 25.0

25.1 to 50.0

50.1 to 100.0

100.1 to 200.0

200.1 to 500.0

Greater than 500.0

110° 108° 106° 104° 102°

40°

38°

Figure 3–11. Absolute percentage difference between observed and predicted mean-monthly streamflow at all streamgages used in 

the analysis for the month of November.
Appendixes 1–3 53

Base modified from ESRI ArcGIS Online, 2013

State Plane, Colorado, Central

North American Datum of 1983

0 50 100 MILES

0 50 100 KILOMETERS

COLORADO

UTAH

WYOMING

ARIZONA NEW MEXICO

TEXAS

OKLAHOMA

KANSAS

NEBRASKA

EXPLANATION

Hydrologic region boundary

December absolute percentage 

difference

0 to 25.0

25.1 to 50.0

50.1 to 100.0

100.1 to 200.0

200.1 to 500.0

Greater than 500.0

110° 108° 106° 104° 102°

40°

38°

Figure 3–12. Absolute percentage difference between observed and predicted mean-monthly streamflow at all streamgages used in 

the analysis for the month of December.
For more information concerning this publication contact:

Director, USGS Colorado Water Science Center

Box 25046, MS 415

Lakewood, CO 80225

(303) 236–6901

Or visit the Colorado Water Science Center Web site:

http://co.water.usgs.gov

Publishing support provided by:

Denver and Rolla Publishing Service Centers 

Kohn and others—Evaluation of Mean-Monthly Streamflow-Regression Equations for Colorado, 2014—SIR 2015–5016

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20155016