Extreme hydrological events precipitation, floods and droughts : proceedings of the International Symposium held at Yokohama, Japan, 20-23 July 1993 ; jointly organized by the International Association of Hydrological Sciences (IAHS) and the International Association of Meterology and Atmospheric Physics (IAMAP) and co-sponsored by the United Nations Educational Scientific and Cultural Organization (UNESCO) and the World Meteorological Organization (WMO)