| To secure water for irrigation、 hydroelectric power and drinking、 the dam reservoirs in cold、 snowy regions need to be maintained at high water levels in spring、 because snowmelt is the predominant water resource in these regions. For this reason、 heavy rainfall combined with snowmelt、 an event that has become increasingly likely with the acceleration of global warming、 is problematic to flood regulation of dam reservoirs in early spring.The Hoheikyo Dam basin in southern Sapporo、 Hokkaido Prefecture has been threatened twice by unseasonably heavy spring rainfall. In mid-May of 2000、 heavy rain combined with snowmelt to cause a particularly large and unexpected inflow、 and flood regulation of the dam became temporarily impossible. In late April of 2000、 about 1.7 times as much rain fell as during a similar number of days in mid-May of the same year. However、 the peak inflow in the mid-May event was 300 m3/s、 versus 210 m3/s for that in late April. This means that outflow in late April was 0.7 times that in mid-May. To properly quantify the above situation、 the storage effect of soil and snow cover on runoff should be considered、 which requires the modeling of long-term hydrologic processes.These processes must be clarified not only for flood prediction under heavy rain combined with snowmelt but also for adaptive water management throughout the watershed. Hydrologic processes related to snow play a particularly important role in cold、 snowy regions such as Hokkaido、 northern Japan、 so the snow accumulation condition and the snowmelt amount need to be properly estimated、 as does evapotranspiration according to the conditions of ground surface and vegetation、 in order to clarify the basin-wide water budget. The Two-layer model proposed by Kondo et. al. that considers the heat balance in the atmosphere、 in the vegetation layer and at the ground surface was applied to estimate hydrologic processes、 and the long-term runoff was calculated by the Tank model. The model evaluates hydrologic processes in relation to the conditions of snow pack and snowmelt amount. Based on the calculated snow accumulation condition and snowmelt amount、 the snow storage amount was provided as the initial condition of a flood runoff model to reproduce the relationship between rainfall and runoff. In this way、 the storage routing relating to snow processes was evaluated and its modeling was found to have promise toward rational and practical estimation of the hydrologic processes in cold、 snowy regions. |
