U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2012–3047

April 2012

USGS Hydro-Climatic Data Network 2009 (HCDN-2009)

Printed on recycled paper

Introduction

The U.S. Geological Survey’s (USGS) Hydro-Climatic 

Data Network (HCDN) is a subset of all USGS streamgages 

for which the streamflow primarily reflects prevailing 

meteorological conditions for specified years. These stations 

were screened to exclude sites where human activities, such 

as artificial diversions, storage, and other activities in the 

drainage basin or the stream channel, affect the natural flow 

of the watercourse. In addition, sites were included in the 

network because their record length was sufficiently long for 

analysis of patterns in streamflow over time. The purpose of 

the network is to provide a streamflow dataset suitable for 

analyzing hydrologic variations and trends in a climatic context. 

When originally published, the network was composed of 1,659 

stations (Slack and Landwehr, 1992) for which the years of 

primarily “natural” flow were identified. Since then, data from 

the HCDN have been widely used and cited in climate-related 

hydrologic investigations of the United States. The network 

has also served as a model for establishing climate-sensitive 

streamgage networks in other countries around the world.

After nearly two decades of use without undergoing a 

systematic revalidation, questions have arisen as to whether 

many of the original stations still maintain their climatesensitive status or even remain operational, as some are known 

to have closed. Some watersheds had been altered to the point 

that stations no longer meet the minimal disturbance criteria 

set forth in the original HCDN report. In addition, some sites 

that did not qualify as HCDN sites in 1988 (the last year of 

data evaluation) because their records were too short now have 

sufficiently long streamflow records for climate-sensitivity 

studies. Accordingly, a review of the existing network was 

initiated in 2009 in order to drop old stations and add new ones 

as appropriate.

Related Efforts: GAGES Reference Sites

As the HCDN review began, the USGS National 

Water-Quality Assessment (NAWQA) Program completed an 

analysis of gaged drainage basins to identify reference sites 

in the conterminous United States (Falcone and others, 2010) 

primarily for determining natural streamflows relevant to 

aquatic ecosystems and for evaluating natural versus altered 

flow conditions. The resulting database, referred to as GAGES 

(Geospatial Attributes of Gages for Evaluating Streamflow), 

incorporated basin attributes for 6,785 USGS streamgages, 

and their upstream drainage basins, with at least 20 years of 

complete-year flow record during the period 1950–2007. Basin 

attributes include the percentage of basin area in various land 

cover types (urban, agriculture, forest, and so on); impervious 

surface; population density and road density; climate, geology, 

topography, and soil characteristics; dam storage; percentage 

of streams coded as canals or pipelines; and freshwater withdrawals in the basins. The database also includes comments 

from the State-based USGS Annual Water Data Reports 

(http://wdr.water.usgs.gov/) regarding human-induced modifications and diversions that might affect streamflow.

Reference sites in the GAGES database were identified 

by comparison with other gages within aggregated ecoregions 

(Falcone and others, 2010). In the original (2010) GAGES 

database, 1,512 streamgages were identified as being in current 

“reference” condition, or minimally affected by direct human 

activities. For a gaged basin to receive a reference designation, 

the watershed had to meet three primary criteria: 

1. Hydrologic disturbance of the watershed was less than 

at least 75 percent of all other gaged watersheds in 

its region. Disturbance was determined with an index 

that included geospatial measures of reservoir storage, 

dam locations and density, freshwater withdrawal, 

road density, and the U.S. Environmental Protection 

Agency’s National Pollutant Discharge Elimination 

System (NPDES) discharges. The disturbance index was 

calculated for all gaged watersheds and ranked within 

each of nine major regions. Watersheds that were in 

the lowest quartile of the disturbance index were given 

priority consideration as having potential reference 

quality. 

2. The USGS Annual Water Data Reports did not identify 

the presence of “regulated” streamflows. Some sites 

considered to have minor modifications were retained. 

3. The watershed passed a visual screen using satellite 

imagery (typically Google Earth) of the entire watershed 

for the presence of human activities that suggested flow 

diversions, groundwater withdrawal, and other factors 

known to influence natural streamflows.

The screening of reference streamgages was not targeted to 

a specific attribute of streamflow. For example, human activities 

in a basin may have no net impact on mean annual flow but may 

influence the timing and duration of specific flow magnitudes. 
Under such conditions, the site would be excluded. The intent 

was to identify gaged sites with all attributes of streamflow 

in natural or least-disturbed condition. It is important to note 

that the GAGES reference sites were derived independently of 

the original HCDN; that is, whether a station was part of the 

original HCDN was not considered in the classification process. 

In 2011, NAWQA released an updated GAGES-II database 

(Falcone, 2011). This new version contains data for 9,322 

streamgages, of which 2,057 are identified as reference gages.

HCDN-2009 Sites

Given the close correspondence between HCDN and 

GAGES reference site criteria, with the latter typically being 

more stringent than those originally established for the HCDN, 

the USGS decided to make the updated HCDN a subset of the 

GAGES-II reference network. Reinforcing this decision was 

a desire to avoid potential conflicts arising from differences 

in the stations listed in two similar, though distinct, reference 

networks. Thus, the updated network is referred to as 

HCDN-2009 and is so designated in the GAGES-II database 

(http://water.usgs.gov/GIS/metadata/usgswrd/XML/

gagesII_Sept2011.xml). 

