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This study is to develop an isotopic catchment-effect index (CEI) connecting the physiographic characteristics of stream catchments. A CEI, describing the extent of difference in stable water isotopic compositions (б values) between stream water and local precipitation at any given sampling site, can help in judging whether water resource management should be focused on upstream regions of streams or local hydrology issues. To establish the isotopic CEI, this study measured б values of stream water and derived б18O of local precipitation based on regional isotopic altitude gradient at montane catchments of various sizes. Results indicate that the CEI is strongly related to catchment physiographic characteristics, such as length of main stream, mean area, mean elevation, perimeter and slope. These characteristics are considered important indices of streamflow. Based on mathematical regression modeling describing the relationships between CEI and respective physiographic factors, CEI values can predict respective physiographic factors and vice versa. Moreover, according to the multiple equations derived in this study, catchments of larger size and steeper slope give elevated CEI values while greater stream length reduces the CEI's value. A greater CEI value indicates that local stream water is principally sourced from upstream reaches rather than contributions from local precipitation. In addition, CEI values are greater in winter than in summer resulting from monsoon effect. Consequently, this study establishes CEI as a useful descriptor of the physiographic characteristics of catchments. [ABSTRACT FROM AUTHOR]