Abstract The production of irrigated rice is increasingly challenged by freshwater scarcity. Water saving technologies such as Alternate Wetting and Drying (AWD) allow sustaining production levels under reduced water availability. Before implementing such innovations on a large scale, their hydrological impact on the system needs to be assessed. This study investigated the applicability of the water management tool WEAP (Water Evaluation and Planning System) for evaluating the effects of AWD on water use and water resources at field and irrigation system level for two different case studies in Central Luzon, the Philippines. In the first study, the Zeigler Experiment Station (ZES) of the International Rice Research Institute (IRRI) was used for parameterization of WEAP and field-scale assessment of AWD, making use of the availability of spatially and temporally highly resolved data. In the second study, WEAP was applied to an irrigation scheme in the Philippines, the Angat-Maasim River Irrigation System (AMRIS) to assess effects on up- and downstream water resources using lower resolution data. Simulated AWD implementation during the dry season reduced water requirements by 12–27% and 34.3% on ZES and AMRIS, respectively. Additionally, AWD implementation enhanced streamflow in main and lateral canals, and thus increased water availability in the entire irrigation system. We also conducted an ex-ante assessment of future freshwater availability assuming reduced precipitation due to climate change effects alongside with irrigation supply at current levels. WEAP showed that water levels in the reservoir will substantially decline under these circumstances leading to severe water stress in AMRIS. Implementing AWD in such a scenario improved water availability in the system by up to 50%. WEAP proved to be a suitable tool for upscaling different irrigation techniques and assessing their impact on water resources on a large scale. Limitations of the approach and future possibilities for improvements are discussed.