Stations included in HCDN-2009 meet the following 

criteria: (1) they are identified as being in current “reference” 

condition according to the GAGES-II classification; (2) they 

have at least 20 years of complete and continuous discharge 

record through water year 2009; (3) they have less than 

5 percent impervious surface area as measured using the Federal 

National Land Cover Data (NLCD) 2006; and (4) they were not 

eliminated during a review by participating State USGS Water 

Science Centers. The designation “HCDN‐2009” signifies 

that the data from these stations have been quality assured 

and validated as conforming to the HCDN criteria through 

water year 2009. Because the HCDN-2009 is drawn from the 

GAGES-II database the HCDN data adhere to all the criteria 

that were applied to the GAGES database, which notably 

include the requirement that watersheds are contained within 

Figure 1. Location of the 743 HCDN-2009 stations by State and territory.

Alaska

Puerto Rico

Hawaii
U.S. boundaries, and watershed size is limited to 50,000 km2

. 

Future updates of the HCDN will occur periodically and will be 

designated by the year for which the most recent quality assured 

and validated data are available. 

HCDN-2009 is composed of 743 stations, and their geographic distribution is shown in figure 1. Records cumulatively 

span the period 1898–2009 and contain a total of 41,292 water 

years1 of complete data. There are 594 stations having at least 

40 years of record through the end of water year 2009, 302 with 

at least 60 years, 107 with at least 80, and 4 with at least 

100 years (fig. 2). The 10-year period 1960–1969 experienced 

the largest gain in stations, with 162, while the preceding two 

decades (1950–1959 and 1940–1949) were second and third, 

respectively, with an increase of 114 and 110 (fig. 3). Nearly 

two-thirds of the 743 stations (471) began operation during the 

40-year period 1930–1969. 

1

Water year is the period October 1 to September 30 and is designated by the 

year in which the period ends.

743

663

594

425

302

196

107

31

4

>– 20 >– 30 >– 40 >– 50 >– 60 >– 70 >– 80 >– 90 >– 100

800

700

600

500

400

300

200

100

0

Number of stations

Years of record

Figure 2. Number of HCDN-2009 stations grouped by number 

of years with complete record ending in 2009. All stations have 

at least 20 years of both complete (365+ days in a year) and 

continuous (unbroken) record ending in 2009.

13

56 60

85

110 114

162

59

9

74

1900–1909

1910–1919

1920–1929

1930–1939

1940–1949

1950–1959

1960–1969

1970–1979

1980–1989

1990

180

160

140

120

100

80

60

40

20

0

Number of stations

Beginning year of record

Figure 3. Number of HCDN-2009 stations grouped by the 10-year 

period when records were first collected.

Although the USGS has published HCDN-2009 as 

the listing of stations conforming to clearly defined criteria 

reflecting a dominant sensitivity to weather and climate (that 

is, the least sensitivity to confounding human activities), it also 

acknowledges that all streamgages reflect, to varying degrees, 

sensitivity to meteorological conditions. Thus, there are likely 

many stations that, although not passing the screening for 

inclusion in HCDN-2009, are nonetheless acceptable for use in 

climate-related hydrologic studies. Researchers may, therefore, 

wish to evaluate other stations for use in their studies, particularly in areas where there are few HCDN-2009 stations. 

Care should be exercised in doing so, however, especially for 

studies focused on low streamflow, where relatively small 

perturbations associated with human activities can have a 

significant effect on the observed discharge. For detailed 

information regarding the character and quality of individual 

USGS streamgages, please contact the relevant State USGS 

Water Science Center.
Details about the stations comprising the HCDN-2009 are 

available online at http://water.usgs.gov/osw/hcdn-2009/. The 

Web site has links to the interface of the USGS National Water 

Information System (NWISWeb) for retrieving streamflow data, 

and to the spatial data for the GAGES-II stations (including 

HCDN-2009). The latter includes a point shapefile of all 9,322 

GAGES-II sites with summary attributes, a spreadsheet of the 

basin characteristics and variable descriptions, shapefiles of 

the basin boundaries, and Arc line coverages of the mainstem 

streamlines. 

References Cited

Falcone, J.A., 2011, GAGES–II, Geospatial Attributes of Gages 

for Evaluating Streamflow [digital spatial dataset], available 

at http://water.usgs.gov/GIS/metadata/usgswrd/XML/

gagesII_Sept2011.xml.

Falcone, J.A., Carlisle, D.M., Wolock, D.M., and Meador, M.R., 

2010, GAGES: A stream gage database for evaluating natural 

and altered flow conditions in the conterminous United 

States: Ecology, v. 91, no. 2, p. 621, a data paper in Ecological Archives E091-045-D1, available at http://esapubs.org/

Archive/ecol/E091/045/metadata.htm.

Slack, J.R., and Landwehr, J.M., 1992, Hydro-climatic data network—A U.S. Geological Survey streamflow data set for the 

United States for the study of climate variations, 1874–1988: 

U.S. Geological Survey Open-File Report 92–129, 193 p.

An HCDN-2009 site, Bear Creek near Oaks, North Dakota, USGS station 06470800, April 2, 2009. 

For additional information, contact:

Harry F. Lins

U.S. Geological Survey

415 National Center

Reston, VA 20191

(703) 648-5712

hlins@usgs.gov