Reference evapotranspiration was calculated using five various types of estimating methods. The Hargreaves and Thornthwaite methods produced close, strongly-correlated ETo values to those yielded by the standard Penman-Monteith method. For these reasons and due to their simplicity in calculations, i.e., few parameters needed, these two methods are recommended for in order to determine crop water as well as irrigation water requirements for the Gallikos watershed. Reference evapotranspiration isolines as yielded by the Hargreaves method showed a characteristic pattern, by decreasing from the eastern to the western part within the Gallikos watershed. Crop water requirements divided crops into more groups. These crops can be an alternative for the present situation which is mainly characterised by wheat cropping. For shortage in irrigation water supply special scenarios, not developed here, should be followed.