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HUC10 Shell: What is it, why we created it, and how is it used?

Figure 1. Appalachian National Scenic Trail showing HUC10 shell and area of focus (dark blue).
Figure 1. Appalachian National Scenic Trail showing the outline of the HUC10 shell.


The HUC10 shell (Figure 1), or the general frame of reference used to establish an area of interest around the Appalachian National Scenic Trail (APPA), is the "outer" boundary of all HUC10 hydrologic units that are in close proximity to the APPA land base. The HUC10 shell is based on units defined by the USGS at the fifth level of the Hydrologic Unit Code (HUC) system, with each being given a discrete 10-digit code (HUC10).

Hydrologic Unit System

USGS defines a hydrologic unit to be "…a drainage area delineated to nest in a multi-level, hierarchical drainage system. Its boundaries are defined by hydrographic and topographic criteria that delineate an area of land upstream from a specific point on a river, stream or similar surface waters. A hydrologic unit can accept surface water directly from upstream drainage areas, and indirectly from associated surface areas such as remnant, non-contributing, and diversions to form a drainage area with single or multiple outlet points…"

Hydrologic Units Are Not Watersheds

Though hydrologic units are sometimes referred to as watersheds, this is not always true because hydrologic units do not always include all upstream components of a true watershed (Omernik 2003). This is acknowledged by USGS in the following statement: "…Hydrologic units are only synonymous with classic watersheds when their boundaries include all the source area contributing surface water to a single defined outlet point…"


Even though hydrologic units, like watersheds, are based on surface water patterns, hydrologic units must also remain within a specified size range. At the HUC10 scale, the size ranges from 40,000 - 250,000 acres (NRCS 2008). Units that exceed the specified size range are truncated. While this occurs with great frequency throughout the HUC system, it is somewhat less likely to happen in the HUC10 shell for three reasons:

  1. HUC's that originate at the 'top' of a 'classic' watershed are watersheds themselves, up to the point that the HUC size criteria is reached (Figure 2, watersheds 1 and 2). Subsequent HUC's, those that do not originate at the top of a classic watershed, are not considered to be watersheds themselves because not all upstream elements are included in the subsequent HUC's (Figure 2, watershed 3)
  2. the midline of the shell, where the APPA is generally located, marks the divide between watersheds that flow in opposite directions (i.e., HUC's that originate at the midline of the HUC10 shell are watersheds), and
  3. there is typically only one hydrologic unit between the midline and the outer boundary of the HUC10 shell.
Figure 2. HUC10 units.
Figure 2. HUC10 units. Hydrologic units 1 (not fully visible) and 2 are true watersheds, and converge with one another at point "A." Hydrologic unit 3, which terminates at point "B," is not a true watershed because it does not include all upstream surface water components. To be considered a true watershed, hydrologic unit 3 would need to be combined with hydrologic units 1 and 2.

While units inside the HUC10 shell are generally complete watersheds (Figure 2, watersheds 1 and 2), the HUC10 shell does contain some 10-digit HUC units that illustrate the caution raised by Omernik (2003; Figure 2, watershed 3). The inclusion of units that are not true watersheds does not diminish the value of the HUC10 shell, however, because the utility of the HUC10 shell is not dependent on each element actually being a true watershed. Rather, the HUC10 shell is based on the proximity of hydrologic units, as defined by USGS, to the Appalachian National Scenic Trail. Thus, any HUC10 element within the prescribed distance of the APPA land area (see below) was used to create the HUC10 shell, regardless of whether the unit was, or was not a true watershed.

What Defines the HUC10 Shell?

The HUC10 shell is the distal boundary of all HUC10 units that are within 5 miles of the APPA land area, and was originally developed to identify water resource data needed to describe the condition of water resources on APPA (Argue et. al. 2011). Following this initial application, the shell was quickly expanded for other purposes, including the acquisition of forest health data. We did consider other levels of the HUC system and found that a boundary based on HUC08 units was too expansive (Figure 3), while an area of interest based on HUC12 units was too small (Figure 4). A boundary that is too large incorporates more data, but much of those data originate far from the APPA and at some point cease to be representative of resources found on APPA. Conversely, a smaller area of interest might ensure that the available data are more representative of APPA resources, but the volume of available data rapidly decreases. The latter is the case for a shell based on HUC12 units.

Figure 3. HUC08 units with outline of HUC10 shell.
Figure 3. HUC08 units with outline of HUC10 shell.

