Creating Hydrologic Unit Boundaries for Wyoming: A Semi-Automated Approach


Wendy L. Berelson
Paul A. Caffrey
Jeffrey D. Hamerlinck

Abstract


The Spatial Data and Visualization Center at the University of Wyoming is using a customized ArcView approach to define hydrologic unit boundaries in a statewide coordinated effort to develop the national Watershed Boundary Dataset (WBD) for Wyoming. The first phase of the process is semi-automated using National Elevation Dataset (NED) derived watersheds created by USGS EROS Data Center. These watersheds are used to create draft boundaries at a 1:100,000-scale. The second phase of the process uses the DRG-Es (Digital Raster Graphic – Enhanced) as a backdrop for digitizing at 1:24,000-scale and the National Hydrography Dataset (NHD) as a reference for watershed naming.

Background

Beginning in the early 1970’s the USGS delineated four levels of nested watersheds across the United States (Seaber, P.R., Kapinos, F.P., and Knapp, G.L., 1987). The four levels are regions, sub-regions, basins and sub-basins (basins and sub-basins were formerly referred to as accounting units and cataloging units). Hydrologic units are nested within each other from the smallest (sub-basin) to the largest (basin). Each level has a two digit hydrologic unit code (HUC). These delineations are often referred to as 8-digit or 4th-level HUCs.

More recently, Federal Agencies have been coordinating development of digital spatial water data under the charter of the Federal Interagency Subcommittee on Spatial Waters Data (SSWD). The SWDD has identified the National Watershed Boundaries Dataset (WBD) as an important dataset to include in the National Spatial Data Infrastructure (Rea, 2000).

When completed, the WBD will provide a high resolution, nationally consistent, seamless base layer for watersheds within the United States. The WBD will further divide sub-basins into watersheds and sub-watersheds also known as 5th and 6th level hydrologic units. The watersheds and sub-watersheds will consist of 10 and 12 digit HUCs, respectively. Mapping of larger-scale watersheds will provide accurate watershed boundaries for use by federal and state agencies in local resources planning and management.

Wyoming’s Approach

The Spatial Data and Visualization Center (SDVC) at the University of Wyoming is leading Wyoming’s coordinated effort to create a statewide watershed boundary dataset. Other participants in the multi-agency effort include the Wyoming Department of Environmental Quality (DEQ), Bureau of Land Management (BLM), Natural Resources Conservation Service (NRCS), United States Geologic Survey (USGS) and United States Forest Service (USFS), as well as representatives from surrounding states. Individuals from the above organizations make up Wyoming's Interagency Hydrologic Unit Group (IHUG).

To facilitate the creation of this data layer, SDVC has developed several tools in ArcView to use during the two phases of the project. Phase I involves delineating watersheds at 1:100,000-scale using watersheds derived from the National Elevation Dataset (NED). Phase II involves digitizing the watersheds at 1:24,000-scale using the aggregated NED derived draft 100,000-scale watersheds and the Digital Raster Graphics Enhanced (DRG-E) as background data layers. The majority of process steps use ArcView 3.2 however, ArcInfo 8.0 is used for creating topology and conflating data.

Protocol for the delineation follows the Federal Standards for Delineation of Hydrologic Unit Boundaries (Natural Resources Conservation Service, 2001). Sizing standards call for the creation of 1:24,000-scale 5th level (40,000-250,000 acres) and 6th level (10,000-40,000 acres) level hydrologic unit boundaries. Boundaries are being delineated on a basin-by-basin priority scheme developed by the IHUG, and dictated in part by available funding and management needs.
 

Phase I Methods –1:100,000-scale Semi-Automated Draft Delineations

Process Steps

Draft 1:100,000-scale (100K) watershed boundaries are created using watersheds derived by the USGS EROS Data Center (EDC) from the National Elevation Dataset (Verdin, 2000). EDC used a 5,000 pixel threshold to delineate the watersheds which average around 2000 acres. SDVC has developed tools to aggregate the derived watersheds and to attribute the polygons. These customized tools were created in ArcView using Avenue scripting language.

