A robust water balance model has been tested for predicting soil moisture levels and supplemental irrigation requirement of a rainfed region of Bangladesh. The predictions were used for improving the understanding of the impacts of rainwater harvesting on rainfed agriculture. The climate data (i.e., rainfall, temperature, evaporation, and evapotranspiration) were used as inputs for predicting the variations in soil moisture. Soil moisture levels under rainfed and supplementary irrigation conditions were compared. Results showed that rainwater harvesting i.e., rain water storage tanks during rainy seasons can be potentially useful for storing rainwater, which can be utilized for enhancing crop land soil moisture during dry seasons for enhancing crop yield. The study presented here will be useful for improving and disseminating rainwater harvesting approaches for enhancing water availability in rainfed regions.