Selection Method

Creating buffers around the APPA land area has historically supported a variety of needs, ranging from determining the population within a specified distance from APPA, to identifying scientific data. Typically, such buffers were established around the APPA using uniform distances like, 5, 10, or 15-miles. While relatively easy to construct, buffers based on distance alone are arbitrary and tend to miss important ecological features. Creating an area of interest around APPA based on hydrologic units is advantageous because hydrologic units are derived from natural landscape features.

The term 'area of interest' is preferred over buffer because it more accurately describes the purpose of the shell, which is to identify an area where data used to make inferences about the condition of ecological resources on APPA itself are found. This is a key point because unlike other parks, we rely on existing data to describe the resources on APPA, thereby avoiding the great cost and difficulty associated with collecting 'new' APPA data. For example, annually collecting forest health data on APPA lands would be prohibitively expensive and logistically challenging, whereas data collected by the U.S. Forest Service (USFS) Forest Inventory Analysis (FIA) program are freely available.

In defining an area of interest around APPA, we believe the HUC10 shell balances two fundamental requirements. First, the HUC10 shell is large enough to ensure ample data availability, and second the shell describes a boundary that maintains relevance to the resources that characterize the APPA itself. The other two boundaries we considered, HUC08 and HUC12, were not able to simultaneously fulfill both requirements.

Figure 4. HUC12 units with outline of HUC10 shell.
Figure 4. HUC12 units with outline of HUC10 shell.

It is important to understand that the HUC10 shell is a starting point. There is no guarantee that the HUC10 shell includes all items of interest related to APPA, or that it excludes all items that are not of interest. For some projects a larger or smaller area of interest may be needed, but the HUC10 shell is a good starting point, is adequate for most applications, and using the HUC10 shell encourages consistency between otherwise disparate projects.

Beyond a Boundary

The utility of the HUC10 shell has expanded beyond just establishing an outer boundary of an area of interest. The HUC10 shell can be partitioned by the internal HUC elements themselves, by state boundaries, by elements of the Bailey (1994) ecoregional system, or by any other system that is pertinent to the analysis being performed. Finding an appropriate balance depends on the project in question. One approach that has been successful is to divide the HUC10 shell using units appropriate for the given analysis, but then only analyze that portion of the elements that actually intersect the land area itself (Table 1, Figure 5). We have done this to analyze forest health data using ecoregional subsections. The subsection scale is particularly useful for forest health analysis because there are sufficient FIA plots within each subsection, and when the FIA plots are sampled the USFS programmatically associates all data by subsection. Despite the clear advantages of the subsection approach, to analyze all 50 subsections that are within the HUC10 shell would be a large task. We overcame this problem by limiting the analysis to only those subsections that directly intersect the land administered by the APPA, which reduces the number of subsections to 20 (Table 1, Figure 5).

Figure 5. Ecoregional subsections that intersect the APPA land area (blue), and subsections that do not intersect the APPA land area (red).
Figure 5. Ecoregional subsections that intersect the APPA land area (blue), and subsections that do not intersect the APPA land area (red).

Another consideration is the spatial area appropriate for the analysis being conducted. The USFS, the agency responsible for the FIA program, states that "…there is one forest plot approximately every 6,000 acres…" and that "…FIA data are statistically useful from the National scale down to areas of about 200,000 acres…" Given that, our forest health analysis based on FIA data within the 20 intersecting subsections is well within the specified 200,000 acre threshold, but attempting to do a similar analysis based on individual HUC10 units would not be advisable because the average size of units associated with the HUC10 shell is 113,300 acres (Table 2) – well below the 200,000 acre limit recommended by USFS. Maintaining a minimum size is another justification for filtering subsections, or any other approach to dividing the HUC10 shell, to only those elements that intersect the land area. Of the 30 subsections that do not intersect the land area (Table 1), most are generally small, ranging from 9.86 to 558,885 acres with a mean (x̄) of 158,869 acres and a median of 129,236 acres. Had the non-intersecting subsections been included in our forest health analysis, our ability to make meaningful inferences about the condition of resources in several of the smaller subsections would have been questionable. Other data analysis projects are likely to have similar constraints that will dictate the aerial thresholds that must be observed.

Sampling Pack

To go along with the HUC10 shell, we have developed a series of spatially balanced data sets that are designed to help researchers develop sampling strategies within the HUC10 shell. These datasets, known collectively as the Appalachian Trail GIS sampling pack, rely on the GRTS (Generalized Random Tessellation Stratified) design methodology, and are available at the following LINK. The sampling pack includes a copy of the HUC10 shell.

