DATA DEVELOPMENT FOR THE NATIONAL WATERSHED BOUNDARY DATASET: MAPPING MISSOURI S 12-DIGIT HYDROLOGIC UNITS Terry Barney and Mark Caldwell Natural Resources Conservation Service The Missouri Natural Resources Conservation Service (NRCS) and Missouri Department of Conservation (MDC) have completed a major revision of Missouri s hydrologic unit boundaries. This document provides an overview of the revised mapping and describes the criteria applied to the delineation process. Missouri s hydrologic unit mapping has historically been based on expansions of the United States Geological Survey s (USGS) Hydrologic Unit Code (HUC) system. The USGS HUC system divides the United States into four nested levels of drainage units. In the mid-1990 s, the Missouri NRCS added 5 th and 6 th levels to the USGS HUC system following the procedures for watershed delineation outlined in the original draft of NRCS National Instruction 170-304. Guided by a new federal interagency standard for watershed boundary delineation, the current 5 th (11-digit) and 6 th (14-digit) levels have been revised to create new 5 th (10-digit) and 6 th (12-digit) levels featuring enhanced stream network/watershed segmentation and new coding. Missouri s new 12-digit map coverage is part of a larger NRCS effort to create a seamless, consistent and accurate 12-digit hydrologic unit national map, based on sound hydrologic and mapping principles. The 12- digit datasets from each state will be incorporated into a national database called the Watershed Boundary Dataset (WBD). THE HYDROLOGIC UNIT HIERARCHY The primary concept underlying the USGS HUC system and the NRCS (interagency) 12-digit system is that of drainage areas organized in a nested hierarchy by size. A hierarchical hydrologic unit code comprised of 2 digits for each level is assigned to uniquely identify each drainage unit. Drainage units are referred to by their level number or the number of digits representing the level. The USGS HUC system has four levels Level 1 (2-digit), Level 2 (4-digit), Level 3 (6-digit) and Level 4 (8-digits). The NRCS 12-digit system subdivides each USGS Level 4 (8-digit) drainage area into smaller Level 5 (10-digit) units and each 10-digit unit into smaller Level 6 (12-digit) units. In addition to a level number and code, there is a nomenclature for the units within each level: Level 1 A two digit code representing major river drainage systems. For example, 10 - Missouri River. Drainage units referred to as REGIONS. Level 2 A four digit code representing the drainage areas of major tributaries within a region For example, 1028 Chariton-Grand-Little Chariton. Drainage units referred to as SUB- REGIONS. Level 3 A six digit code representing the larger river drainages within a sub-region. For example, 102801 Grand River. Drainage units referred to as BASINS. Level 4 An eight digit code representing the larger drainage systems within a basin. For example, 10280102 Thompson River. Drainage units referred to as SUB-BASINS. Level 5 A ten digit code representing the drainage areas of the larger tributaries within a subbasin. For example, 1028010216 No Creek. Drainage units referred to as WATERSHEDS. Level 6 A 12-digit code representing the smaller drainage areas within a watershed. For example, 102801021603 Crooked Creek. Drainage areas referred to as SUBWATERSHEDS.
