Forests with a tropical monsoon climate are often influenced by extreme climatic phenomena. We investigated the differences in actual water content of surface soils in a lowland evergreen forest (EF) and deciduous forest (DF) in Cambodia, yearly from 2014 to 2016. The average soil water content values were higher for lowland EF than for DF, showing a distinct difference in the dry seasons. A major reason for this were the differences in soil type and thickness: Acrisols have an average thickness of 7 m, whereas Plinthosols, Arenosols, and Leptosols have an average thickness of 1 m; thicker, fine-textured soils contain and supply substantial amounts of water even in dry seasons. In the lowland EF site, some ponds were near the stream water during an average year’s dry seasons because the groundwater levels were close to the soil surface year-round. However, in extreme climate, such as “El Niño” in February 2016, all ponds in the EF dried up, and the surface soils contained no water at several points of higher elevation. At the lowland DF site, even during “El Niño,” the surface soil water content did not reach zero; this could be due to their high water-holding capacity and low evapotranspiration, and the effect of no understory vegetation. On average, the lowland DF site showed lower values than the lowland EF site, especially in Leptosols, which are quite thin. Soil water content fluctuation is important for understanding Cambodia’s unique forest landscape; specifically, lowland EF and DF form a mosaic in a similar monsoon climate. We presented the variations of soil water content spatially, seasonally, and interannually, which were mainly explained by soil thickness and partially by soil type and texture. The spatial variations in the 4-ha plot corresponded well with the tree species characteristics.