Introduction

Increasing demand for the limited water resources of the United States continues to put 

pressure on water-resource agencies to balance the competing needs of ecosystem health 

with municipal, agricultural, and recreational uses. In 2007, the U.S. Geological Survey 

(USGS) identified a National Water Census as one of six pivotal future science directions 

for the USGS in the following decade (U.S. Geological Survey, 2007). The envisioned 

USGS National Water Census would evaluate large-scale effects of changes in land use 

and land cover, water use, and climate on water availability, water quality, and human and 

aquatic ecosystem health.

The passage of the SECURE (Science and Engineering to Comprehensively 

Understand and Responsibly Enhance) Water Act (SECURE Water Act, 2009) was a 

key step towards implementing the USGS National Water Census. Section 9508 of the 

Act authorizes a “national water availability and use assessment program” within the 

USGS (1) to provide a more accurate assessment of the status of the water resources of 

the United States; and (2) to develop the science for improved forecasts of the availability of water for future economic, energy production, and environmental uses.

The USGS WaterSMART Program

Initial funding for the USGS to begin working on the National Water Census came 

with the approval of the U.S. Department of the Interior’s WaterSMART (Sustain and 

Manage America’s Resources for Tomorrow) Initiative (U.S. Department of the Interior, 

2010). The WaterSMART Initiative provides funding to the USGS and the Bureau of 

Reclamation to achieve a sustainable water strategy to meet the Nation’s water needs. 

WaterSMART funding also allowed the USGS to begin the national Water Availability 

and Use Assessment, as called for under the SECURE Water Act, for the purpose of:

•	 providing estimates of the distribution and abundance of freshwater 

resources, including assessments of 

water use for human, environmental, 

and wildlife needs;

•	 evaluating factors affecting water 

availability (for example, energy 

development, agricultural practices, 

increasing population, climate change); 

•	 estimating undeveloped potential water 

resources, such as saline and brackish 

water and wastewater; and

•	 developing data and information 

needed to forecast likely outcomes 

of water availability, water quality, 

and aquatic ecosystem health due to 

changes in land use and cover, natural 

and engineered infrastructure, water 

use, and climate.

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2012–3114

September 2012 Printed on recycled paper

—The Colorado River Basin Focus-Area Study
Solar

radiation

Precipitation

Air temperature

Recharge zone

Lower zone

Rain

Surface runoff to stream

Interflow or

subsurface

flow to stream

Subsurface recharge

Groundwater flow to stream

Groundwater

sink

Rain

Groundwater

recharge

Throughfall

Snowmelt

Groundwater recharge

Snowpack

Groundwater

Reservoir

Subsurface

Reservoir

Soil-Zone Reservoir Impervious-Zone Reservoir

Plant canopy

interception

Evaporation

Evaporation

Transpiration

Evaporation

and

Transpiration

Sublimation

U.S. Department of the Interior

U.S. Geological Survey

Circular 1344

Estimated Use of Water in 

the United States 

in 2005

In keeping with the scientific, nonregulatory mission of the USGS, water availability in the Colorado River Basin will be assessed by investigating components 

of a regional “water budget.” A water budget is analogous to a household budget 

or checking account where understanding the amount entering and leaving the 

account indicates the balance remaining in the account for other uses. In its simplest form, a water budget can be written as follows:

Extent of the Colorado River Basin. Upper Basin outlined in red.

Conceptualized water budget (from Healy and others, 2007).

Sublimation: means a substance transforms from 

a solid to a gas without passing through a liquid stage. 

The classic example is the disappearance of dry ice 

(solid carbon dioxide to gaseous carbon dioxide). 

In many low humidity areas, snow and ice can 

sublimate leaving little if any water in the soil.

WaterSMART Focus-Area Studies

To address the goals of the DOI 

WaterSMART Program, while at the same 

time moving the USGS toward a National 

Water Census, the USGS proposed to start 

three “focus-area” studies. The locations for 

the three studies were chosen by the DOI: the 

Delaware River Basin (Del., N.J., N.Y., Pa.), 

the Apalachicola–Chattahoochee‒Flint River

Basins (Ala., Ga., Fla.), and the Colorado River

Basin (Ariz., Calif., Colo., Nev., N. Mex., 

Utah, Wyo.). The initial plan for each of these 

WaterSMART Focus-Area Studies is a 3-year

effort, beginning in 2012, to develop and test 

methods and techniques for meeting the goals 

of the authorizing legislation. The remaining 

sections of this Fact Sheet address only those

activities that are proposed for the Colorado 

River Basin Focus-Area Study.

