U.S. Geological Survey National Water Census: 

Colorado River Basin Geographic Focus Area Study

The U.S. Geological Survey’s (USGS) concept of a national 

census (or accounting) of water resources has evolved over the last 

several decades as the Nation has experienced increasing concern 

over water availability for multiple competing uses. The implementation of a USGS National Water Census was described in the USGS 

2007 science strategy document (U.S. Geological Survey, 2007) that 

identified the highest priority science topics for the decade 2007–17. 

In 2009, the SECURE Water Act (Public Law 111–11, subtitle F) 

authorized the USGS to create a Water Availability and Use 

Assessment Program for the Nation, and in 2012, the Department 

of the Interior WaterSMART initiative provided funding to begin 

implementation of the USGS National Water Census (NWC).

Generally, the USGS NWC approaches water-availability 

assessment in terms of a “water budget” (fig. 1). The water-budget 

approach seeks to better quantify the inflows and outflows of water, 

as well as the change in storage volume, both nationally and at a 

regional scale and, by doing so, provides critical information to 

managers and stakeholders responsible for making water-availability 

decisions. The NWC has two primary components: Topical Studies 

and Geographic Focus Area Studies. Topical Studies do research 

on methods that can provide nationwide estimates of particular 

water-budget components at the subwatershed scale. Some 

examples of NWC Topical Studies include estimation of streamflow 

at ungaged locations; periodic quantification of evapotranspiration; 

and water use related to development of unconventional oil and gas. 

These efforts are planned to include additional topics in the future. 

Geographic Focus Area Studies (FASs) assess water availability 

and use within a defined geographic area, typically a surface-water 

drainage basin, to increase the understanding of factors affecting 

water availability in the region. In the FASs, local stakeholder input 

helps the USGS identify what components of the water budget are 

in most need of additional understanding or quantification. Focus 

Area Studies are planned as 3-year efforts and, typically, three FASs 

are ongoing in different parts of the country at any given time.

The Colorado River Basin Geographic Focus Area Study

The Colorado River Basin (CRB) (fig. 2) and the Delaware 

and Apalachicola-Chattahoochee-Flint (ACF) River Basins were 

selected by the Department of the Interior for the first round of FASs 

because of the perceived water shortages in the basins and potential 

conflicts over water supply and allocations. After gathering input 

from numerous stakeholders in the CRB, the USGS determined 

that surface-water resources in the basin were already being closely 

monitored and that the most important scientific contribution could 

be made by helping to improve estimates of four water-budget 

components: evapotranspiration losses, snowpack hydrodynamics, 

water-use information, and the relative importance of groundwater 

discharge in supporting streamflow across the basin. The purpose 

of this fact sheet is to provide a brief summary of the CRB FAS 

results as the study nears completion. Although some project results 

are still in the later stages of review and publication, this fact sheet 

provides an overall description of the work completed and cites the 

publications in which additional information can be found.

Figure 1. Components of a simple water budget for part of a watershed.

Modified from Healy (2007).

Figure 2. The Colorado River Basin. The Upper Colorado River Basin

is outlined in black. 

Aquifer Bedrock

NOT TO SCALE

Unsaturated zone

Surface-water outflow, exported

water (pipelines, canals)

Groundwater 

 outflow

Surface-water inflow,

imported water

(pipelines, canals)

Groundwater

inflow

Evapotranspiration

Precipitation

Water table

IDAHO

UTAH

ARIZONA

MEXICO

NEW MEXICO

COLORADO

NEVADA

CALIFORNIA

WYOMING

UPPER

BASIN

LOWER

BASIN

Las Vegas

Denver

Santa Fe

Albuquerque

Lees Ferry

Hoover

Dam

Glen

Canyon

Dam

Salt Lake

City

Cheyenne

Phoenix

Tucson

Yuma

San Diego

Los AngelesAll

American

Canal

PACIFIC OCEAN

Gulf of California

Colorado River

Colorado R

Green River

Green River

Virgin R

Yampa River

Gila River

Gila River

Santa Cruz R

San Pedro R

Salt River

Little Colorado R

San Juan River

Grand Canyon

0 100 200 300 MILES

0 100 200 300 KILOMETERS

Base modified from U.S. Geological Survey data

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2015–3080

December 2015
Evapotranspiration 

Evapotranspiration (ET)—the combination of water transpired 

from plants and water that evaporates from soil or open water 

bodies—is an important component of a water budget. The USGS 

NWC program has advanced new methods of remote-sensingbased actual ET (ETa) estimation through the development of the 

