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Extensive deforestation in tropical regions may significantly influence the hydrological cycle. However, subsurface runoff processes in thick soil layers in humid tropical forests are poorly understood; thus, the impact of land-use changes in such regions remains unclear. To understand runoff generation mechanisms in the humid tropics, we monitored groundwater and soil moisture dynamics in a forested hillslope in Sumatra, Indonesia. We also conducted field and laboratory experiments to determine soil hydraulic characteristics and used the results to simulate vertical infiltration and groundwater recharge. Although the soil is categorized as silty clay loam, the high infiltrability and high water retention capacity of the soil enabled infiltration during storm events and recharge to groundwater. Within the 4-5 m thick soil layer at the foot of the hillslope, the shallow groundwater table quickly responded to rainfall and did not drop below a depth of 2-3 m, possibly due to continuous flow contributions from the upslope. Overall, this study demonstrates the importance of subsurface flow and vertical infiltration in thick soil layers in humid tropical regions. [ABSTRACT FROM AUTHOR]