In Reply Refer To:
Mail Stop 415

September 24, 2012

OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 12.07

SUBJECT: Guidance on Determination and Revision of Watershed Drainage Areas

Drainage areas stored in the Nathional Water Information System (NWIS) database for the vast majority of U. S. Geological Survey (USGS) streamgages were determined by delineating watershed divides on maps, mostly using the USGS 1:24,000 topographic map series. Consequently, the accuracy of these drainage areas is dependent on the accuracy of topographic and positional features on the maps. Considering that maps and methods have inherent inaccuracies, conventional practice has been to only report drainage areas to a limited number of significant figures. Initial guidance on determination and publication of drainage areas was provided by the Federal Inter-Agency River Basin Committee (1951). This guidance was reiterated and expanded on by Novak (1985, pp. 43-46), and states that, if available maps permit this precision, drainage areas should be published to the nearest square mile for areas greater than or equal to 100 square miles (mi2); to the nearest tenth of a square mile from 10 to 100 mi2, and to the nearest hundredth of a square mile for areas less than 10 mi2. This practice has been adequate for most practical applications and introduces no more than 1 percent error into any subsequent computations. However, a disadvantage of this practice is that, when streamgages are close together, sometimes their drainage areas may be listed as the same because the incremental area between upstream and downstream streamgages is insufficient to increment the rounded downstream drainage area.

The development of new topographic and positional mapping technologies such as LiDAR and Global Positioning Systems and computational technologies such as Geographic Information Systems have raised questions about the conventional practice of limiting precision of drainage area determinations. A corollary question is when should a previously published drainage area be revised?

Precision of reported drainage areas

With the development of newer technologies and the availability of the 12-digit Watershed Boundary Dataset (WBD), drainage areas may be determined easier and faster, and computations may have more accurate and precise horizontal and vertical positions at higher resolutions than drainage areas determined from manual methods used in the past. In theory, results could be reported accurately to several more significant figures when the source data justify that precision. However, the same principles that led to the guidelines above still apply to the reporting of drainage areas, regardless of improvements in the accuracy of data sources and methods. Most applications, including annual data reports, should not or need not require publication of drainage areas with more significant digits than the existing guidance allows. However, drainage areas can be published with greater precision in rare cases where the additional precision is needed to differentiate among stations and the data sources and computation methods can support it. Another exception to the reporting rule is when drainage areas are provided by cooperating agencies. In those cases, drainage areas should be reported to the same degree of precision used by the agencies involved.

Drainage area revisions

The data source and computation method of drainage area determinations, if known, should be recorded in the DRAINAGE AREA element in the Site Information Management System (SIMS). This is particularly important because there is no existing means to document the source or method in NWIS. The explanation of source will be variable and can be as detailed as appropriate, and examples may include:

Novak (1985, p. 46) states that published drainage areas should be revised as soon as possible when found to be grossly in error (more than 10 percent). When errors are less than 10 percent, Novak states only that minor revisions should not be made before coordinating the figures with other USGS offices and other agencies, where applicable. He also acknowledged that when a drainage area for a station is revised, it may also be necessary to revise the figures for some upstream and downstream stations to maintain compatibility among the total and intervening areas.

Office of Water Information Technical Memorandum 12.04 announces the availability of the Watershed Boundary Dataset (WBD) as the authoritative data for hydrologic unit boundaries, and it strongly encourages use of the WBD, or its derivatives, when modeling or describing a basin and to document the methods of basin delineation. The WBD boundaries have been incorporated into the National Hydrography Dataset (NHD), which will host the basin boundaries for streamgages that are indexed to the NHD. New basin-boundary delineations for streamgages should align with the upstream boundary from the most up-to-date version of the WBD at the time of revision – if the alignment is consistent with the improved accuracy.

Drainage area revisions should continue to be based on the suitability of data and the need, but with the improvements in data sources and delineation methods the threshold for considering a revision should be a change of at least two percent. Revisions of two percent or less are allowed when justified by data and need. All revisions must be reviewed and documented by an experienced hydrographer.

New drainage areas for large groups of stations often are determined as parts of flow-regionalization studies and other types of studies, and comparisons are made between the new and previously published drainage areas to identify inconsistencies. The new delineations for stations with differences in drainage areas of more than two percent should be compared with the previous delineations, topographic maps, and aerial photography to determine if the new delineation is erroneous or if the old drainage area figure should be revised. In general, decisions to revise a drainage area should be site-by-site at the discretion of an investigator and based on suitability of data and need. However, if the data and methods used to determine new drainage areas for a study are considered superior to the data and methods used previously, then mass revision of the group of stations can be considered after first quality assuring the new drainage areas. Unless required by a specific program or application, Water Science Centers are not required to systematically review and revise drainage areas for all sites under their responsibility.

Katherine Lins //s// Katherine Lins
Chief, Office of Water Information

References:

Federal Inter-Agency River Basin Committee, 1951, Inter-agency coordination of drainage area data: Bulletin 4, 48 p., November 1951.

Novak, C.E., 1985, WRD data reports preparation guide: U.S. Geological Survey Open-File Report 85-480, 331 p., on line at http://pubs.usgs.gov/of/1985/0480/report.pdf.