A Study on the Characterization Methods of Ecological Water Requirements in Rivers in the Southern Plain Region, China

Determining the ecological water requirements is essential for the health of river ecosystems, with the concept of ecological flow being commonly used to characterize the status of rivers and lakes. However, the low hydraulic connectivity of channels in the river systems of the southern plain region makes ecological flow monitoring challenging. Consequently, the ecological water level is often used as a substitute for ecological flow to represent ecological water requirements. Nevertheless, the conceptual and intrinsic differences between these two parameters call into question the validity of this substitution, for which there is a lack of thorough verification. Building upon this, this study selected three representative hydrological stations—Jinsuozhen Station, Sihong Station, and Shuanggou Station—in the Jiangsu plain as control sections for investigation. A comprehensive analysis of the correlation and periodicity characteristics between meteorological-hydrological factors is conducted, with multiple methods employed to calculate the alteration degree of Indicators of Hydrologic Alteration (IHA) metrics. The Tennant method is further applied to evaluate whether the proposed ecological water level meets the multi-year average water level requirements. The results demonstrate that:(1) Significant positive correlations and periodicity characteristics exist between meteorological and hydrological factors in the study area. (2) The calculation of the overall alteration degrees was performed using three distinct methodologies: traditional IHA-RVA, entropy weight method, and histogram matching approach (HMA). The results are outlined below: Jinsuozhen Station (41.57%, 34.95%, 18.31%), Sihong Station (20.36%, 14.05%, 13.13%), and Shuanggou Station (26.24%, 16.98%, 12.54%). The results obtained from this study suggest that the overall hydrological alteration level across all stations is relatively low. (3) The evaluation results of the selected appropriate ecological water level at the three hydrological stations' control reaches demonstrated good performance, meeting the multi-year average water level requirements. This finding suggests that the current hydrological regime remains stable and intact, thereby providing a healthy environment conducive to the development of in-stream flora and fauna. Consequently, within river systems of the southern plain, ecological water level can function as a reliable surrogate for ecological water requirements, effectively supplanting ecological flow as a means to evaluate the ecological status of rivers and lakes.