We examined the impact of both seasonal and year-to-year variations in precipitation on simulated soil moisture dynamics at a tropical rain forest (TRF) site and a tropical seasonal forest (TSF) site in Southeast Asia, between which there is a clear difference in the precipitation regime, through a probabilistic ecohydrological model. All model parameters have apparent physical meaning and were obtained by field observations. Rainfall statistics as the primary model forcing terms were constructed from long-term rainfall records, and their analysis revealed a close relationship between drought and El Niño events at the TRF site and a long-term drying trend at the TSF site. The model results further demonstrated that the studied ecosystem's robustness (mainly concerning the plant water availability) are attributed to functional factors such as soil texture for the TRF site and rooting depth and the dry season use of water from the preceding wet season for the TSF site.