The first set of tools allow an individual to select the desired derived watersheds and aggregate them to the appropriate acreage for 5th level or 6th level watersheds (Figure 1). After selecting the watersheds to aggregate, the sum tool is used to calculate the total acreage (Figure 2). The user can repeat the selection process until the acreage and watershed outlet are acceptable. Once the user is satisfied with the selection, the polygon is attributed with the last two digits of the hydrologic unit code (HUC) number at the appropriate level. This is done by using the 5th or 6th level tools, which assign the 10 and 12 digit codes, respectively (Figure 2). The attributes are later used to dissolve the smaller NED-derived watersheds into the larger 5th and 6th level watershed polygons that make up the draft 100K boundaries. The 4th level tool is used to assign the 4th level HUC by querying the 250k hydrologic unit boundary coverage (Figure 2). The final process steps for Phase I involve running several scripts that manipulate the attribute table to append the 4th level HUC number to the two digits of the 5th and 6th level HUC to get the final 10-digit and 12-digit draft HUC.
 

100k Aggregation example
Figure 1. 100K aggregation example.
24k attribution tools

Figure 2. 24k attribution tools.

A comments tool was created to allow the user to describe artifacts in the watershed primarily resulting from areas of low relief (e.g. edit outlet, non-contributing area). In areas where there are errors in the NED and therefore in the watershed boundaries, the aggregated polygons are edited so that the boundary is more clearly defined (U.S. Geological Survey, 2000). Areas where editing is needed may be identified by querying the comment field.

To aid in determining the appropriate locations for watershed boundaries and outlets the 100K NHD is used as a backdrop and where necessary the 24K DRG-E’s are reviewed. For determination of some outlet locations a screen capture of the area in question is sent to the IHUG and other regional agency representatives with local knowledge for consultation.

Coordination

Once the draft 100K watershed boundaries are completed for an IHUG priority area, shapefiles and/or maps are made for review and comment by the IHUG and other cooperators (e.g. USFS, NPS, NRCS field offices and surrounding states). A coordinating meeting is then held with the IHUG, cooperators and the public to discuss recommendations from reviewers. Any necessary edits are made to the linework on a master map.

QA/QC

After the review meeting the 100K linework is updated. The updated linework is checked against the master review maps to verify that all of the changes were made. The attribute numbers are then reviewed to ensure that the HUCs are numbered from upstream to downstream.

Phase II Methods– 1:24,000-scale Delineations

Digitizing

Watershed boundaries at 24K are created in ArcView with a line shapefile using heads-up digitizing. The 100K draft lines are used as a guide for the location of the boundary and the contour lines from the enhanced digital raster graphics  (DRG-E) are used for selecting the actual location to digitize the line (Figure 3). Where necessary, the Digital Orthophoto Quarter Quads (DOQQs) are used to resolve watershed boundary questions.

Digitizing QA/QC

All of the digitized lines are reviewed internally by SDVC. A 12-meter buffer is used to validate that linework meets national map accuracy standards (US Geological Survey 1999). Errors found during the internal review are corrected. An external review is then held by the IHUG. The IHUG reviews a random 10% of the linework after which any additional changes are made. To date, the errors found have totaled less than 3% of the lines reviewed.
 

Example of 24K digitized line with 100K draft line and DRG-E backdrop.

Figure 3. Example of 24K digitized line with 100K draft line and DRG-E backdrop.

After review, the 24K linework and the 100K draft boundaries are converted to coverages in Arc/Info. The 100K HUC number attribute is then conflated to the 24K linework using the PUT command in ArcEdit. As required by the federal guidelines, double precision is used for all processes.

Attribution Tools: Naming

SDVC has developed several tools in ArcView to facilitate attribution of the hydrologic units. Using Dialog Designer, a customized interface was created to help the user name watersheds. First, the user selects a watershed or sub-watershed. The100K NHD name attribute is queried and a list of the stream names from the NHD is returned (Figure 4). The streams in the list are ranked according to the NHD stream level attribute. For clarifying the location of the streams, the user can select the stream on the list to highlight the stream in the view. Using the listed information the user can determine the appropriate name for the watershed. The user enters the name for the watershed and the name is assigned. In some locations there are no NHD name attributes and the DRG-E is consulted to aid in selecting the appropriate name.

