YIELD-BASED, IRRIGATED ONION CROP COEFFICIENTS

Abstract

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 toconserve limited water resources. Information about crop coefficient (Kc), daily reference evapotranspiration (Eto), anddaily evapotranspiration (Et) is important for irrigation scheduling. A gradient sprinkler line source irrigationexperiment was conducted to determine the Kc for irrigated onion. The Kc curve obtained from this study corresponds tothe typical Kc curve described by Doorenbos and Kassam (1986), with a maximum Kc of 1.09 associated with amaximum yield of 73 000 kg ha 1 , obtained under non-moisture-stress conditions. Growing degree days (GDD) werecorrelated to the Kc values, which were fitted to a third-order polynomial. The results were integrated to determine themaximum Kc associated with maximum yield. A quartic equation was developed using GDD and relative peak Kc topredict a Kc for a selected yield level. This equation was developed to help onion growers to establish the Kc curve forparticular yield. The simulated Kc curve obtained from this method for the non-moisture-stress yield level of 73 000 kgha 1 was similar to the one measured. This new method is practical and gives farmers an idea of the predicted seasonalirrigation water requirements for their onion crop. In addition, this method is simple and easy to use to scheduleirrigation events for onion.