The Colorado River Basin WaterSMART 

Focus-Area Study

The Colorado River is a critical water supply for much of the Southwestern United States. 

The River supplies water to more than 25 million people and irrigates more than 3 million

acres of cropland across seven “basin states.” 

Increasing population, decreasing streamflows, 

and the uncertain effects of a changing climate 

urge a better understanding of water use and 

water availability in the Colorado River Basin. Waterflow In – Waterflow Out = 

Change in Water Storage

The water-budget components studied in the Colorado River 

Basin will be those where understanding and quantification are 

lacking, and the USGS can contribute the largest incremental 

increase in scientific knowledge. In consultation with basin 

stakeholders, the USGS has identified three water-budget components where understanding could be improved and the USGS has 

relevant expertise: (1) current water use—in particular the “consumptive” use of water; (2) regional and field scale assessments 

of water losses from evapotranspiration and snowpack sublimation; and (3) groundwater discharge to streams, and the relative

importance of the regional groundwater flow system in annual 

streamflow volumes. Better quantification of these components of 

the basin water budget will contribute to the overall assessment of 

water availability in the Colorado River Basin, thereby complementing ongoing work by other agencies.

Water Use in the Colorado River Basin

Many regional stakeholders and water-supply managers in the United States have indicated an important need for water-use data to implement their management strategies, yet 

there is a lack of information in this area (National Research Council, 2002). What data do 

exist are often inconsistent and out of date. Improved and regionally consistent databases 

of water-use information will be one of the most essential outcomes of the National Water 

Census and its geographic Focus-Area Studies.

By renewing effort on quantification of water use, the USGS will be better able to 

describe how humans move, use, consume, and dispose of the water they withdraw, 

divert, or impound and to integrate that information with our understanding of natural 

flows in the environment. Through this integrated approach, we hope to describe how 

human use of water and natural flows influence one another. This effort requires that we 

understand the sources from which water is withdrawn (both surface water and groundTo integrate water-use data with natural-flow data, the USGS will strive to improve 

information on the location of the points of water withdrawal, the source from which the 

water is withdrawn, the location of the points of demand to which the water is delivered, 

and the transit losses of water on the way to that location. Finally, as this information

improves nationally, it will allow a mass-balance estimation of the volume of the eventual 

“return flows” to the environment. Owing to the large area encompassed by the Colorado 

River Basin, the USGS intends to work at multiple spatial and temporal scales to develop 

information across the entire basin, as well as demonstrate analysis and interpretive capabilities at selected subbasin scales. The Focus-Area Study will compile and integrate location information for public supply (large and very large systems, which include population 

served greater than 10,000) and for irrigation water uses—the two largest water uses in the 

Colorado River Basin. Water-use information for the industrial, mining, and thermoelecwater), the demand that the water is used to satisfy, the 

transport of the water to the demand location (including transportation losses), the amount of water that is 

“consumed” in satisfying the demand, and the volume 

and location of water returned to the environment, 

either as return flows to surface water or recharge to 

groundwater systems. Each of these steps has a strong 

geospatial component: We need to know which watersheds are losing flows, which are gaining flows, and 

the net exchange between watersheds.

Some of the many uses of water in the United States (from Kenny and others, 2009).

tric sectors will be compiled where available to provide as complete a summary 

as possible for the major water uses in the 

Colorado River Basin.

Evapotranspiration and Snowpack 

Sublimation in the Colorado River 

Basin

Two important water-budget components that are critical to understanding 

water supplies in the Colorado River Basin 

are evapotranspiration from natural and 

cultivated land, and loss of water storage 

from the mountain snowpack through 

sublimation. Despite their recognized 

importance, evapotranspiration (ET) and 

sublimation remain difficult to measure 

and are poorly quantified for the Colorado 

River Basin. An improved understanding 

of these processes in the varied landscapes 

of the Colorado River Basin will improve 

our ability to calculate water budgets 

across the region. 

