Hydrologic landscape regions of the United States

Hydrologic landscape regions (HLRs) in the United States were delineated by using geographic information system (GIS) tools and statistical methods including principal components and cluster analyses. The GIS and statistical analyses were applied to land-surface form, geologic texture (permeability of the soil and bedrock), and climate variables that describe the physical and climatic setting of 43,931 small (roughly 200 square kilometers) watersheds in the United States. The analyses then grouped the watersheds into 20 noncontiguous regions (the HLRs) on the basis of similarities in land-surface form, geologic texture, and climate characteristics.

This hydrologic landscape regions dataset contains for each of the 43,931 watersheds the (1) watershed identification number, (2) land-surface form, geologic texture, and climate characteristics for each watershed, and (3) hydrologic landscape region number for each watershed.

The HLRs derived in this study are specific to several choices made in the analysis: (1) the particular set of variables used, (2) details in the statistical analyses, and (3) details in the GIS analyses. Using a different set of watershed characteristics would have affected the derived HLR map. In addition, averaging the variables over smaller areas would have produced a map with finer spatial detail. Changing any of these factors likely would lead to other regional maps that could be equally valid and useful.

The methods used to define HLRs are expected to be sensitive to the spatial scale of the analysis. In the study described herein, the spatial extent of the analysis covered all 50 States; this was the appropriate spatial scale for the purpose of identifying HLRs to help design a national water-quality assessment. Satisfying a different objective may require a different spatial scale of analysis and might result in a different set of regions.

The concept of hydrologic landscapes proposed by Winter (2001) represents the natural landscape and climate factors expected to affect hydrologic processes. There are important water-resources management activities, such as reservoir and canal construction, irrigation, and subsurface drainage, that have a significant effect on hydrologic processes. These water-resources management activities were not included as factors in defining the HLRs.

Cited references:

Winter, T.C., 2001, The concept of hydrologic landscapes: Journal of the American Water Resources Association, v. 37, p. 335-349.