Rainfall and Runoff Observations in the Subtropical Forest of Okinawa Island, Japan

Abstract

We set up two experimental catchments to provide an improved understanding of hydrological processes in a subtropical forested area in the northern part of Okinawa Island, Japan. We calculated runoff using water level data (recorded by a pressure-type water level gauge installed in a box culvert) and a discharge rating curve (derived from in situ observations). Water balance calculations for 2010 showed that the rainfall, runoff and evapotranspiration losses (= rainfall – runoff) were 3403.6 mm, 2285.7 mm and 1117.4 mm, respectively. This result was in agreement with previous results from other forested experimental catchments in this region. Direct runoff, as a proportion of event total rainfall, can be expressed by the empirical equation ( Q direct = 0.0048, P event 1.7971, R 2 = 0.9599). When P event was 100 mm or less, the ratios of Q direct to P event were less than 15% in general. When P event exceeded 100 mm, the ratios were 20% - 30%.