Abstract
Oku-Tone basin (northern part of Gunma Prefecture) is an area of heavy snowfall and located in the upper basin of the Tone River. The dam operation which utilizes the snowmelt flowing into the reservoir plays a major role in supplying water to the Tone river basin in spring and early summer. This basin supplies drinking water for 29 million people living in the Tokyo Metropolitan area and five prefectures, and drought has occurred in this basin every two or three years for the last twenty years. An important concern is that future climate change may affect the frequency of occurrence of droughts. For this reason, research on the potential of cloud seeding to enhance orographic snowfall was conducted. If successful, this would allow for a more stable supply of water resources in this region.
The research utilizes numerical models to investigate the effectiveness of orographic snow cloud modification for the drought mitigation. In this paper we quantitatively evaluate the effect of snow augmentation by cloud seeding on snowmelt volume and water storage in the dams of the Oku-Tone basin from December 2006 through June 2007 by using a non-hydrostatic atmospheric model, a snow model and a runoff model. The results show that cloud seeding through the winter has the potential to increase snowmelt volume flowing into Yagisawa and Naramata dams by 17 % and 20 %, respectively. In addition, water storage volumes on June 30, when full reservoir capacity is shifted to the below full reservoir capacity of summer in preparation for flood events, is increased from 70 % to 100 % in Yagisawa dam and from 83 % to 93 % in Naramata dam. For the virtual drought case where the total water storage on 1 December 2006 is assumed to be as low as 80 % of mean water storage on 1 December for the last 15 years, it is shown that cloud seeding could markedly reduce the drought period. The present study shows that the combination of snowpack augmentation by cloud seeding and improved dam management will be very effective to mitigate drought disaster and to secure the water resources in the Kanto area in the future where less snowfall over the mountain areas and extreme events such as longer non-precipitation periods are expected as a result of climate change.
