Runoff Characteristics of a Small Watershed in a Tropical Rainforest in Peninsular Malaysia

Hydrological observations were conducted at a small tropical rainforest watershed (part of the Bukit Tarek experimental watershed (BTEW)) to clarify its runoff characteristics.During storm periods, rainfall of <30 mm was converted to stormflow with an efficiency of <10%, and the effects of the initial runoff rate on stormflow were not clear.Stormflow may have been determined in the stream channel and riparian areas as the source area.However, stormflow generation strongly depended on antecedent wetness, which was represented by the initial runoff rate during large storm events (rainfall > 30 mm).When the initial runoff rate exceeded 0.1 mm h -1 (wet conditions), all rainfall except for ~30 mm contributed to stormflow.When the initial runoff rate was below 0.07 mm h -1 (dry conditions), the loss exceeded 30 mm.This behavior was caused by the retention of rainwater in soil, with all rainfall beyond this loss contributing to stormflow.The discharge duration depended on rainfall variation.The stream regimen at the BTEW varied within the range observed for forested watersheds in Japan.The relationship between the plentiful runoff/scanty runoff ratio and recession coefficient at the BTEW, which is underlain by sedimentary rocks, was similar to that observed for a watershed underlain by sedimentary rocks in Japan.Runoff characteristics were evaluated using a conceptual hydrological model (HYCYMODEL), and the discharge responses to simulated rainfall obtained using this model well agreed with the observed discharge.