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Introduction

As part of an effort to map terrestrial ecosystems, the U.S. Geological 

Survey (USGS) has generated topographic moisture potential classes to be used 

in creating maps depicting standardized, terrestrial ecosystem models for the 

conterminous United States (Sayre and others, 2009), using an ecosystems 

classification developed by NatureServe (Comer and others, 2003). A biophysical stratification approach, developed for South America (Sayre and others, 

2008) and now being implemented globally (Sayre and others, 2007), was used 

to model the ecosystem distributions. Substrate moisture regimes strongly 

influence the differentiation and distribution of terrestrial ecosystems, and 

therefore topographic moisture potential is one of the key input layers in this 

biophysical stratification.

The method used to produce these topographic moisture potential classes 

was based on the derivation of ground moisture potential using a combination of 

computed topographic characteristics (CTI, slope, and aspect) and mapped 

National Wetland Inventory (NWI, 

http://www.fws.gov/wetlands/Data/DataDownload.html) boundaries. This 

method does not use climate or soil attributes to calculate relative topographic 

moisture potential since these characteristics are incorporated into the ecosystem model though other input layers. All of the topographic data used for this 

assessment were derived from the USGS 30-meter National Elevation Dataset 

(NED, http://seamless.usgs.gov/index.php) including the National Compound 

Topographic Index (CTI, http://edna.usgs.gov/Edna/datalayers/cti.asp). The 

CTI index is a topographically derived measure of slope for a raster cell and the 

contributing area from upstream raster cells, and thus expresses potential for 

water flow to a point. In other words CTI data are “a quantification of the 

position of a site in the local landscape,” where the lowest values indicate ridges 

and the highest values indicate stream channels, lakes and ponds (USGS, 2003). 

These CTI values were compared to independent estimates of water accumulation by obtaining geospatial data from a number of sample locations representing two types of NWI boundaries: freshwater emergent wetlands and freshwater 

forested/shrub wetlands. Where these shorelines (the interface between the NWI 

wetlands and adjacent land) occurred, the CTI values were extracted and a 

histogram of their statistical distributions was calculated. Based on an evaluation of these histograms, CTI thresholds were developed to separate periodically 

saturated or flooded land, mesic uplands (moderately moist), and uplands. After 

the range of CTI values for these three different substrate moisture regimes was 

determined, the CTI values were grouped into three initial topographic moisture 

potential classes. As a final step in the generation of this national data layer, the 

uplands classification was subdivided into either very dry uplands or dry 

uplands. Very dry uplands were defined as uplands with relatively steep, 

south-facing slopes, and identification of this class was based on the slope and 

aspect datasets derived from the USGS NED. The remaining uplands that did 

not meet these additional criteria were simply re-classified as dry uplands. The 

final National Topographic Moisture Potential dataset for the conterminous 

United States contains four classes: periodically saturated or flooded land (CTI 

≥ 18.5), mesic uplands (12 ≤ CTI < 18.5), dry uplands (CTI < 12), and very dry 

uplands (CTI < 12, Slope > 24 degrees and 91 degrees ≤ Aspect ≤ 314 degrees). 

This map shows a smoothed and generalized image of the four topographic 

moisture potential classes. Additional information about this map and any of 

the data developed for the ecosystems modeling of the conterminous United 

States is available online at http://rmgsc.cr.usgs.gov/ecosystems/.

References Cited:

Comer, Patrick, Faber-Langendoen, Don, Evans, Rob, Gawler, Sue, Josse, 

Carmen, Kittel, Gwen, Menard, Shannon, Pyne, Milo, Reid, Marion, 

Schulz, Keith, Snow, Kristin, and Teague, Judy, 2003, Ecological systems 

of the United States—A working classification of U.S. terrestrial systems: 

NatureServe, Arlington, Va. Accessed July 21, 2009 at 

http://www.natureserve.org/library/usEcologicalsystems.pdf.

Sayre, Roger, Yanosky, Alberto, and Muchoney, Doug, 2007, Mapping global 

ecosystems—The GEOSS approach, Group on Earth Observations, ed.: 

London, U.K., Tudor Rose. 

Sayre, Roger, Bow, J., Josse, C., Sotomayor, L., and Touval, J., 2008, Terrestrial 

ecosystems of South America, Campbell, J.C., Jones, K.B., and Smith, J.H., 

eds., in North America Land Cover Summit—A Special Issue of the 

Association of American Geographers: Association of American Geographers, p. 131–152.

Sayre, Roger, Comer, Patrick, Warner, Harumi, and Cress, J.J., 2009, A new 

map of standardized terrestrial ecosystems of the conterminous United 

States: U.S. Geological Survey Professional Paper 1768, 17 p. Accessed 

July 20, 2009 at http://pubs.usgs.gov/pp/1768. 

U.S. Fish and Wildlife Service, 2008, National Wetlands Inventory (NWI): U.S. 

Fish and Wildlife Service. Accessed Feb 24, 2009 at 

http://www.fws.gov/wetlands/Data/DataDownload.html. 

U.S. Geological Survey, 2003, Elevation Derivatives for National Applications 

(EDNA)—Compound Topographic Index (CTI): U.S. Geological Survey. 

Accessed Feb. 24, 2009 at http://edna.usgs.gov/Edna/datalayers/cti.asp. 

U.S. Geological Survey, 2004, The National Map—National Elevation Dataset 

(NED): U.S. Geological Survey. Accessed Jan. 2006 at 

http://seamless.usgs.gov/index.php.

Terrestrial Ecosystems—Topographic Moisture Potential of the Conterminous United States

By Jill J. Cress, Roger Sayre, Patrick Comer,1 and Harumi Warner2

2009

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Map 3086

Version 1.0

SCALE 1:5 000 000

100

100

100

100

0

0 200

200 300

300 400

400 500 MILES

500 KILOMETERS

Map projection: Albers Equal Area Conic

NAD 83 DATUM

Suggested citation:

Cress, J.J., Sayre, Roger, Comer, Patrick, and Warner, Harumi, 2009, Terrestrial 

Ecosystems—Topographic moisture potential of the conterminous United States: 

U.S. Geological Survey Scientific Investigations Map 3086, scale 1:5,000,000, 1 

sheet.

Publishing support provided by: 

Denver Publishing Service Center

Manuscript approved for publication on June 29, 2009.

For more information concerning this publication, contact:

USGS Central Region Geospatial Information Office

Federal Center

Box 25286

Denver, CO 80225

1-888-ASK-USGS or

Jill Cress

Rocky Mountain Geographic Science Center

U.S. Geological Survey

(303) 202-4275

jjcress@usgs.gov

Any use of trade, product, or firm names is for descriptive purposes only 

and 

does not imply endorsement by the U.S. Government.

Although this information product, for the most part, is in the public 

domain, it also contains copyrighted materials as noted in the text. 

Permission to reproduce copyrighted items for other than personal use 

must be secured from the copyright owner. 

For sale by U.S. Geological Survey Information Services

Box 25286, Federal Center, Denver, CO 80225

1-888-ASK-USGS

This and other USGS products are available on the World Wide Web at:

http://store.usgs.gov/

This report is available at:

http://pubs.usgs.gov/sim/3086

1 Ecology Department, NatureServe, Boulder, Colo.

2 Parallel Inc., Lakewood, Colo.

Prepared in collaboration with NatureServe

Periodically saturated or flooded land

Mesic uplands

Dry uplands

Very dry uplands

EXPLANATION

Topographic Moisture Potential Classes

Other Features

Open water

State boundaries