Unit of Division Intersects APPA Do Not Intersect APPA Total
HUC08 62 7 69
HUC10 176 60 236
HUC12 430 787 1,217
Province 3 2 5
Section 7 12 19
Subsection 20 30 50
Table 1. Example units of division for the HUC10 shell.

Is the HUC10 Shell Representative?

We developed the HUC10 shell to help identify data that are representative of resources on APPA itself. If sufficient data were available from APPA managed lands we would not need to look for representative data, but after a thorough data mining effort we determined that using data from outside the APPA land area was the only affordable way to characterize the resources on APPA. The FIA data set provides a good example of data scarcity. Out of 23,710 FIA forest plots within the HUC10 shell only 235 are on APPA lands. Other datasets, such as water resources, exhibit the same pattern where there is an abundance of data available from within the HUC10 shell but relatively few data from the APPA land area.

Unit of Division Acres
Range Mean (x̄) SD
Min Max
HUC08 68,069
HUC10 39,630
HUC12 1,720
Provinces 700,908
Section 3,563
Subsection <10
Table 2. Units of division, range, average, and standard deviation of different land area divisions inside the HUC10 shell.

With this strategy we assume that resources within the HUC10 shell, but not within the land area managed by APPA, are substantially the same as the resources found on the APPA. In making this assumption we do recognize that it is unrealistic to think that in all instances the entire HUC10 shell can be treated as a single unit, which is why we advocate for dividing the HUC10 by ecoregions or some other way to create smaller units and overcome this limitation.

In one respect, we know that at the 'whole' HUC10 scale the two zones (On APPA vs. Off of APPA) do differ. Because the HUC10 shell is based on hydrologic units, with the trail generally following the ridge line that longitudinally bisects the shell, it is reasonable to assume that ground elevations will generally decrease as you move away from the trail centerline. This is perhaps the single instance where the conclusions reached at the scale of the entire HUC10 are transferable to smaller scales. Following a comparison between On vs. Off APPA elevations we have found that the median elevation of lands off of APPA is significantly lower than the median elevation of lands on APPA (Figure 6a).

Figure 6. a - Comparison of median elevations from 'Off' APPA to 'On' APPA (p = 0.000000000005643); b - Comparison of mean (x̄) elevations (m) 'On' APPA vs. 'Off' APPA, and overall mean (x̄).
Figure 6. a - Comparison of median elevations from 'Off' APPA to 'On' APPA (p = 0.000000000005643); b - Comparison of mean (x̄) elevations (m) 'On' APPA vs. 'Off' APPA, and overall mean (x̄).

Other Considerations

The process we followed to generate the HUC10 shell may have a rational basis, but it isn't perfect. When looking at the HUC10 shell some users express concern that some areas that seem to extend too far from APPA are nonetheless included. While there are examples of small extensions from APPA, that is not true of the boundary in general. It is also important to recognize that the boundary is the result of a process that was free of arbitrary manipulation, and while it might seem advantageous to simply remove extensions that have no apparent relationship with the APPA, that would introduce an arbitrary element into the process and thereby violate one of the most important tenets upon which we relied to create the shell. Consequently, NETN has avoided tampering with the original conformation of the HUC10 shell by retaining all outer boundary components, regardless of how far they might extend from the footpath.

Literature Cited

Argue, D.M., Pope, J.P., and Dieffenbach, Fred, 2011, Characterization of the water quality of headwater streams along the Appalachian National Scenic Trail and within adjacent watersheds, Maine to Georgia: U.S. Geological Survey Scientific Investigations Report 2011-5151, 63 p., plus CD-ROM. (Also available at

Bailey, R.G.; Avers, P.E.; King, T.; McNab, W.H, eds. 1994. Ecoregions and subregions of the United States (map). Washington, DC: U.S. Geological Survey. Scale 1:7,500,000. Colored. Accompanied by a supplementary table of map unit descriptions compiled and edited by McNab, W.H. and Bailey, R.G. Prepared for the USDA Forest Service.

Natural Resource Conservation Service (NRCS). 2008. Delineation of 10- and 12-Digit Hydrologic Unit (HU) Boundaries. SD-FS-54. (Available at

Omernik, J.M. 2003. The Misuse of Hydrologic Unit Maps for Extrapolation, Reporting, and Ecosystem management. Journal of the American Water Resources Association. (JAWRA) 39(3):563-573.

Last Updated: December 30, 2016 Contact Webmaster