Operational Simplified Surface Energy Balance (SSEBop) model 

(Senay and others, 2013). The SSEBop ETa model uses land surface 

temperature (LST) derived from satellite imagery and integrates 

these data with local weather datasets (for example, net radiation, 

air temperature, humidity, wind speed, and air pressure) to produce 

estimates of ETa at the time of each satellite overpass. Time periods 

between satellite overpasses are interpolated using potential ET data 

derived from the land-based weather datasets.

The SSEBop ET model uses LST from both MODIS (1,000 meters

[m]) and Landsat (100 m) satellite sensors, thus producing estimates 

of ET at two spatial scales. The finer scale resolution substantially 

increases the computational time required to produce the estimates. 

Currently (2015), 1,000-m ETa estimates have been completed for 

the conterminous United States for the year 2000 to the present as 

monthly and annual totals. These datasets are archived and disseminated 

through the USGS Geo Data Portal at http://cida.usgs.gov/gdp/. Because 

of the large computational requirements, 100-m ETa estimates are 

produced on a region-specific basis. In 2014, the first ever CRBwide, 100-m scale ETa estimate was completed for 2010 using 

Landsat 5 and 7 imagery (Singh and others, 2014) followed by a 

2013 CRB-wide ETa estimate using Landsat 8 (G.B. Senay, USGS, 

written commun., Oct. 26, 2015). These ETa estimates were validated 

using a combination of methods including land-based eddy-covariance 

flux towers, annual-scale basin water balance ET datasets (Velpuri 

and others, 2013), and field-reported data. The 100-m resolution 

ETa estimates were used by the CRB FAS to quantify agricultural 

irrigation consumptive water use on a field-by-field scale and are being 

incorporated into the irrigation category for the basinwide water-use 

compilation discussed in the Water-Use Information section (fig. 3). 

Snowpack Hydrodynamics

Snow is an essential resource in the Western United States, 

providing water for drinking, irrigation, industry, energy production, 

and ecosystems across much of the region. In the mountains 

of the Western United States, most precipitation falls as snow, 

which accumulates in seasonal snowpacks that serve as large 

natural reservoirs.

Understanding the spatial distribution of snowpack water content 

is essential for forecasting annual snowmelt streamflow. As part of the 

CRB FAS, the USGS has been conducting studies of factors affecting 

snowpack distribution, snowmelt, and losses of snowpack water due to 

sublimation in the Upper Colorado River Basin (UCRB). These studies 

have shown that peak snowmelt in Colorado is occurring 2–3 weeks 

earlier than it did during the late 1970s (Clow, 2010). These changes 

in snowmelt timing have been traced to a combination of decreased 

snowfall, warm springtime air temperatures, and deposition of 

windblown dust onto mountain snowpacks (Clow, 2010; Painter 

and others, 2012; Clow and others, 2015). Also, as part of the FAS, 

the USGS has developed methods to make physical measurements 

of snowpack sublimation. Continuous measurements at multiple 

sites indicate a seasonal loss of 2–30 percent of annual snow water 

equivalent (SWE), depending on a variety of conditions including 

elevation, aspect, temperature, and wind speed (G. Sexstone, USGS, 

written commun., Nov. 19, 2015) (fig. 4).

In addition, the CRB FAS compared the output from the 

National Weather Service’s Snow Data Assimilation (SNODAS) 

program (National Operational Hydrologic Remote Sensing Center. 

