Hydrologic Regionalization of Watersheds: Methodology Development

Shih-Min Chiang
Senior Environmental Specialist, Office of Deputy Administrator, Environmental Protection Administration, 41, Sec. 1, Chung-Hwa Rd., Taipei, Taiwan. E-mail: smchiang@sun.epa.gov.tw

Ting-Kuei Tsay
Professor, Dept. of Civil Engineering, National Taiwan Univ., 1, Sec. 4, Roosevelt Rd., Taipei, Taiwan.

Stephan J. Nix
Professor, Dept. of Civil and Environmental Engineering, Northern Arizona Univ., Flagstaff, AZ 86011.

Hydrological behaviors of watersheds play an important role in water resources planning and management. It is costly to obtain hydrological information by setting gauge stations for every watershed. Hydrological regionalization, the classification of gauged watersheds into regions according to preset criteria, provides a way to extend information from gauged watersheds to ungauged ones. The preset criteria are generally based on streamflow or watershed and climatic variables. Regionalization techniques provide a mechanism to determine the hydrologic behaviors of gauged watersheds. Streamflow and watershed variables describe streamflow properties such as monthly flows or streamflow parameters and watershed characteristics. A mathematical model (e.g., a time series model) estimates the streamflow parameters. Watershed variables describe the watershed characteristics. If a regionalization scheme is successful, strong relationships between streamflow properties and watershed variables can be realized. These relationships can be utilized to develop useful streamflow information at ungauged watersheds featuring characteristics similar to one of the groups.

The purpose of this paper is to develop a regionalization scheme to classify watershed into regions and identify the regional membership. This scheme used 16 streamflow parameters estimated by a time series model to classify 94 watersheds into 6 regions by cluster analysis. The classified regions seem to be separated by physiographical boundaries, especially the two main clusters. Discriminant analysis tests the significance of the cluster difference; thus, each cluster represents one hydrologic region. Principal component analysis interprets the regional differences and similarities. The regional membership is mainly identified by the watershed variables of elevation, forest area, channel slope, and precipitation based on the calculation of the scores of canonical discriminant variates. This emphasizes the importance of the hydrologic regionalization and the identification of the specific characteristics in each region.

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