Developing a spatially explicit estimate 

of ET and sublimation across the region 

will reduce uncertainty in the estimated 

water budget and allow more accurate 

calculation of other residual terms in the 

equation (for example, consumptive use 

from agricultural irrigation, irrigation 

efficiency, surface-water return flows, or 

groundwater recharge). Methods that use 

satellite-based remotely sensed data will 

be developed and implemented, and these 

regional estimates will be compared to 

ground-based measurements for calibration and verification. Derivative products 

produced during this effort (for example, 

land-cover distribution) also can be integrated into the assessments of water use 

and groundwater that are additional goals 

for this Focus-Area Study.

Evapotranspiration

Evapotranspiration from irrigated croplands and native vegetation (for example, 

riparian ecosystems) is a significant component of the water budget in the Western 

United States. Its quantification across 

the landscape is essential for estimating 

regional water budgets and for calculating consumptive water use on agricultural 

lands. The USGS Focus-Area Study will

apply remote sensing techniques for calculating the surface energy balance that will 

provide estimates of ET across the landscape. An assessment of the entire Colorado River Basin will use MODIS (1-kilometer resolution) to provide monthly and 
Groundwater Discharge to Surface Water

Groundwater is an important resource influencing water availability in the Colorado River Basin. Aquifers store water during rainfall and snowmelt (aquifer recharge), 

thus providing a reliable source of often high-quality water. Groundwater discharge to 

streams and wetlands is a critical component of regional water supply in that it supports 

year-round surface-water flows (baseflows) for multiple uses and sustains ecosystems 

and aquatic environments in the Colorado River Basin. Compared to surface water, 

however, groundwater storage and flow are difficult to measure, so that the groundwater 

seasonal ET estimates from 2000 to 2011. 

Estimates of ET for individual irrigated 

agricultural fields will be developed using 

LANDSAT thermal imagery (30-meter

resolution) and will compare scenes from 

2005 and 2010. This approach has many 

advantages over statistical interpolation 

between flux towers and climate stations, 

or estimates made from indirect proxies 

like consumption of electricity by irrigation pumps. The WaterSMART Colorado 

River Basin Focus-Area Study provides

the opportunity to develop, test, and 

evaluate remote-sensing techniques before 

performing other regional or national 

applications. Maps and on-line datasets 

will be made available to the public.

Snow-Water Assessment

Snow accumulation in high elevation 

settings of the Western United States is an 

integral component of the western regional 

water cycle providing water for drinking, 

irrigation, industry, energy production, 

and ecosystems. In the mountains of the 

Western United States, seasonal snowpacks act as a large natural water-storage 

reservoir providing, on average, 70 to 

80 percent of annual surface-water runoff 

(Doesken and Judson, 1996). In the Upper 

Colorado River Basin, the percentage is 

even higher, with 85 percent of streamflow 

derived from melting snow (Edwards and 

Redmond, 2005). The quantity of water 

stored in seasonal snowpacks is expressed 

as the snow water equivalent (SWE). 

Springtime SWE is one of the most important data inputs to hydrologic models used 

to forecast runoff in the Western United 

States because it is the main source of 

water to streams during late spring and 

early summer (Clark and Hay, 2004; Slater 

and Clark, 2006). Snowpack sublimation, 

which is analogous to evaporation from 

land surfaces or water bodies, represents 

an important, but poorly quantified, loss 

of water from the snowpack (Hood and 

others, 1999). Sublimation represents 

one of the major uncertainties in runoff 

forecast models, and is thought to be 

particularly important during drought 

years, when water is scarce. As part of the 

WaterSMART Focus-Area Study in the

Colorado River Basin, the USGS intends 

to develop regional SWE and sublimation 

estimates using moderate- and highresolution gridded models and groundbased validation measurements to inform 

hydrologic modeling studies, water availability studies, and water-use assessments.

components of the Colorado River Basin 

water budget are less well understood. 

The rate and spatial distribution of aquifer 

recharge, discharge, and use are critical for 

understanding long-term water availability 

in the basin.

In many areas, groundwater and 

surface water are hydraulically connected 

and can be considered a single resource 

(Winter and others, 1998). Drought, 

climate change, and human manipulations, such as surface-water regulation or 

groundwater pumping, can have major 

effects on the linkage between groundwater and surface water, resulting in changes 

in the direction and volumes of flow. 