MISSOURI HYDROLOGIC UNIT HIERARCHY Table 1 summarizes the number of Missouri hydrologic units by level. Note the changes in the NRCS and interagency nomenclature and the increase in the number of units and decrease in average unit size as one moves down through the hierarchy. Figure 1 shows the geographic distribution of levels across the state. Table 1. Missouri 12-Digit Hydrologic Unit Summary By Level Figure 1. Spatial Distribution of Missouri s 8-, 10- and 12-Digit Hydrologic Units 2
Table 2 breaks down the hierarchical coding for the Silver Fork subwatershed located in Boone County, Missouri. Figures 2, 3 and 4 illustrate the nested hierarchical spatial structure of the 12-digit mapping using the Silver Fork subwatershed. Figure 2 shows the location of Silver Fork s 8-digit sub-basin, one of five 8- digit units on the lower Missouri River between Kansas City and St. Louis. Figure 3 locates Silver Fork s 10- digit watershed Perche Creek and Figure 4 identifies the eight 12-digit subwatersheds within Perche Creek, including Silver Fork. SILVER FORK SUBWATERSHED HYDROLOGIC UNIT CODING 10 Missouri River REGION (Level 1) 1030 Lower Missouri River SUB-REGION (Level 2) 103001 Missouri River Platte to Gasconade BASIN (Level 3) 10300102 Missouri River Mainstem Glasgow to Hermann SUB-BASIN (Level 4) 1030010209 Perche Creek WATERSHED (Level 5) 103001020902 Silver Fork SUBWATERSHED (Level 6) Table 2. 12-Digit Hierarchical Coding Scheme Figure 2. Silver Fork s 8-Digit Sub-Basin M0. River Mainstem Glasgow to Hermann 3
Figure 3. Silver Fork s 10-Digit Nested Watershed Perche Creek 4
Figure 4. 12-Digit Subwatersheds Nested Within the Perche Creek Watershed MISSOURI HYDR0LOGIC UNIT MAPPING BACKGROUND HISTORY Missouri, like many other states, has mapped watersheds beyond the USGS 8-digit level but never to a common, federal interagency standard. The 11-digit and 13-digit coverages were based on hydrologic criteria but delineated and digitized to in-house specifications. The 14-digit mapping and digitizing was guided by the original draft of NRCS National Instruction 170-304, Mapping and Digitizing Watershed and Subwatershed Hydrologic Unit Boundaries. Table 3 summarizes Missouri s hydrologic unit history from the release of the USGS HUC mapping to the current 12-digit coverage. Table 3. Missouri Watershed and Subwatershed Mapping Historical Summary 12-DIGIT MAPPING In 2000, the Missouri NRCS and Missouri MDC began a revision of the NRCS 14-digit hydrologic unit mapping under the auspices of a signed Memorandom of Understanding. The mapping revision began prior to the release of the interagency standard and adopted an early draft of the new standard in 2001. A draft 12-digit coverage was completed in March, 2003, under the finalized standard released in March, 2002. The draft will go through a national certification process, conducted by the NRCS through its National Cartography and Geospatial Center (NCGC), prior to official release of the data. 5
The draft data set is a complete digital hydrologic unit boundary coverage compiled to the 12-digit, subwatershed level for the State of Missouri. The data set consists of geo-referenced digital boundary lines and associated attributes for each 12-digit subwatershed. The 10-digit watershed and 12-digit subwatershed hydrologic unit boundaries provide a uniquely identified and consistently interpreted subdivision of the state s 66 large USGS 8-digit subbasins. Compiled and digitzed, at a scale of 1:24,000, to National Map Accuracy Standards, the coverage features enhanced stream network/watershed segmentation and new sequential coding. Each polygon is attributed with codes for its 8-, 10- and 12-digit levels, name, size, type and modifications. Each arc is attributed with the highest level hydrologic unit it represents, the base map source the arc was delineated on and the metadata ID code that identifies which metadata file applies to the arc. NEW INTERAGENCY STANDARD In March, 2002, the Federal Geographic Data Committee s (FGDC) Spatial Water Data Subcommittee released a final set of interagency federal standards for mapping watershed boundaries, FGDC Proposal, Version 1.0 Federal Standards for Delineation of Hydrologic Unit Boundaries. This standard is a highly modified set of delineation guidelines based on the original NRCS National Instruction 170-304 that served as NRCS s policy on delineating and digitizing watershed and subwatershed boundaries from 1992-2001. Table 4 profiles the Spatial Water Data Subcommittee responsible for the development of the interagency standard. Table 4. Interagency Development of FGDC Proposal, Version 1.0 6
