Increasing of demand for water, allocation of limited water resources, climatic variability, degradation of water in the environment and developing of policies for sustainable water use are issues of increasing concern in the Kontsa irrigation project. This study aimed at assessing the effects of selected irrigation methods on water demand and allocation among farmers in Kontsa irrigation project. The study employed both primary and secondary data. The primary data were collected by interviewing 80 farmers, key informant interviews and observation of the project area while secondary data were collected from different agencies of Ethiopia. For this study Water Evaluation and Planning (WEAP) model was used to model the current situation of irrigation water demand and also to create scenario for future irrigation water demand. The model was set up for a current account year in 2015 and last year of scenarios in 2040 based on the available data. Then the irrigation water demand of the project area was modeled while giving consideration for existing and planned developments in the area. The result from the current situation of irrigation water demand indicated that the demand was satisfied fully and the unmet demand under the base year (2015) was zero. Additionally, irrigation expansion scenario was created and the result of this scenario indicates the increment of irrigation water demand as compared to the base year and the reference scenario. The study also revealed that furrow irrigation and plastic buckets were the main irrigation techniques employed by farmers in the study area.