Thus, the groundwater source and flow 

paths for discharge to streams can be an 

important component of the water budget 

and can be critical to water availability in 

times of small surface-water runoff. Fundamental to understanding and managing 

water resources in the Colorado River 

Basin is an understanding of groundwater 

discharge to surface water in the basin 

(Rosenberry, 2008).
A Web application is under development as part of the 

USGS National Water Census, modeled on the USGS StreamStats platform, 

for delivering water availability information at scales that are relevant to the user.

Vision: Select a stream location 

where information is desired 

and automatically generate the 

contributing surface-water 

basin boundary.

Compile data on water-accounting 

components for delineated area.

Work with the online tool to 

construct your local water budget.

Access and analyze available 

trend information.

Goal: To provide detailed 

water budget information for 

any user-defined drainage 

basin nationwide.

Screen capture of USGS StreamStats Web interface from Utah.

Groundwater discharging to streams 

arrives at the stream interface via local, 

intermediate and regional groundwater 

flow paths (Freeze and Cherry, 1979;

Winter and others, 1998; Stolp and 

others, 2010; Gardner and others, 

2011) and is often a mix of water from 

multiple flow paths. During times of 

drought, surface water often has a 

greater component of discharge from 

regional groundwater flow paths with 

longer residence times and often differing geochemistry (Hornberger and 

others, 1998).

Understanding groundwater flow 

paths and residence times provide 

important information about vulnerability of water resources to natural 

and anthropogenic changes. Local 

groundwater flow systems with shorter 

residence times are more immediately 

susceptible to water-quality effects 

related to changing surface conditions 

and human activities. Conversely, discharge volumes from regional flow systems with long groundwater flow paths 

and residence times of possibly 1,000s of 

years would respond much more slowly 

to short-term hydrologic or climate 

changes and surface-related influences 

(Tetzlaff and Soulsby, 2008; Gardner 

and others, 2011). Effective management of water resources in support of 

ecosystem health requires an understanding of groundwater and surface-water 

interactions over a range of spatial and 

temporal scales.

Due to the large areal extent of the 

Colorado River Basin, the initial groundwater investigation in the Focus-Area

References Cited

Clark, M.P., and Hay, L.E., 2004, Use of medium-range weather forecasts 

to produce predictions of streamflow: Journal of Hydrometeorolgy, v. 5, 

p. 15–32.

Doesken, N.J., and Judson, A., 1996, The Snow Booklet—A guide to the 

science, climatology and measurement of snow in the United States: 

Department of Atmospheric Sciences, Colorado State University, 

Fort Collins, Colo., ISBN #0-9651056-1-x, June, 84 p.

Edwards, L.M., and Redmond, K.T., 2005, Climate factors on Colorado 

River Basin water supply, in Colorado River Basin climate—Paleo, 

present, future: Special publication for Association of California Water 

Agencies and Colorado River Water Users Association Conferences, 

California Department of Water Resources, p. 14–22.

Freeze, R.A., and Cherry, J.A., 1979, Groundwater: Englewood Cliffs, N.J.,

Prentice-Hall, Inc., 604 p.

Gardner, W.P., Harrington, G., Solomon, D.K., and Cook, P., 2011, Using 

terrigenic 4He to identify and quantify regional groundwater discharge 

to streams: Water Resources Research, v. 47, 13 p. (also available at

http://www.agu.org/pubs/crossref/2011/2010WR010276.shtml).

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.

Hornberger, J.M., Raffensperger, J.P., Wiberg, P.L., and Eshleman, K.N., 

1998, Elements of physical hydrology: Baltimore, The Johns Hopkins 

University Press, 302 p.

Hood, Eran, Williams, Mark, and Cline, Don, 1999, Sublimation from a 

seasonal snowpack at a continental, mid-latitude alpine site: Hydrological Processes, v. 13, p. 1781–1797.

Kenny, J.F., Barber, N.L., Hutson, S.S., Linsey, K.S., Lovelace, J.K.,

and Maupin, M.A., 2009, Estimated use of water in the United States 

in 2005: U.S. Geological Survey Circular 1344, 52 p., accessed

September 25, 2012, at http://pubs.usgs.gov/circ/1344/.

