ABSTRACT: Ground water is a vital resource in the Yun-Lin area of Taiwan. A substantial amount is continuously extracted, creating adverse effects such as land subsidence and seawater intrusion. Minimizing these negative impacts depends on regulating the rate of groundwater withdrawal. An optimal yield must be determined to establish a sound water management policy. A wide range of safe yields for Yun-Lin have already been proposed based on constant hydrological and hydrogeological parameters. By extending the results of those investigations, this study presents a decision analysis model. The optimal yield concept is introduced as well. The proposed model incorporates a probability density function for rainfall recharge and a loss function, derived from fluctuations in the ground water table. Through decision analysis, the optimum yield is obtained by minimizing the expected value of the loss function. The optimal yield varies monthly because the probability density function is time dependent. Analysis results suggest that the cumulative optimum yield of ground water in the area is 1.26 × 108 m3/year. If the probability distribution function for rainfall recharge is modified as new precipitation data become available, the above suggested yield may require revision in the future.