Development of an integrated hydrological-irrigation optimization modeling system for a typical rice irrigation scheme in Central Vietnam

In Central Vietnam, farmers report an increasing occurrence frequency of water shortages for irrigation during dry seasons. Particularly during the summer-autumn rice season, water is often insufficient to irrigate the entire rice production areas, and thus restricting rice productivity significantly. In this study, a coupled hydrological-irrigation optimization modelling system is developed to optimize irrigation strategies for a typical rice irrigation system in Central Vietnam. The model consists of a fully distributed hydrologic model, which simulates the inflow to a reservoir, and an irrigation model, which optimizes the rice irrigation technology, i.e. Alternate Wetting and Drying (AWD) or Continuous Flooding (CF), irrigation schedule and cropping area under given water constraints. Irrigation strategies are derived based on different initial reservoir water levels at the beginning of the cropping season as well as different maximum water releases. The simulation results show that the initial level of water in the reservoir is crucial: Water levels of less than 90% do not provide sufficient water to irrigate the entire cropping area, whereas a level of 70% restricts the cropping area to 75%. AWD is able to reduce the water input, ranging from 4% to 10%. The adoption of AWD therefore has the potential to irrigate a larger area and may help to increase the profit of the farmers. However, the benefits of AWD can only be achieved after significant investment in the canal system and the reservoir outlet. Since the robustness of the optimization results (performance variability) is crucial for decision support, we estimated the impact of different computing environments on the solutions. Only limited performance variability is found, giving confidence in the robustness of the model for decision support.