Effects of Water Diversion on Hydrological and Vegetation Dynamics in a Downstream Wetland Ecosystem

ABSTRACT The Oze region in central Japan was largely spared from large‐scale hydropower development; however, a small‐scale water diversion scheme for electricity generation was implemented in the mid‐20th century. This study applies the process‐based NICE‐BGC model to the Lake Ozenuma–Ozegahara Marsh basin to evaluate the long‐term impacts of upstream water diversion on hydrological processes and downstream vegetation dynamics over the past 57 years. Model results indicate that alterations in lake water balance and diversion practices have significantly influenced downstream hydrology, including discharge to the Nushiri River and water level dynamics in Ozegahara Marsh. Following water diversion, lake levels exhibited increased seasonal variability and more rapid recovery during spring (April–June), leading to changes in riverine inflow to the marsh. Despite the marsh's flow‐regulating capacity, simulations suggest that long‐term changes in surface water and groundwater regimes have contributed to vegetation shifts, in addition to climatic influences. These effects are particularly pronounced in the eastern marsh during the earlier part of the simulation period. The results further indicate expansion of high‐moor vegetation ( Oxycocco–Sphagnetea ) towards riparian zones and an increase in intermediate marsh vegetation ( Moliniopsietalia japonicae ), consistent with previously reported observations. The model captures spatially explicit relationships between hydrological change and vegetation succession across major plant communities. These findings provide a basis for assessing basin‐scale hydrological alteration, identifying groundwater‐dependent wetland ecosystems and informing adaptive management strategies for wetland conservation under altered hydrological regimes.