Water allocation in the Chrey Bak catchment faces the management problem that can cause water scarcity. Thus, water management and allocation for agriculture is conducted using the WEAP model to predict water demand. The study depends on collected data such as, stream flow (discharge), irrigation water need for each irrigation scheme in the Stung Chrey Bak catchment which exist of 7 irrigation schemes. There are two scenario is created; the reference scenario, and annual increase in irrigation demand: 5% of irrigate area in dry season is increase annually. These scenarios are made to answer the question “what if”. To achieve this, water discharge in each stream was generated from the SWAT model. The results showed that water demand is reported enough for all scenarios in wet season. In contrast, the dry season rice meets water shortage especially, Tang Krasang irrigation scheme. This scheme lacks of 64 thousand m3 2014 and 49 thousand m3 in 2025. Chrey Bak and Trapaeng Trabek, both of these schemes meet water scarcity only in annual increase in irrigation demand. The Chrey Bak irrigation scheme lacks of 368 m3 in 2017 and 24 thousand m3 in 2025. In Trapaeng Trabek, water scarcity occurs in 2017 to 2025 the same as Chrey Bak irrigation scheme. In 2017 this scheme lacks of 2 thousand m3 in 2017 and 118 thousand m3 in 2025. It is recommended that farmer should understand how to manage water, improve irrigation system, and water use efficiency. Furthermore, the officer of PDoWRAM should enlarge the existing reservoir in Tang Krasang irrigation scheme because the old one is not effort to store large amount of water to guarantee the sustainable use of water for irrigation.