In Reply Refer To:
Mail Stop 415

June 1, 2011

Office of Surface Water Technical Memorandum No. 11.06

Subject: Regional Streamflow Analyses

Regional analyses of various streamflow characteristics have been widely used by the U.S. Geological Survey as a means for providing streamflow information at ungaged sites. Regional analyses typically involve the use of multiple-regression methods to relate streamflow characteristics at streamgages to various basin and climatic variables measured for the streamgages. Most are undertaken by local Water Science Centers (WSCs) in cooperation with various state and local agencies and thus tend to be based predominantly on streamflow, basin, and climatic data within state boundaries. WSCs have been encouraged to use data from adjoining states when performing regional streamflow analyses, but the use of data from other states generally has been limited to streamgages near state borders. As a result, estimation equations developed by adjoining WSCs may provide widely varying results for watersheds that cross state borders, which may cause reduced user confidence in the reliability of the equations. As more WSCs begin to implement the StreamStats program (http://water.usgs.gov/osw/streamstats/) for providing information from regional analyses, the need for providing consistent streamflow data from state to state becomes more crucial. This memorandum sets forth a new policy for defining study areas for regionalization studies.

Defining Study Areas
Fundamentally, surface-water hydrologic processes are better characterized by watershed boundaries than by state or political boundaries. Accordingly, this memorandum establishes the policy that study areas for regional analyses of streamflow should be based on large watersheds, as defined by hydrologic units (Seaber and others, 1987), such as sub-regions (4-digit HUCs) or accounting units (6-digit HUCs). Watersheds of this size commonly include drainage areas that extend into multiple states, and thus studies based on these watersheds will affect multiple WSCs. Efforts to facilitate studies on this basis are strongly encouraged and should be coordinated among adjacent WSCs as necessary.

Studies for large watersheds will not always be possible due to financial and political limitations among the states. When studies for large watersheds are not practical, the definition of a study area is to be based on a collection of cataloging units (8-digit HUCs). This approach is consistent with the StreamStats application, in that geospatial datasets required to compute basin and climatic characteristics for StreamStats are compiled at the 8-digit HUC level. For a particular state, the study area will include the cataloging units contained entirely within the state and cataloging units where the majority of the basin area is contained within the state. At the discretion of the WSC and its cooperators, cataloging units that are less than half-contained within a state may be included in the regional analysis to achieve complete state-wide coverage of regression equations. Under this policy, it is likely that some of the streamgage information collected by a WSC in one state will be incorporated into the regional study for an adjacent state.

Multi-state and major river basin studies are encouraged. WSCs participating in multi-state studies will need to reach an agreement on how the work is divided, relying on the WSC Directors and potentially the Water Science Field Team Surface Water Specialists (formerly Regional Surface Water Specialists) to aid in crafting the work agreement. Although the additional coordination efforts will require more time and effort in the early stages of a regional analysis of streamflow, the final regression product will be hydrologically consistent across political boundaries.

Review of Interpretive Statistics
Any interpretive streamflow statistics, such as low- and peak-flow frequency statistics, that are computed by a WSC for streamgages serviced by an adjacent WSC must be reviewed and agreed upon by the adjacent WSC. Funding for this activity should be built into the project budget.

Datasets for Basin Characteristics
The resolution of available Digital Elevation Models (DEMs) and other geospatial data needed to delineate drainage boundaries and determine basin characteristics for regional studies may vary within a study area. OSW staff and StreamStats development team staff can provide guidance on selection of geospatial datasets and how to address potential issues resulting from the use of geospatial data with multiple scales and accuracies. While state and local data sets may better describe parameters of interest within the state boundaries, comparable data is often not available in adjacent states, making multi-state analyses very difficult. Mixing of different data sets to compute basin characteristics for a regression analysis can create bias in estimates of explanatory variables, and hence in predictions from regression equations. Mixing of data sets may be appropriate if reasonably consistent results can be obtained. When state and local data are used, the study report should document analyses to show that differences between the datasets do not create problems with the regression. WSCs are encouraged to consider the use of national or regional GIS data sets in preference to state and local datasets to avoid issues caused by mixing data sets.

WSCs considering regional analyses of streamflow are encouraged to meet with interested cooperators as early in the planning process as possible to explain the need for defining study units based on watershed boundaries rather than state boundaries and to explore the best means for doing so. To encourage multi-state efforts, the Office of Surface Water is willing to provide a very limited level of funding to assist multi-state regional analysis projects. The ability of OSW to provide assistance funding to multi-state regional assessments will be determined on an annual basis, dependent upon the appropriated funding for the National Streamflow Information Program.

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Steve Blanchard //signed//
Chief, Office of Surface Water (http//water.usgs.gov/osw)
Delaware River Master (http//water.usgs.gov/osw/odrm/)

U.S. Geological Survey 703-648-5629
National Center, MS415 571-216-1423 (cell)
12201 Sunrise Valley Drive 703-648-6693 (FAX)
Reston, VA 20192 sfblanch@usgs.gov

References:
Seaber, Paul R., Kapinos, F. Paul, and Knapp, George L., 1987, Hydrologic Unit Maps: U.S. Geological Survey Water Supply Paper 2294