Naming tool interface.

Figure 4. Naming tool interface.

Attribution Tool: Downstream HUC

Another set of tools was created in ArcView to facilitate selection of the downstream HUC (Figure 5). 24K hydrography does not exist for most of Wyoming. Since the watershed boundaries are digitized at 24K scale, it was determined it would be more efficient to select the upstream and downstream hydrologic unit (HU) visually than to use the 100K NHD and try to automate the process. The downstream tool is selected and the user is prompted to pick the downstream HU. The user then selects the upstream tool and is prompted to select the upstream HU. The downstream number is then assigned to the upstream, currently selected, HU. The user is also prompted as to whether they are assigning the downstream HU to a 5th or 6th level HU.
 

Downstream hydrologic unit interface
Figure 5. Downstream hydrologic unit interface.

Attribution Tool: Others

The above noted attributes are added as IHUG priority areas are completed. Other attributes will be added once the entire state has been delineated at the 24K-level. The additional polygon attributes will include acres, states and non-contributing areas. Arc attributes required by the Federal Standards will also be added. Tools to facilitate this additional attribution will be created as needed.

Attribute QA/QC

HUC number attributes are verified using several processes. Currently, a script is run that checks for duplicate attributes. HUC numbers are verified by autolabeling and then visually checking the 24K HUC labels against the 100K HUC labels. Several scripts are in the process of being written which will automate the verification process for checking the HUC attribute as well as the other attributes.

Conclusion

The methods and tools noted above have provided an efficient and effective way for delineating watersheds at 1:24,000-scale in Wyoming. Using the NED-derived watersheds to create our draft 1:100,000-scale HUBs has saved a considerable amount of time by eliminating the need to manually create hard copy boundaries. Using the DRG-Es has allowed us to speed up the process of digitizing the boundaries at 24K scale. The extensive QA/QC methods used by SDVC ensures that our linework is of the highest quality. The Wyoming Watershed Boundary Dataset is scheduled for completion in December 2001. For information please visit www.sdvc.uwyo.edu/huc.

Acknowledgments

Funding for this project was provided by SDVC the Wyoming Department of Environmental Quality and the USDI Bureau of Land Management. We could like to thank the members of the IHUG and other cooperators for providing assistance.

References

National Resources Conservation Service. 2001. Draft Federal Standards for Delineation of Hydrologic Unit Boundaries.

Rae, Alan. 2000. Development of a National Watershed Boundary Dataset. In Proceeding of the Twentieth Annual Esri International Users Conference.

Saber, P.R, Kapino, F.P, and Knapp, G.L. 1987. Hydrologic Unit Maps: U.S. Geological Survey Water-Supply Paper 2294, 62p.

U.S. Geological Survey. 1999. National Mapping Accuracy Standards. U.S. Geological Survey Fact Sheet FS-171-99.

U.S. Geological Survey. 2000. Creating a Standardized Watersheds Database for the Lower Rio Grande/Rio Bravo, Texas. U.S. Geological Survey Open-File Report 00-065, p.10.

Verdin, Kristine. 2000. Development of the National Elevation Dataset - Hydrologic Derivatives (NED-H). In Proceeding of Twentieth Annual Esri International Users Conference.

Author Information

Wendy L. Berelson
Assistant Research Scientist
University of Wyoming
Spatial Data and Visualization Center
P.O. Box 4008, University of Wyoming
Laramie, WY 82071-4008
Phone: 307-766-2735 Fax: 307-766-2744
e-mail: berelson@uwyo.edu WWW: www.sdvc.uwyo.edu

Paul A. Caffrey
Assistant Research Scientist
Spatial Data and Visualization Center
P.O. Box 4008, University of Wyoming
Laramie, WY 82071-4008
Phone: 307-766-2735 Fax: 307-766-2744
e-mail: caffrey@uwyo.edu

Jeffrey D. Hamerlinck
Associate Director
Spatial Data and Visualization Center
PO Box 4008, University of Wyoming
Laramie, WY  82071-4008
Ph: 307.766.2532  FAX: 307.766.2744
Email: itasca@uwyo.edu