Computing the crop water production function for onion

Onions are a major irrigated crop in New Mexico. An excessive amount of water is generally applied, because the crop is shallow-rooted and requires frequent irrigation to achieve good yields. Onions under deficit irrigation have a decrease in evapotranspiration and yield. Consequently, farmers need to use the water production function (wpf) for onions to estimate water requirements at different locations for selected yield goals. The wpf is the relationship between yield and water applied. The same relation can be expressed in terms of evapotranspiration, in which case the production function is known as the evapotranspiration production function (Etpf). A gradient sprinkler line source onion experiment was conducted in 1986 and 1987 at Farmington New Mexico and a linear Etpf determined. The linear Etpf was expressed as a relative Etpf and the yield response factor (Ky), which represents the slope of relative Etpf, was calculated for onions at Farmington, NM and found to be 1.52, compared to 1.5 obtained by [Doorenbos, J., Kassam, A.H., 1986. FAO Irrig. Drain., Paper 33, Rome, Italy] for onions stressed at the yield formation period.