Assessing the ecological hydrology of natural flow conditions in Taiwan

There is a growing use of hydrologic indicators to describe the flow needs for organisms in riverine ecosystems. These indicators use hydrologic statistics as a foundation to understand flow variability and how this variability is related to the response of riverine ecosystems to natural and altered flow regimes. The Taiwan ecohydrology indicator system (TEIS) was developed to identify hydrologic statistics most appropriate to Taiwan fisheries. We provide a rigorous evaluation of hydrologic statistics used in the TEIS for 52 long-term flow records from 23 undisturbed watersheds in Taiwan. We have used the TEIS indicators for general flow, flow duration, and flow frequency to assess the natural flow regime conditions in these target watersheds. The correlation coefficients between TEIS statistics and physiological variables (area and elevation) for the target watersheds were also calculated. The expected high correlations between watershed area and flow related statistics were found. Elevation was correlated with frequency statistics. Cluster analysis was used to characterize relationships among TEIS statistics in the target watersheds and then group watersheds with similar characteristics. Both K-mean and SOM clustering methods categorized the watershed statistics into three clusters and supported the assessment of potential redundancy in the hydrologic statistics. Although this analysis identified a high level of information redundancy in hydrological statistics, the actual information redundancy was reduced through the consideration of species life history and ecological requirements because these requirements demand calculation of all statistics that define habitat needs. This analysis supports the use of advanced cluster analysis techniques to supplement the analysis of hydrologic statistics, and uses station grouping and ecological interpretations to evaluate the natural flow regimes in Taiwan.