Ice-covered conditions of Lake Kuttara, a mountainous lake in Hokkaido, were explored in four winters of 2013 – 2016. Following the Jan-March mean air temperature (-4.23 ℃ in 2013, -3.50 ℃ in 2014, -1.06℃ in 2015 and -2.41 ℃ in 2016), the lake was completely ice-covered for 83 days, 52 days and 33 days in 2013, 2014 and 2016, respectively, but mostly open in 2015. One-dimensional model for simulating temporal change of ice thickness after complete ice cover was designed by considering the heat budget in three layers of snow, ice and water. Simulated ice thicknesses were very reasonable to observed ones in 2013 and 2014. For detecting a sensitivity of the numerical simulation, the ice thickness under condition of +0.28 ℃ in air temperature after a decade was calculated for 2013, 2014 and 2016. Calculated mean ice thicknesses were by 9.1 %, 57.0 % and 83.3 % smaller than the original ones in 2013, 2014 and 2016, respectively. The ice thinning thus prevails in relatively warm winter.

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In the river basins exposed to volcanic activity, volcanic deposits and water quality change of springs may affect the water environment qualitatively. On June 29, 2015, an eruption occurred in Owakudani at Mt. Hakone, and subsequently, strong turbidity was observed in the Owakusawa River and the Hayakawa River. To clarify the cause of these phenomena, river waters and rainwaters were collected in the Hayakawa River basins include Owakusawa River every one to three months for analysis. As for the Owakusawa River, high electrical conductivity was observed after the eruption and subsequently decreased gradually. The ratio of Cl- concentration in anions in the Owakusawa River was found to be high immediately after the eruption but decreased after one year. The geochemical composition of rainwater was affected by volcanic gas, and it was found that the rainwater at the observation points closer to Owakudani, the main fumarole, contains more dissolved components.

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An elaborated model for the blue coloration of Aoike Pond, Aomori Prefecture, was proposed. In the　elaborated model, water’s density fluctuation scattering coefficients and a volume scattering function were referred from literatures, and calculation methods for solid angles were refined. In addition, formulas for light intensities produced by irregular reflection at lake bottom and by density fluctuation scattering in lake water were described on the bases of light flux. Image analyses enabled us to determine a scaling factor that connected the observed RGB values in the images of Aoike Pond to the theoretically predicted ones, and a sensitivity factor of the digital camera was separated from the scaling factor. As a result, there were highly linear relationships between the observed and theoretically predicted RGB values, and thus the observed colorations were reproduced fairly well in wide areas on the images. Hence, the elaborated model was capable of quantitatively describing the blue colorations with high validation.

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