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ABSTRACT: The use of watersheds to conduct research onland/water relationships has expanded recently to include bothextrapolation and reporting of water resource information andecosystem management. More often than not, hydrologic units(HUs) are used for these purposes, with the implication that hydro-logic units are synonymous with watersheds. Whereas true topo-graphic watersheds are areas within which apparent surface waterdrains to a particular point, generally only 45 percent of all hydro-logic units, regardless of their hierarchical level, meet this defini-tion. Because the area contributing to the downstream point inmany hydrologic units extends far beyond the unit boundaries, useof the hydrologic unit framework to show regional and national pat-terns of water quality and other environmental resources can resultin incorrect and misleading illustrations. In this paper, the implica-tions of this misuse are demonstrated using four adjacent HUs incentral Texas. A more effective way of showing regional patterns inenvironmental resources is by using data from true watersheds rep-resentative of different ecological regions containing particularmosaics of geographical characteristics affecting differences inecosystems and water quality.(KEY TERMS: watersheds; hydrologic units; watershed manage-ment; ecoregions; ecosystem management.)
Omernik, James M., 2003. The Misuse of Hydrologic Unit Maps for Extrapola-tion, Reporting, and Ecosystem Management. J. of the American WaterResources Association (JAWRA) 39(3):563-573.
INTRODUCTION
“It does not occur to most Americans that a goodmap raises more questions than it answers –that the question of why things are locatedwhere they are raises important intellectualissues, with immediate serious implications”(Peirce Lewis, 1985, pg. 471).
For many decades, watersheds have been usedappropriately in the study of characteristics related tonatural and anthropogenic activities and their associ-ations with the quality and quantity of water atspecific points on a stream and on particular waterbodies. This use has expanded recently to includeextrapolation and reporting by means of hydrologicunits, particularly since the development of a U.S.Geological Survey (USGS) digital framework and apaper, “Hydrologic Unit Maps” (Seaber et al., 1987).Magnusson (2001) stressed the importance of catch-ments (watersheds) as the basic unit of managementin terrestrial biomes and urged their use in conserva-tion biology courses. Although one cannot dispute thevalue of watersheds as the basic unit of study forland/water relationships, use of the watershed as anideal unit for ecosystem management is not wellunderstood and can be inappropriate. As with anygeographic unit used to study and extrapolate scien-tific information, the strengths and limitations ofwatersheds must be clarified, and the limitations ofsurrogates such as hydrologic units need carefulexamination.
According to Seaber et al. (1987, pg. 1), “TheHydrologic Unit Maps show drainage, hydrography,culture, and political and hydrologic unit codes(HUCs), thus providing a standard geographic andhydrologic framework for detailed water-resource andrelated land-resource planning.” The hydrologic unitsshown on these maps are commonly referred to asHUCs, even though the codes are merely identifiersfor the units at their particular hierarchical level. Inthis paper, these hydrologic units will be referred toas HUs. The HU framework is hierarchical in which
1Paper No. 02011 of the Journal of the American Water Resources Association. Discussions are open until December 1, 2003.2Geographer (retired), U.S. Environmental Protection Agency, National Health and Environmental Effects Research Laboratory, Western
Ecology Division, 200 S.W. 35th Street, Corvallis, Oregon 97333 (E-Mail: [email protected]).
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JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATIONJUNE AMERICAN WATER RESOURCES ASSOCIATION 2003
THE MISUSE OF HYDROLOGIC UNIT MAPS FOR EXTRAPOLATION,REPORTING, AND ECOSYSTEM MANAGEMENT1
James M. Omernik2
units of roughly similar size have been mapped atseveral levels. For the conterminous United States 21units have been defined at the first level, 222 havebeen mapped at the second level, 352 at the thirdlevel, and 2,150 at the fourth level. Although exami-nation of the publication by Seaber et al. (1987)reveals that not all of these HUs shown on the mapsare true watersheds, most proponents or users of theframework believe watersheds and HUs to be synony-mous, or at least so similar that the HU frameworkcould be termed a watershed framework (e.g.,USFWS, 1995; Jones et al., 1997; Ruhl, 1999; Alexan-der et al., 2000; Graf, 2001; USEPA, 2001). Moreover,because hydrologic units and watersheds seldom cor-respond to areas containing similar mosaics of geo-graphic phenomena associated with differences inwater quality and quantity, the logic of using theseunits for most “water-resource and related land-resource planning” is questionable (Omernik and Bai-ley, 1997; Griffith et al., 1999). This is not to say thathydrologic units have no use in the management ofwater resources. For some specific subjects or issues,such as flood control and fish management, they canbe helpful. They are also useful for research on track-ing water quality characteristics as they move down-stream (Smith et al., 1997; Alexander et al., 2000).
