A six-year field study was conducted to evaluate water management practices of level-terraced paddy fields draining by surface drainage pipe. Field monitoring was conducted to measure rainfall, irrigation, and drainage. Field data revealed that rain water was not efficiently used in some cases. The runoff curve number (CN) was estimated from measured rainfall and runoff data to quantify runoff potential of paddy fields managed by surface drainage pipes. The CN values were determined from the maximum retention storage values at probabilities of 10, 50, and 90%, from a fitted lognormal distribution. The CN values at the antecedent moisture condition (AMC) I, II, and III were found to be 69, 86, and 95, respectively. CN values implied that runoff potential of paddy field managed by drainage pipes would be higher than those of paddy fields using weir type drainage outlets. Appropriate drainage facilities should be introduced for water management and saving labor.