Author(s): Masayasu Irie; Masaki Okuda; Yuichi Miyabara; Yusuke Nakatani; Masashi Toyota

Linked Author(s): Masayasu Irie, Masashi Toyota

Keywords: Diurnal stratification; Shallow lake; Bottom heat transfer; SCHISM; Simulation

Abstract: Shallow lakes are highly susceptible to the impacts of climate and climatic changes because of their small depth. Lake Suwa, Japan is surrounded by mountains and farmlands, and nutrients derived from chemical fertilizers used for agriculture and domestic wastewater caused eutrophication of the lake before the construction of a regional sewerage system, resulting in large outbreaks of Microcystis and other environmental degradation. In recent years, the water quality has been improved. As a result, a large amount of phytoplankton has been suppressed, while a large number of aquatic plants have been proliferating, and the ecosystem in the lake itself has been changing. In order to conduct numerical simulations of water environment in such a water body, the model first needs to ensure the reproducibility of water temperature. However, it is challenging to simulate thermal stratification in summer, and the model performance to represent the temperature distribution is very sensitive to boundary and meteorological conditions. We incorporated a bottom heat transfer model into a three-dimensional hydrodynamic and biogeochemical model SCHISM-ICM (Zhang et al., 2016). Sensitivity analysis was conducted by changing the bottom boundary conditions and the temperature of inflowing river water. Finally, we examined the impacts of model performance in the thermal stratification on biogeochemical modeling by comparing the distributions of dissolved oxygen. The new diabatic model simulated the bottom temperature better than the conventional adiabatic model, and hypoxia was formed longer and more stably due to the stronger vertical stratification.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221509

Year: 2022