Exploring the assessment scale for small watersheds in the Han river basin using an integrated ecosystem health index

• A novel segment-based method for river health assessment is developed. • The R-IEHI comprises aquatic life, water quality and hydrologic regime. • The R-IEHI can significantly distinguish different levels of disturbance intensity. • The R-IEHI is applicable to other small watersheds in subtropical regions. The Han River is a crucial water source for the South-to-North Water Diversion Project; however, it also faces a range of ecological and environmental challenges. Currently, there is a lack of comprehensive, index-based assessments that integrate multiple biological groups to evaluate the ecological health of small watersheds. This study selects five representative rivers and develops a comprehensive evaluation index at both the sample scale and the river segment scale. The index incorporates three categories of indicators: aquatic life, water quality and hydrologic regime. To construct the river integrated ecosystem health index (R-IEHI), we first selected multiple biological group indicators, including commonly used evaluation groups such as fish, macrobenthos, and phytoplankton. When constructing the river segment-based assessment index, we first developed the F- S O/E index based on species richness and river segment length. We then combined this with several other group-specific indices to create a simplified comprehensive evaluation index. The calculation of the R-IEHI score involves first normalizing each individual index, followed by a weighted summation. Both the sample-based and river segment-based indices can significantly distinguish different levels of disturbance intensity. However, since rivers are continuous systems, the river segment-based evaluation index is better suited for assessing the health of small watersheds. The river segment-based comprehensive evaluation index, applicable to other small watersheds in subtropical regions, can provide scientific data to support the protection of the Han River water source area.