In snow-covered areas, slope failures induced by snowmelt water may occur. In this study, we attempted to develop a warning method for snowmelt disasters using the effective rainfall index, which reflects the analyzed snowmelt amount (hereinafter referred to as the effective snowmelt index) and we verified the appropriate half-life of the effective snowmelt index by comparing it with the observation results of groundwater response. As a result, it was confirmed that a strong correlation between them under the conditions that the half-life of the index was set from 24 hour to 96 hour. In addition, through case studies, we found that the warning method using the effective snowmelt index and snow depth as evaluation indices could effectively provide warning of snowmelt disasters.