NRCS has directed each state to compile a draft 12-digit watershed map, based on the FGDC s interagency standard. Each state s draft coverage will be subjected to a national certification process conducted by NRCS through its National Cartography and Geospatial Center (NCGC) prior to official release. The certification process will consist of two phases. Phase 1 will be a provisional verification conducted by the NCGC staff and interagency review team. A successful provisional review will initiate a Phase 2 final verification conducted by the FGDC Subcommittee on Spatial Water Data. Upon final approval of a data set by the FGDC, the Chairman of the Subcommittee on Spatial Water Data will send a letter to the state s NRCS State Conservationist stating the hydrologic unit coverage is approved, meets the national guidelines and will be placed in the national Watershed Boundary Dataset (WBD) database. MISSOURI MAPPING CRITERIA FGDC PROPOSAL, VERSION 1.0 PRIMARY CRITERION: DRAINAGE AREA In compiling watershed boundaries over large geographic areas, attempts to rigorously and consistently apply a narrowly defined watershed concept to the delineation process are quickly confounded. Thus, most projects are guided by a combination of delineation criteria with special emphasis placed on those that best meet the user s primary needs. FGDC Proposal, Version 1.0 has imbedded many of the widely accepted watershed delineation concepts into its guidelines and procedures for maximum flexibility but does strongly emphasize drainage unit size and numbers. This is perfectly understandable, given the objective of developing a national coverage based on the USGS HUC system. Size constraint is one of the least complicated ways to develop and maintain an ordered hierarchy of drainage areas. However, this unit sizing approach often discourages more desirable hydrologic breaks, based on other criteria, because they create too many nonconforming drainage units in terms of unit size and total unit numbers. MISSOURI MAPPING PRIMARY CRITERION: STREAM NETWORK/WATERSHED SEGMENTATION The Missouri 12-digit mapping has tried to balance the interagency standard s emphasis on unit size with an equal emphasis on properly segmented watershed boundaries. In 1999, responding to partnership critiques of its widely distributed and heavily utilized 14-digit hydrologic unit coverage, the Missouri NRCS initiated an interagency planning effort to revise its 14-digit coverage. Drawing on its combined experience with the Missouri NRCS 14-digit coverage, the planning group identified stream network/watershed segmentation as the most important criteria to apply to the revision mapping effort. The 12-digit compilation process drew on a combination of five primary delineation criteria to support the development of properly segmented hydrologic units within a hierarchical structure. DELINEATION BY DRAINAGE AREA First and foremost, the Missouri 12-digit coverage is a hierarchical system based on the USGS 8-digit HUC system. Nested systems require watersheds to decrease in size and go up in number as one moves down through the hierarchy. This concept, as historically applied in the USGS HUC system and currently incorporated in FGDC Proposal, Version 1.0, expects the number of drainage units in each level to fall within a fixed range and the size of each unit to fall within a fixed range. DELINEATION TO STREAM CONFLUENCES The 12-digit Missouri coverage has placed most of its drainage unit outlets at stream confluences. A unique watershed can be determined for any point on a stream. This is often done to determine effective drainage areas and discharge estimates for impoundments and other control structures. However, for multi-purpose watershed mapping, confluences with larger streams make a more intuitive and practical focal point for outlets. 7
DELINEATION BY STREAM HIERARCHY Implicit in the placement of outlets at stream confluences with larger streams is the idea that drainage breaks should be made that reflect the stream hierarchy. The natural network of streams forms a hierarchy of channels. Shorter streams with small drainage areas near the watershed divides join to form larger streams that inherit all of the drainage areas of the feeder streams, which in turn join with larger streams, etc., leading to the main channel (largest stream) in the watershed. This natural hierarchy is referred to as stream order and various systems have been devised to describe this phenomenon. DELINEATION BY STREAM NETWORK (SEGMENTATION) This concept is a combination of delineating to confluences and delineating by stream hierarchy. It forces more precise delineation of watersheds feeding significant main channel tributaries and requires the delineation of main channel drainage areas between tributaries. This concept assumes that because streams form a nested hierarchy, their watersheds can be delineated in a hierarchy directly reflecting that of the streams. Watersheds thus delineated can be thought of as a series of hierarchical drainage segments. The Missouri 12-digit coverage s 5 th and 6 th levels are a product of strictly enforced segmentation. Proper segmentation allows all flow above and below a drainage unit outlet to be accounted for at each level. When segmentation is ignored or poorly executed, it is impossible to ask questions such as What is the main channel drainage area above the outlet of this tributary? or We know chemical