The model simulation is a simplification of the field processes. Irrigation scheduling is important for developing best management practices for irrigated areas. In metekel zone understanding how much and when to irrigate their crops is proplems of farmers.  Therefore, this study was conducted to determine the crop water requirement and irrigation scheduling of  onion  for the study area to solve the problem. Crop data,  soil physical and chemical data, collected long-term daily climatic data and irrigation water quality data, used for crop water requirement and irrigation scheduling using CropWat and AquaCrop models. Reference evapotranspiration was estimated using the FAO Penman Monteith method. The minimum ETc of onion was 401.9mm in Bullen and the maximum Crop water requirement of onion was 495.6 mm in Guba using CropWat Model. But the maximum ETc of  onion was 523.7 mm  in Pawe and minimum ETc was 355 mm in Wembera  using AquaCrop model. The irrigation scheduling with a fixed interval time  criterion of 5 days with 20 irrigation events has been determined. Among the performance indicators, NSE values of onion were 0.9,12. 0.88, 0.91, 0.86, 0.85, RMSE values of onion were 8.3, 9.2, 8.17, 8.15, 5.8, and in Pawe, Mandura, Guba, Bullen, Wembera respectively. This  indicated  that Aqua Crop model  used to simulate irrigation scheduling  and  ETc  of onion as CropWat in Metekel zone.

Keywords: AquaCropCrop water requirement, Climate data., CropWat