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. A second gradient drip line- source irrigation experiment was conducted at Las Cruces, NM, during 1994–1996 to determine a wpf as related to applied water for drip irrigated onions. The irrigation treatments were 40, 60, 80, 100, and 120% of calculated nonstressed evapotranspiration determined from the sprinkler line source experiment. The wpf was curvilinear because excess water was applied to the different irrigation levels in the experiment in order to keep the base plate of the onions wet so root growth would continue. The result was that part of the applied water went to deep drainage rather than to evapotranspiration. The wpf was corrected for the amount of irrigation water lost as deep drainage and expressed as evapotranspiration versus yield (Etpf) by using reference evapotranspiration measured at Las Cruces and season crop coefficients for selected yield levels measured at Farmington, NM. Maximum onion yield at Las Cruces under the drip irrigation system was 20% higher than measured at Farmington using the sprinkler system. The results indicate that high onion yield are achievable using a drip system compared to a sprinkler system but a larger amount of applied water goes to deep drainage using a drip system compared to a sprinkler system to achieve maximum yield.