Rainfall-runoff characteristics in a tropical forested catchment, Puchong, Selangor, Malaysia

This study aims to clarify the contributions of pre-event water to storm runoff using environmental tracers (dis-solved inorganic ions and stable isotopes) in a tropical forested catchment in Puchong, Selangor, Malaysia. We performed intensive sampling campaigns of stream water and throughfall for two storm events in July and November 2018. The discharge showed a low peak of 0.13 mm/h in event 1, with 18 mm of total rainfall, whereas event 2, with 50 mm of total rainfall, showed a quick discharge peak of 1.17 mm/h and a slow recovery of 0.39 mm/h. The nitrate concentration in the stream water during event 2 was higher than that in event 1. The temporal variations in nitrate ions indicate that subsurface water provided a dominant storm-flow in event 2. Hydrograph separations using silicate as a tracer revealed that pre-event water was the dominant com-ponent of the storm hydrograph (58-98%). Our results sug-gest that pre-event water plays an essential role in storm runoff of headwaters in humid tropical regions.