X is coming out of this tributary, what other permitted sites above this tributary are discharging the same chemical to this point on the main channel?. With proper segmentation, these questions become simple point-in-polygon queries. Proper segmentation also allows watershed coding to proceed in a strict pattern of drainage sequence. One can identify and code the drainage units such that these three watersheds drain to this watershed, etc. The trade-off with this approach is the creation of numerous and typically undersized units on main channels. This runs counter to the drainage area concept favoring unit sizing over strict segmentation. Proper segmentation requires the delineation of relatively small drainage units on main channels at both the 10- and 12-digit levels. Frequently, two or more tributaries, each with their own delineated watershed, join the main channel within short distances of each other. Quite often, these necessary but small drainage areas fall below established minimum mapping sizes and compromise the primary goal of the drainage area concept nearly equally sized units at each level of the drainage hierarchy. Figure 5 illustrates the segmentation concept at the 12-digit level. The 12-digit subwatershed named Perche Creek Middle 1 is a small mainstem drainage area between the much larger Silver Creek and Rocky Fork/Bear Creeks tributaries. ( See Figure 4 for a full view of the watersheds) DELINEATION BY WATERSHED DIVIDE This is a widely accepted and frequently used criterion that is seldom debated and incorporated into most watershed delineation schemes, including FGDC Proposal, Version 1.0. In combination with delineating to a confluence or single outlet, this approach produces the classic watershed. Drainage areas are clearly defined by surface topographic divides (a k a breaks or ridges). Water flows from higher to lower elevations within drainage areas separated by divides. There are no subdivided upstream areas. The divides are identified by elevation contours on topographic maps, making delineation of the upper and lateral boundaries precise. The level of precision achieved in delineating from the crisp lateral edges to the outlet point and locating boundaries on broad divides is extremely dependent upon the scale and contour interval of the topographic map base. 8
FIGURE 5. Segmented 12-Digit Subwatersheds in the Perche Creek Watershed In Figure 7, the Perche Creek 12-digit subwatersheds have been subdivided into properly segmented 14-digit (Level 7) drainage units. A properly segmented level makes additional subdividing much easier. Note the very small mainstem units delineated on Perche, Rocky Fork and Hinkson Creeks to maintain correct segmentation at the 14-digit level. 9
Figure 7. 12-Digit Perche Creek Subwatersheds Divided Into 14-Digit (Level 7) Units 10
SUMMARY OF MISSOURI 12-DIGIT HYDROLOGIC UNIT MAPPING Missouri has completed a revision of its 14-digit hydrologic unit map under a Memorandum of Understanding between the Missouri Department of Conservation, Fisheries Division and the Missouri Natural Resources Conservation Service, Resources Inventory and Assessment Section. The revision is based on federal interagency standard FGDC Proposal, Version 1.0 Federal Standards for Delineation of Hydrologic Unit Boundaries. This standard sets forth requirements for expanding the USGS 4 level (8-digit) HUC system to 6 levels (12-digits). Current 5 th (11-digit) and 6 th (14-digit) levels have been revised to create new 5 th (10-digit) and 6 th (12-digit) levels. The 12-digit revision keeps all of the original USGS 8-digit sub-basin boundaries. Significant portions of the 14-digit hydrologic unit line work are retained. The revised mapping features enhanced stream network /watershed segmentation. All 10- and 12-digit coding is new. All 10- and 12-digit hydrologic units are named. The revised map consists of approximately 430 10-digit watersheds with an average size of 101,811 acres and 1,981 12-digit subwatersheds averaging 22,259 acres in size. The final draft 12-digit coverage, including proposed edge matching to adjoining states has been submitted to the NRCS National Cartography and Geospatial Center (NCGC) for certification in conjunction with the Federal Geographic Data Committee s Spatial Water Data Subcommittee. Upon certification, the Missouri 12-digit coverage will be placed in the national Watershed Boundary Dataset (WBD). The objective of the WBD is a seamless, consistent and accurate 6 level (12-digit) national map based on sound hydrologic and mapping principles. Primary delineation criteria: - 1:24,000 scale USGS 7.5 minute quadrangle mapping base meeting NMA standards - all drainage divides based on surface topography and selected man-made features - stream network/watershed segmentation strictly enforced - boundaries cross streams at confluences with larger streams - no delineations down stream centerlines - hydrologic units numbered sequentially from upstream to downstream - consistent numbering (coding) scheme no gaps 11