2004) to field-based measurements at multiple locations in the 

CRB. The SNODAS program provides moderate-resolution model 

estimates of snow water content at a daily time step. The CRB FAS 

showed that although SNODAS performed well in forested areas, 

it was less accurate in areas above the tree line where snow is 

redistributed by wind (Clow and others, 2012). Results of these 

studies and ongoing work will help improve runoff forecast models 

used by water managers in the Western United States.

109°50'

32°10'

32°05'

32°00'

109°45' 109°40' 109°50' 109°45' 109°40'

A B

Annual ETa

EXPLANATION

July 6, 2013, ETa

EXPLANATION

1,500 mm

750 mm

50 mm

Alfalfa

 field

8 mm

4 mm

0 mm

Alfalfa

 field

0 1 2

KILOMETERS

4 6 0 1 2

KILOMETERS

4 6

Figure 3. Zoomed-in view of the spatial distribution of (A) annual evapotranspiration (millimeters per year) and (B) evapotranspiration (millimeters 

per day) on the day of satellite overpass (July 6, 2013) in a selected part of the southern Willcox irrigation district in Arizona. Modified from G.B. Senay, 

USGS, written commun., Oct. 26, 2015. [ETa, actual evapotranspiration; mm, millimeter] 
Figure 4. Meteorological and continuous sublimation monitoring 

station, Upper Colorado River Basin (photograph by David Clow, 

U.S. Geological Survey).

Figure 5. Colorado River diversion (water use) at the Imperial Dam,

which crosses the Colorado River along the Arizona-California border

near Yuma, Arizona (photograph from Bureau of Reclamation). 

Water-Use Information

The USGS defines water use as the interaction of humans 

with the hydrologic cycle. Human water use is a major component 

of a regional water budget. Water-use data collected by the USGS 

describe water that is withdrawn from a source and delivered 

to a “customer” and is reported for the following categories of 

use: public supply, domestic, commercial, industrial, irrigation, 

livestock, mining, aquaculture, hydroelectric and thermoelectric 

power generation, and wastewater returns. The USGS has published 

national compilations of water-use data since 1950 (at 5-year 

intervals) as part of the National Water Use Information Program 

(Maupin and others, 2014). 

Water-use data for the CRB FAS are being compiled as annual 

total withdrawals by source and include the water-use categories 

listed previously. Data are being aggregated for 8-digit Hydrologic 

Unit Code (HUC-8) watersheds within the Upper and Lower 

Colorado River Basins rather than by county/State, which is the 

format used for USGS national water-use compilations. Water that 

is withdrawn and applied within the hydrologic basin boundaries 

of the CRB are reported as a water use at the point where water is 

taken from the source. Additionally, water that is exported outside 

the basin boundaries (“interbasin transfers”) is summarized for 

municipal, industrial, and irrigation uses. In order to look back at 

recent trends in water use in the CRB, data are being reaggregated 

for HUC-8 watersheds from 1985 to 2010, in 5-year intervals, with 

consumptive use for thermoelectric power generation and irrigation 

for 1985–95, 2010, and sporadically for 2000 and 2005. These 

data are intended to enhance hydrologic studies by representing 

the effects of water use on the water budget and, therefore, water 

availability. Water-use compilations at this scale and number of 

categories is not planned to be continued into the future (fig. 5).

Groundwater Discharge to Streams

Historically, management of water resources in the CRB has 

focused on surface-water supplies; however, groundwater and 

surface water are interconnected, and groundwater discharge to 

streams sustains surface-water flows in much of the UCRB. In light 

of recent droughts and anticipated changes in climate, there is a 

need to understand the link between groundwater and surface water. 

The objective of this component of the CRB FAS is to quantify 

groundwater discharge to streams across the UCRB.