The fact that most (about 55 percent) HUs are nottrue watersheds has been stated in the literature(e.g., Omernik and Griffith, 1991; Omernik, 1995;Omernik and Bailey, 1997; Bryce et al., 1999; Griffithet al., 1999). However, authors have not explicitlydefined the serious implications of considering themsynonymous, and of extrapolating and reporting datagathered from HUs to show patterns in the status ortrends of environmental resources (particularly waterquality) on regional or national scales. This paperdescribes these implications, and suggests a way inwhich information gathered from true watershedsand other reference areas could be used effectively toshow national and regional patterns in environmentalresources.
DISTINGUISHING BETWEEN WATERSHEDSAND HYDROLOGIC UNITS
Watersheds are defined as topographic areas with-in which apparent surface water and subsurfacewater drain to a specific point on a stream or to awater body such as a lake (Omernik and Bailey, 1997;Griffith et al., 1999). Although mapped confluencesare commonly used to define watersheds, there areliterally an infinite number of points along streamsfrom which watersheds can be defined. Whereas there
is little disagreement over this definition, the defini-tion of hydrologic units is not as clear, and certainlynot as well understood. The hydrologic units identi-fied on the U.S. Geological Survey framework (Seaber et al., 1987) can be defined as watersheds, and down-stream segments of watersheds, many of whichinclude adjacent intersticies, or areas that lie inbetween topographic watersheds (Omernik and Bai-ley, 1997). The downstream points of over half ofthese hydrologic units drain areas that are greaterthan the areas defined by their hydrologic unit bound-aries. Hence, the hydrologic unit maps are not mapsof watersheds.
Characteristics of spatial geographic phenomenasuch as soils, geology, vegetation, land use, physiogra-phy, and climate can be mapped to show regions with-in which there is less variability compared to otherregions. Similarly, these maps can show a particularvariability of a characteristic that makes a region dif-ferent from other regions. It is also possible to compilea map of hydrologic characteristics, such as karst, ordifferences in lake density patterns, lake types, andstream drainage networks. However, since streamsare linear units rather than spatial ones, it is not pos-sible to map the watersheds of streams and attain afinal map completely covered with watersheds of simi-lar size. A national map of HUs (6-digit, 8-digit, orany other hierarchical level) contains roughly 45 per-cent true watersheds. At the coarsest 2-digit level(Water Resource Regions), the percentage is only 33percent. The remaining HUs comprise downstreamsegments of watersheds, or groups of adjacent water-sheds, along a sea coast, lake, estuary, or major river.
How does this understanding of watersheds andHUs affect use of the HU framework to extrapolatewater quality data gathered from downstream seg-ments, or any other part of an HU, to illustratenational or regional patterns in status or trends? Tograsp the problem, first consider the ecologicalregions (ecoregions) and 8-digit HUs that cover Texasand parts of adjacent states (Figure 1). The ecoregionson this figure comprise regions within which there issimilarity in the mosaic of biotic, abiotic, aquatic, andterrestrial ecosystem components, with humans con-sidered part of the biota (Omernik, 1995; Omernik etal., 2000). Considering the 8-digit HUs that are com-pletely or partly in Texas, Figure 2 illustrates thosethat are true watersheds and those that are not. Only48 percent of the HUs in this region are true water-sheds.
