347 / 2024-02-29 18:18:01

Application of atmospheric, hydrodynamic, and biogeochemical models to enhance simulation accuracy of water temperature and hypoxia in a shallow lake (Kasumigaura, Japan)

hypoxia,atmospheric model,biogeochemical model,water temperature,Kasumigaura

Hypoxia in shallow lakes is affected by daily weather conditions. Located in the eastern part of Japan, Kasumigaura is a severely eutrophic lake, experiencing significant nutrient influx from surrounding agriculture and livestock farming activities. Hypoxia occurs annually near the bottom of the lake and significantly impacts the lake environment and ecosystems. Predicting the occurrence of hypoxia is crucial for devising effective strategies for lake improvement. The shallow depth of the lake prevents the development of long-term thermal stratification. Previous studies have shown that the duration of diurnal stratification exerts the most significant influence on the occurrence of hypoxia in the lake. The stratification behavior is primarily dictated by wind speed and duration. The intricate spatial distribution arising from seasonal winds, in combination with land-sea and land-lake breezes, necessitates precise representations of wind force conditions. Such accuracy is crucial for enhancing model performance in predicting diurnal stratification. Nevertheless, the limited number of observation stations poses a challenge, and the existing wind observations are the sole source, which is not capable of adequately representing the distribution.

This study used an atmospheric model, the Scalable Computing for Advanced Library and Environment (SCALE), to obtain high-resolution wind fields over the lake. The global model output was downscaled to a horizontal distribution with a resolution of 1 km using nesting. The results showed that the wind direction aligned with the observations, and the model accurately simulated small-scale wind gyres. Although the discrepancy in wind speed was approximately one m/s throughout the year, the model obtained accurate distributions of not only normal wind speeds, which contributed to the formation of diurnal stratification, but also when typhoons occurred. We performed three-dimensional hydrodynamic and biogeochemical modeling after bias correction for the wind speed discrepancy. Compared to the case with the coarse wind field created with ground observations, the case with the simulated wind field changed the frequency of occurrence of water temperature stratification. It also improved the diurnal temperature stratification in summer. These results suggest that a high-resolution wind field is essential for simulating diurnal stratification and bottom hypoxia in shallow-water lakes, where meteorology significantly influences flow and water quality.