The water budget and discharge processes in a seasonal tropical watershed were analyzed. The watershed has very stable base stream flows even in the late dry season and very quick direct runoff during rains. A tentative runoff error correction method applying an existing lumped runoff model was proposed in this paper and showed good agreement with the correct runoff error. After correcting runoff data, the annual average rainfall and runoff during the 11 years of 1998–2008 were calculated respectively as 1870.4 mm and 1229.2 mm. The average annual water loss was 641.2 mm. Distribution measurements of topsoil depth taken using a knocking cone penetration meter showed that this watershed has a deep topsoil layer (5.3 m average). Groundwater tables are apparent only in the lower area of the watershed. A saturated swamp area is a permanent feature near the weir. Results suggest that the stable base flow in this watershed was generated by return flow of soil-water infiltration into the thick topsoil and fractured bedrock.