Mesoscale spatial distributions and temporal variations of oxygen and hydrogen isotope compositions in surface waters were observed on the Kii Peninsula, Japan, which receives more than 3000 mm of precipitation annually. The enriched isotopic values were distributed along the western, southern, and eastern coasts. To explain this, we proposed a new parameter, early dropped rainwater frequency (ERF), and defined it as the number of times that early dropped rainwater was first observed in a domain. ERF can be considered a characteristic of rainfall, and it is a useful parameter for clarifying mesoscale isotopic distribution. A mass balance simulation suggested the early dropped rainwater determined the specific isotopic value of stream water in a watershed.