National Research Council, 2002, Estimating water use in the United 

States—A new paradigm for the National Water-Use Information 

Program: Washington D.C., National Academy Press, ISBN 0-309-08483-0,

176 p.

Rosenberry, D.O., 2008, The effect of river regulations and ground-water 

discharge on the ecology of riparian corridors of the Colorado River 

and tributaries, in Integrating groundwater science and human well-being: 

Proceedings of the 36th IAH Congress, Toyama, Japan, 10 p.

Slater, A.G., and Clark, M.P., 2006, Snow data assimilation via an 

ensemble Kalman filter: Journal of Hydrometeorology, v. 7, no. 3,

p. 478–493.

SECURE Water Act, 2009, Omnibus Public Land Management Act, Subtitle 

F, Secure Water: Public Law 111-11, March 30, 2009, 123 Stat. 1329–1346,

accessed September 20, 2012, at http://www.gpo.gov/fdsys/pkg/

PLAW-111publ11/pdf/PLAW-111publ11.pdf.

Stolp, B.J., Solomon, D.K., Suckow, A., Vitvar, T., Rank, D., Aggarwal, P.K., 

and Han, L.F., 2010, Age dating base flow at springs and gaining streams

using helium-3 and tritium—Fischa-Dagnitz system, southern Vienna

Basin, Austria: Water Resources Research, v. 46, no. 7.

Tetzlaff, D., and Soulsby, C., 2008, Sources of base flow in larger 

catchments—Using tracers to develop a holistic understanding of runoff 

generation: Journal of Hydrology, v. 359, nos. 3 and 4, p. 287‒302.

U.S. Department of the Interior, 2010, WaterSMART: Departmental Highlights, p. 19–25.

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.

Winter, T.C., Harvey, J.W., Franke, O.L, and Alley, W.M., 1998, Ground

water and surface water―A single resource: U.S. Geological Survey

Circular 1139, 79 p.

Study will evaluate only the groundwater systems of the Upper Colorado River Basin 

(UCRB). Indeed, less work has been done to understand the groundwater/surface-water 

interactions in the upper basin. For understanding groundwater discharge at the scale of

the UCRB, two components need to be considered: (1) identifying key stream reaches 

where substantial groundwater discharge occurs, from what aquifers, and how discharge 

varies temporally and spatially; and (2) understanding the groundwater flow paths supplying water to these discharge points.

The proposed approach for the groundwater assessment in the UCRB is a hierarchical, nested approach applying multiple techniques and tracers to estimate groundwater 

discharge to specific stream reaches. The study will have three significant components: (1) to revise our understanding of the hydrogeologic framework controlling 

groundwater movement and to identify locations where local groundwater discharge 

is likely to be a significant source of surface water in rivers and streams; (2) to determine groundwater ages, residence times, and flow paths in selected stream reaches, 

which will require a geochemical reconnaissance survey of the main tributaries in areas 

identified as having substantial groundwater discharge to streams; and (3) to develop

modeled estimates of groundwater discharge to streams and to map these discharges 

at a subwatershed scale across the entire UCRB.

Information Delivery

The results of the Colorado River Basin Focus-Area Study will be distributed publicly in topic-specific databases, maps, and published information products. The longterm vision for delivering water budget information from the USGS National Water 

Census is to develop an interactive map-based internet application that would allow 

user-defined queries of nationally consistent data. This application is currently (2012) 

under development but will await the broad application of methods and data collection 

activities being piloted in the WaterSMART Focus-Area Studies. The activities and

methods being pursued in the Colorado River Basin and the other Focus-Area Studies

will be evaluated for regional and national application and may be implemented nationwide as part of the National Water Census.

Contact Information

For additional information about the USGS WaterSMART Program or the Colorado River

Basin Focus-Area Study, contact:

Eric J. Evenson, Coordinator Bret Bruce, Coordinator

National Water Census	 Colorado River Basin Focus-Area Study

Water Mission Area U.S. Geological Survey

U.S. Geological Survey Rocky Mountain Regional Executive’s Office

810 Bear Tavern Road, Suite 206 P.O. Box 25046, MS911

West Trenton, NJ 08628 Denver, CO 80225

Phone: 609-771-3904	 Phone: 303-236-4902

Email: eevenson@usgs.gov Email: bbruce@usgs.gov