Next, let us examine four contiguous 8-digit HUsthat occupy similar ecoregions (Figure 3). All fourHUs are centered on the East Central Texas Plains(33), called the “Claypan Area,” which once was large-ly covered by a post oak savanna and is now a region
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of pasture and range. The HUs also occupy, albeiteach to a different extent, parts of the Texas Black-land Prairie (32), a region that once was prairie andnow contains a much higher percent of cropland agri-culture than surrounding ecoregions. If these HUswere true watersheds, because they occupy the sameecoregions, one would expect them to have similarcharacteristics regarding water quality and quantityat their downstream points, compared to watershedsof surrounding ecoregions. However, as Figure 4shows, only two of these HUs (B and D) are truewatersheds. The downstream points on the other two
(A and C) drain areas 15 to 22 times larger than theareas defined by the 8-digit HUs. Also, HUs A and Cdrain ecoregions having characteristics far differentfrom those covered by the HUs. Thus, the quality and quantity of water at the downstream sites for A and Care quite different from those of B and D (Figure 5).
Most aspects of water quality and quantity are sim-ilar for HUs A and C, and for B and D, as well (Figure5). There are, however, fairly sharp differencesbetween the values for A and C and those for B andD. Even though the headwaters of the true water-sheds for A and C are much drier than the points
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21 Southern Rockies22 Arizona/New Mexico Plateau23 Arizona/New Mexico Mountains24 Chihuahuan Deserts25 Western High Plains26 Southwestern Tablelands27 Central Great Plains28 Flint Hills29 Central Oklahoma/Texas Plains30 Edwards Plateau31 Southern Texas Plains32 Texas Blackland Prairies33 East Central Texas Plains34 Western Gulf Coastal Plain
35 South Central Plains36 Ouachita Mountains37 Arkansas Valley38 Boston Mountains39 Ozark Highlands40 Central Irregular Plains
Figure 1. Level III Ecoregions and Eight-Digit Hydrologic Unit Codes (HUs) in Texas and Parts of Adjacent States.
farthest downstream, with mean annual precipitationranging from less than 40.6 cm (16 in) in the headwa-ters to greater than 101.6 cm (40 in) at the mouth ofeach watershed (USDA, 2001), the mean annual dis-charges for A and C are still 8 to 11 times greaterthan those for B and D. Mean annual water tempera-tures are about 5ºC less for A and C than for B and D.Similar examples, many with far greater implications,can be found throughout the United States. The prob-lem obviously has important ramifications for usingHUs as spatial units within which to extrapolate datataken from downstream points. Examples of this type
of use can be seen in products of the U.S. Environ-mental Agency’s Office of Water Index of WatershedIndicators project (USEPA, 2001) the goal of whichwas to characterize regional patterns in water qualityconditions and “show existing water quality acrossthe country” (USEPA 1997, pg. 4). Smith et al. (1997)also used the approach to characterize water qualityin the conterminous United States. Because half ormore of the contributing areas to the HUs are oftenincorrectly represented using this approach, mislead-ing illustrations are presented. As can be seen in Fig-ures 3 through 5, an HU that has a drainage area
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True watershedsEight digit HUState boundaryStream
Figure 2. Eight-Digit HUs That Are True Watersheds and Are Completely or Partially in Texas.
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quite different and many times larger than the area itoccupies is unlikely to have water characteristics sim-ilar to that of an adjacent HU that is a true water-shed. If, for example, the values for downstreampoints in HUs A, B, C, and D (HUCs 12090301,12070102, 12080101, and 12070103) are used toproduce maps of nitrate and nitrite, HUs A and Cwould be represented by values that are three to sixtimes less than that of adjacent HUs B and D. Thearea contributing to HU A includes 24 additional HUsand that contributing to HU C includes 21 additionalHUs.
Only data representative of the region from whichthe data were collected should be used to developmaps illustrating patterns in stream quality. Of thefour HUs highlighted on Figures 3 and 4, only datafrom HUs B and D are representative of the regionthey occupy. By including data from A and C andassigning the values taken from their downstreampoints to their respective HUs, a misleading picture ispresented of the quality one might expect in theregion. For maps of water quality, or any environmen-tal resource, the information used to portray the val-ues must be consistent with the spatial variation offactors associated with differences in the quality ofthe resource of interest.