To estimate groundwater discharge to streams in the UCRB, 

methods were developed to use in-stream water-quality data, 

specifically the electrical conductivity (specific conductance, SC)

of stream water, to quantify the fraction of streamflow that originates as groundwater. Using this approach with SC data that were 

continuously measured in 14 streams and rivers in the UCRB, it was 

determined that, on average, 21–58 percent of annual streamflow 

at these sites originates as groundwater discharge (Miller and 

others, 2014). For sites where only occasional measurements of 

SC (referred to as discrete data) have been made, an additional 

method was developed to estimate the percentage of streamflow 

that originates as groundwater (Miller and others, 2015). This 

method was applied at 229 sites in the UCRB. The results show 

that groundwater discharge to streams contributes an average of 

48 percent of total streamflow in the UCRB, with site-specific 

estimates ranging from 12 to 92 percent (Rumsey and others, 2015) 

(fig. 6). Spatially, groundwater discharge to streams is greater in 

upland, mountainous areas that have greater precipitation. The 

last phase of the study, which is currently underway, uses the 

annual estimates of groundwater discharge to streams at these 

229 sites as calibration data in a Spatially Referenced Regression 

on Watershed Attributes (SPARROW) model (Schwarz and others, 

2006) to estimate groundwater discharge to streams in more than 

10,000 stream reaches in the UCRB. 

In Closing

The Colorado River Basin Focus Area Study developed new 

information on important components of the basin water budget 

that can be used to inform water managers and users throughout 

the region. This fact sheet provides an early summary of the USGS 

activities in the CRB as part of the USGS National Water Census 

Geographic Focus Area Study. The following list of references 

guides the reader to a more in-depth discussion on most of the 

information contained herein. Interested readers also are directed to 

the National Water Census online data portal (http://water.usgs.gov/

watercensus/) where existing data and publications can be accessed.

Wind-blown 

snow in the 

Rocky Mountains 

(photograph by 

David Clow, U.S. 

Geological Survey).
42°

38°

110° 105°

Annual baseflow, 

 in 108 m3/yr

EXPLANATION

Subregion

Base map sources: ESRI, USGS, NOAA

0 to 66

67 to 266

267 to 688

689 to 1,719

1,720 to 3,048

Study area

Study area

IDAHO WYOMING

COLORADO

NEW MEXICO

AZ NM

CO

UT

ID WY

ARIZONA

UTAH

Figure 6. Estimates of annual 

groundwater discharge to 

streams (baseflow) at 229 sites 

in the Upper Colorado River 

Basin (UCRB). Groundwater 

discharge to streams contributes 

an average of 48 percent of total 

streamflow in the UCRB. Modified 

from Rumsey and others (2015). 

[m3/yr, cubic meters per year] 

References Cited

Clow, D.W., 2010, Changes in the timing of snowmelt and streamflow in 

Colorado —A response to recent warming: Journal of Climate, v. 23, no. 9, 

p. 2293–2306. (Also available at http://dx.doi.org/10.1175/2009JCLI2951.1.)

Clow, D.W., Nanus, L., Verdin, K.L., and Schmidt, J., 2012, Evaluation of 

SNODAS snow depth and snow water equivalent estimates for the Colorado 

Rocky Mountains, U.S.A: Hydrological Processes, v. 26, no. 17, 

p. 2583–2591. (Also available at http://dx.doi.org/10.1002/hyp.9385.)

Clow, D.W., Williams, M.W., and Ingersoll, G.I., 2015, Influence of windblown 

dust on snowpack chemistry and snowmelt timing in mountains of the 

western U.S., in Proceedings of the 9th International Conference on Acid 

Deposition, October 19–23, 2015, Rochester, N.Y. (Also available at 

http://acidrain2015.org/Documents/AcidRain2015_Proceedings.pdf.)

Healy, R.W., Winter, T.C., LaBaugh, J.W., and Franke, O.L., 2007, Water 

budgets—Foundations for effective water-resources and environmental 

management: U.S. Geological Survey Circular 1308, 90 p. (Also available 

at http://pubs.usgs.gov/circ/2007/1308/.)

Maupin, M.A., Kenny, J.F., Hutson, S.S., Lovelace, J.K., Barber, N.L., 

and Linsey, K.S., 2014, Estimated use of water in the United States 

in 2010: U.S. Geological Survey Circular 1405, 56 p. (Also available 

at http://dx.doi.org/10.3133/cir1405.)

