| Summary: | 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.
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