This example helps clarify the fact that topographicwatersheds are infinite in number. Unlike character-istics such as soils, geology, vegetation, and physiog-raphy, which are amenable to classification,watersheds are spaces associated with points onstreams. There are an infinite number of these pointson streams. To those who suggest managing waterresources and terrestrial biomes by watersheds(catchments), one can ask the question, “For water-sheds of what size?” If the answer is, for instance,roughly 1,000 km2 (386 mi2), what is to be done withthe remainder of the country after the headwaters ofstream systems that are true topographic watershedsof that size have been mapped? If one chooses tobreak these areas into downstream segments ofdrainage basins, such as HUs, these “other” units arenot “watersheds.” They might have been defined bydifferent segments of topographic divides, but they donot enclose the entire drainage areas for the HU’spoints farthest downstream.
As important as it is to distinguish between water-sheds and HUs, it is even more important to recognizethat watersheds and HUs seldom depict areas of simi-lar combinations of characteristics associated withwater quality and patterns in the mosaic of ecosystemcomponents (Omernik and Bailey, 1997; Bryce et al.,1999; Griffith et al., 1999). Although topographicwatersheds always will provide a critical spatialframework for assessing the relative contributions ofnatural and anthropogenic characteristics to the
quantity and quality of water at specific points, theytend to cross regions of similarity in characteristicssuch as soils, geology, physiography, vegetation, cli-mate, and land use – the characteristics associatedwith spatial differences in water quality. This fact hasbeen corroborated in several scientific papers. Cookeand Doornkamp (1974), for example, noted that aland systems approach to environmental manage-ment and planning “is based on the identification ofregional boundaries that are frequently independentof watersheds (catchment divides).” Hornbeck andSwank (1992), recognizing the difference between, aswell as the complementary nature of, watersheds andecoregions (which they called “regional landscapes”),pointed out that watersheds used for ecosystem anal-ysis should be representative of regional landscapes.In writing on the limitations of the watershed frame-work for natural resource management, Hatch et al.(2001) recognized that the geographic characteristicsthat affect soil erosion and water quality often varygreatly within watersheds. They suggested usingecoregions that incorporate these characteristics (e.g.,precipitation, physiography, certain soil properties,and cropping systems).
USING WATERSHED INFORMATIONTO SHOW SPATIAL PATTERNS INENVIRONMENTAL CONDITIONS
The quality and quantity of water at any point on astream reflect the aggregate of the characteristicsupgradient from that point (McMahon et al., 2001).The quality and quantity of water in streams whoseassociated watersheds are completely within a partic-ular ecoregion tend to be comparable, yet differentfrom those in streams of adjacent regions. It followsthen that if regional, state, and national maps depictthe central tendencies and ranges of water qualityand other environmental conditions from representa-tive watersheds within different ecoregions, meaning-ful pictures of the status and trends of water qualityand other ecosystem conditions could result. The keyhere is to ensure the representativeness of watersheddata used to compile these maps. Figure 6 illustratessets of 8-digit HUs that are true watersheds com-pletely, or nearly completely, within specific ecore-gions in Texas. Data from these types of HUs areappropriate for use in compiling maps of water quali-ty for Texas, as well as the United States, if the inter-est is in spatial differences at the level III ecoregion(USEPA, 2002) or Common Ecological Region (CER)(McMahon et al., 2001) level of detail.
In selecting watersheds representative of particu-lar ecoregions, one should not be bound by those that
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happen to be HUs. The watersheds upgradient from apoint on a stream should enclose an area that is most-ly within the same ecoregion as the stream site. Setsof these sites and their associated watersheds, whichcan be termed reference sites and reference water-sheds, should be selected for each region to helpaccount for within region variability (Hughes et al.,1986; Omernik, 1995). These sites and watershedscan be selected using a probability based samplingdesign (Paulsen et al., 1998) or screened to representleast impacted conditions (Omernik, 1995). Referenceareas should be identified for some environmentalconditions or issues (e.g., soil characteristics, forest
condition, land cover change, and wildlife characteris-tics), and for areas where watersheds are eitherimpossible to define or are irrelevant (Hughes andOmernik, 1981; Omernik and Bailey, 1997). A key cri-terion for identifying a reference watershed or area isthat it be representative of the larger region it occu-pies.
