In Hokkaido, we have often experienced ground surface freezing during the winter like as Canada, North America in the subarctic zone. It shows that the effects of frozen soil on the snowmelt runoff got the runoff percentage larger than one of the rainfall runoff, that is, it was similar phenomenon as the urbanizing the basin. The Tomisato dam experimental basin, located in the eastern part of Hokkaido, Kitami City, was set up to clarify the effects of frozen soil on the snowmelt runoff.
The main results and remarks are indicated as follows.
(1) It was pointed out that the swell of long wavelength of hydrograph occurred in the beginning snowmelt season with the more frozen soil depth increasing. On the other hand, this phenomenon Was rarely recognized in the case of few frozen soil depth. Moreover, in this basin the diurnal change of snowmelt runoff, which has often observed in generally basin, was rarely recognized except in the maximum snowmelt season.
(2) By multiple regression analysis, the frost depth is good relationship between freezing index, snow depth and melting index, and the max value of frost depth is dependent upon the accumulate rainfall at autumn season.
(3) To make clear the effects of frozen soil on the snowmelt runoff, we construct 4×3 zoned tank model, which can consider the spatially distributed frost depth and snow depth at this site, by including the above frost dept prediction model. As a result, it is possible in snowmelt season to occur overland flow at the hillslopes because of reducing infiltration capacity of ground surface by soil freezing.
(4) For estimating the snowmelt, we proposed the Degree-Hour method with considering the snowmelt-refreezing at the snow surface layer. By considering the snowmelt-refreezing, it is possible to estimate the delay of snowmelt start.