Abstract
The authorhasobservedand measuredthephysicalproperties ofHitiomi-numa Pond,a smallpond inthe Sarobetsu Mirefor about ten years.There isafloating isletin thepond. Fromthe survey, it was found outthat the floating isletvirtuallysinkseverywinter. In order to understand this sinking mechanism, the authorfixed an absolute-pressurewater levelgauge into the floating islet.Thereby, sinking depths of the islet were continuously measured, while water temperature was monitored at some certain water depths.Meanwhile, the author modifieda prototype model for simulating thermal conditions of a pond.Definingthe differences betweensummer and winter boundary conditions, a new model was built up for quantitatively explaining the mechanisms of water temperature distributionsandthefreezingprocessesof the pond.Numerical simulations by the model allowed us to understand the thermal variations ofthepond, since the model well simulated the freezing process of the pondsurface. Furthermore, it was revealed that there is a close relationship between the freezing process andthe sinking mechanism of the floating islet in winter.