Different hierarchical levels of ecoregions can beused to paint different types of pictures. For example,Level I ecoregions (CEC, 1997) can be used to showregional differences in the ranking of environmentalissues. Forest management and the health of anadro-mous fish would be high on the list in the Marine
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Figure 6. Representative Watersheds Within Level III Ecoregions That Are Completely or Partially in Texas.
West Coast Forests, salinity would rank high in theNorth American Deserts, effects of acidification anddevelopment on lakes and streams would be of majorimportance in the Northern Forests (of the north cen-tral and northeastern United States), and pesticideuse and nitrate contamination of ground water wouldrank high in the Great Plains. Some of these issueswould rank low or would not even make the list in dif-ferent Level I ecoregions. Data from reference water-shed sites and areas representative of Levels I, II, III,and IV ecoregions could be used to show patterns inthe status and trends of many characteristics, includ-ing water quality, agricultural practice/riparian andwater quality associations, and land cover change.
CONCLUSIONS
Much has been said about “the watershed” beingthe appropriate unit for ecosystem management. Ruhl(1999, pg. 520) wrote, “More so than any other ecosys-tem management unit that has been proposed in theliterature, lay people and politicians can easily under-stand the concept of a watershed.” Although most sci-entists do understand the concept of a watershed as itrelates to their basic research projects and use it cor-rectly for those purposes, it is clear that most peopledo not understand the concept as it relates to extrapo-lation, reporting, and management. While refiningand subdividing ecoregions throughout the country, Ilearned from my conversations with state and federalresource managers and university scientists thatmost people are not aware that the USGS maps ofhydrologic units are not maps of watersheds. In addi-tion, most are not aware that it is impossible to devel-op a map of nearly equal sized watersheds. Andfinally, most people are not aware of the problematicramifications of using HUs for extrapolation andreporting.
The following points are key to the effective use ofgeographic units for (1) conducting research on land/water associations and terrestrial ecosystems, (2)mapping patterns in the status and trends of waterquality and other environmental conditions, and (3)structuring environmental management and planningstrategies.
• The importance of using topographic watersheds(catchments) for research on land/water associationscannot be overstated. Where watersheds can bedefined and are relevant, which is over more thantwo-thirds of the conterminous United States (Hughesand Omernik, 1981; Omernik and Bailey, 1997), theyare critical for identifying the characteristics that
affect specific water bodies or particular points onstreams.
• Most HUs are not true watersheds. It is impossi-ble to develop a map completely covered with hydro-logic units comprising watersheds of nearly equalsize. Regardless of the hierarchical scale of the HUsbeing mapped, roughly 55 percent will not define truewatersheds. Many HUs drain areas that extend farbeyond their boundaries, which has serious implica-tions for the use of HU boundaries to illustrate thespatial patterns of data collected at each HU’s down-stream point.
• Watersheds rarely correspond to regions of simi-lar characteristics that affect water quality and quan-tity. Ecoregions, on the other hand, are defined toinclude areas of coincidence in combinations of char-acteristics that reflect similarity in ecosystems andecosystem components.
• Watershed and ecoregion frameworks are com-plementary. Watersheds provide the framework fordetermining the land/water associations, and ecore-gions provide the framework for extrapolating andreporting this information.
ACKNOWLEDGMENTS
I wish to thank Jeffrey Comstock for his help in producing thegraphics for this paper. I am also grateful to Carolyn Adams, Park-er J. Wigington, Glenn Griffith, Ann Puffer, and Susan Christie fortheir helpful reviews. The information in this paper has been fund-ed by the U.S. Environmental Protection Agency. It has been sub-jected to review by the National Health and Environmental EffectsResearch Laboratory’s Western Ecology Division peer review andapproved for publication. Approval does not signify that the con-tents reflect the views of the Agency.
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THE MISUSE OF HYDROLOGIC UNIT MAPS FOR EXTRAPOLATION, REPORTING, AND ECOSYSTEM MANAGEMENT