Miller, M.P., Johnson, H.M., Susong, D.D., and Wolock, D.M., 2015, A new 

approach for continuous estimation of baseflow using discrete water quality 

data—Method description and comparison with baseflow estimates from 

two existing approaches: Journal of Hydrology, v. 522, p. 203–210.

Miller, M.P., Susong, D.D., Shope, C.L., Heilweil, V.M., and Stolp, B.J., 2014, 

Continuous estimation of baseflow in snowmelt-dominated streams and 

rivers in the Upper Colorado River Basin—A chemical hydrograph separation

approach: Water Resources Research, v. 50, no. 8, p. 6986–6999.

National Operational Hydrologic Remote Sensing Center, 2004, Snow Data 

Assimilation System (SNODAS) data products at NSIDC, ver. 1: Boulder, 

Colo., National Snow and Ice Data Center, accessed October 28, 2015, at 

http://dx.doi.org/10.7265/N5TB14TC.

Painter, T.H., Skiles, S.M., Deems, J.S., Bryant, A.C., and Landry, C.C., 2012, Dust

radiative forcing in snow of the Upper Colorado River Basin—1. A 6 year record

of energy balance, radiation, and dust concentrations: Water Resources Research,

v. 48, no. 7. (Also available at http://dx.doi.org/10.1029/2012WR011985.)

Rumsey, C.A., Miller, M.P., Susong, D.D., Tillman, F.D., and Anning, D.W., 

2015, Regional scale estimates of baseflow and factors influencing baseflow 

in the Upper Colorado River Basin: Journal of Hydrology—Regional 

Studies, v. 4, p. 91–107. (Also available at http://dx.doi.org/10.1016/

j.ejrh.2015.04.008.)

Schwarz, G.E., Hoos, A.B., Alexander, R.B., and Smith, R.A., 2006, The 

SPARROW surface water-quality model—Theory, application, and user 

documentation: U.S. Geological Survey Techniques and Methods Report, 

book 6, chap. B3. (Also available at http://pubs.usgs.gov/tm/2006/tm6b3/.)

Senay, G.B., Bohms, S., Singh, R.K., Gowda, P.H., Velpuri, N.M., Alemu, H., 

and Verdin, J.P., 2013, Operational evapotranspiration mapping using 

remote sensing and weather datasets—A new parameterization for the 

SSEB approach: Journal of the American Water Resources Association, 

v. 49, no. 3, p. 577–591. 

Singh, R.K., Senay, G.B., Velpuri, N.M., Bohms, S., Scott, R.L., and Verdin, J.P.,

2014, Actual evapotranspiration (water use) assessment of the Colorado 

River Basin at the Landsat resolution using the Operational Simplified 

Surface Energy Balance model: Remote Sensing, v. 6, no. 1, p. 233–256. 

(Also available at http://dx.doi.org/10.3390/rs6010233.)

U.S. Geological Survey, 2007, Facing tomorrow’s challenges—U.S. Geological 

Survey science in the decade 2007–2017: U.S. Geological Survey 

Circular 1309, 70 p.

Velpuri, N.M., Senay, G.B., Singh, R.K., Bohms, S., and Verdin, J.P., 2013, 

A comprehensive evaluation of two MODIS evapotranspiration products 

over the conterminous United States—Using point and gridded FLUXNET

and water balance ET: Remote Sensing of Environment, v. 139, p. 35– 49. 

(Also available at http://dx.doi.org/10.1016/j.rse.2013.07.013.)

By Breton W. Bruce, David W. Clow, Molly A. Maupin, Matthew P. Miller, 

Gabriel B. Senay, Graham A. Sexstone, and David D. Susong

For more information, contact

Program Coordinator

National Water Availability and Use Science Program

12201 Sunrise Valley Drive

988 National Center

Reston, VA 20192

Or visit the National Water Census Web site at

http://water.usgs.gov/watercensus/

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20153080