Conflicts of interest between irrigation and aquaculture in water use from reservoirs in Vietnam can be resolved when trade-offs in the economic value of water can be quantified over time. Determining these trade-offs can be used as a benchmark for making decisions about managing reservoirs tending to develop rural areas in Vietnam. To solve this problem, a stochastic dynamic programming model was constructed. This model maximizes the expected net present values generated by both agriculture and aquaculture by finding the optimal release paths throughout a year, under conditions of uncertain rainfall. The model was constructed using two main components. First, a dated water production function is used to evaluate responses of crop yields for different levels of applied irrigation. Second, a bio-economic model for reservoir fisheries is employed to estimate fish yields at different levels of water during a harvest season. Using this model, we present a case study of reservoir water management in Vietnam.