Irrigation scheduling is one of the most important tools for developing best management practices (BMPs) for irrigated projects. Proper irrigation scheduling results in the high irrigation water use efficiencies necessary to conserve limited water resources. Information about crop coefficient (Kc), daily reference evapotranspiration (Eto), and daily evapotranspiration (Et) is important for irrigation scheduling. A gradient sprinkler line source irrigation experiment was conducted to determine the Kc for irrigated onion. The Kc curve obtained from this study corresponds to the typical Kc curve described by Doorenbos and Kassam (1986), with a maximum Kc of 1.09 associated with a maximum yield of 73 000 kg ha –1 , obtained under non-moisture-stress conditions. Growing degree days (GDD) were correlated to the Kc values, which were fitted to a third-order polynomial. The results were integrated to determine the maximum Kc associated with maximum yield. A quartic equation was developed using GDD and relative peak Kc to predict a Kc for a selected yield level. This equation was developed to help onion growers to establish the Kc curve for particular yield. The simulated Kc curve obtained from this method for the non-moisture-stress yield level of 73 000 kg ha –1 was similar to the one measured. This new method is practical and gives farmers an idea of the predicted seasonal irrigation water requirements for their onion crop. In addition, this method is simple and easy to use to schedule irrigation events for onion.