Secchi depth (SD) of pure water is the basic value required for quantitative analysis of the SD in of other water bodies. However, it is difficult to determine this value directly through experiments and, to date, no universal value has been theoretically obtained. Previously, we reported an estimation of this value by observing a calculated image of a first-order approximation of light scattering by water molecules, with a focus on improving accuracy. Therefore, in this study, we analogized the nth-order approximation of the luminance formula for light scattering by water molecules, based on first- to fourth-order approximations, to improve accuracy. The calculation time increased with an increase in the order of approximation. We then evaluated the pure water SDs by observing the calculated first- to second-order and first- to third-order approximated images with a realistically possible calculation time.
　For incident angles of 0°, 10°, 20°, …, 80°, and 89°, the color image SDs (CISDs) observed for the first- to third-order approximated tristimulus values for light arriving from the direction of a Secchi disk and the background were in the range of 94.2 ± 2.1 m to 114 ± 6.1 m; at this order, they were considered to have almost converged. The monochromatic image SDs (MISDs) obtained by third-order approximation were in the range of 83.5 ± 1.5 m to 106 ± 4.4 m. As the MISDs were smaller than the CISDs at the angle of incidence, it can be said that our observations of pure water SDs tended to be judged by the color difference rather than contrast.
　The root mean squares of the relative errors of the automatically determined SDs based on the color difference threshold to the CISDs, and similarly determined SDs based on the contrast threshold to the MISDs, were 6.1% and 3.0%, respectively.

　There is a concern that the water temperature in terrestrial areas may increase owing to global warming. Some lakes, such as Lake Kasumigaura, in Japan have experienced an increase in water temperature of approximately 2°C in both summer and winter. Moreover, lakes, such as Lake Kojima, have witnessed a decrease in water temperature in both summer and winter, and the impact of climate change is different in each lake. However, water temperature changes in lakes are mostly observed offshore, especially at the center of the lake. It has not been confirmed whether the effects of global warming on fish and bivalves, that mainly live in lakeshore areas, are from only the water temperature at the center of the lake.
　In this study, we continuously observed the water temperature at five lakeshore sites in Lake Kasumigaura from August to September 2021. The results of the continuous observations showed that the water temperatures observed 182 times at lakeshore sites during the summer exceeded 30°C on an average, whereas the water temperature observed 36 times at the center of the lake was 30°C. Statistical analysis of the water temperature data that exceeded 30°C in August showed that the water temperatures were significantly higher at the three lakeshore sites than at the center of the lake. The daily variations in water temperature were smaller and more stable at the center of the lake; however, the variations were larger at the lakeshore sites, with variations of 7.4°C at some sites.
　As shown in this study, the water temperature and its daily variations differed greatly between the lakeshore sites and the center of the lake. To estimate the effects of global warming on lake ecosystems, it is important to combine the results obtained from lakeshore sites and the center of the lake.