Computing the crop water production function for onion

Onions are a major irrigated crop in New Mexico. An excessive amount of wat er is generally applied, because the crop is shallow-rooted and requires fr equent irrigation to achieve good yields. Onions under deficit irrigation h ave a decrease in evapotranspiration and yield. Consequently, farmers need to use the water production function (wpf) for onions to estimate water req uirements at different locations for selected yield goals. The wpf is the r elationship between yield and water applied. The same relation can be expre ssed in terms of evapotranspiration, ih which case the production function is known as the evapotranspiration production function (Etpf). A gradient s prinkler line source onion experiment was conducted in 1986 and 1987 at Far mington New Mexico and a linear Etpf determined. The linear Etpf was expres sed as a relative Etpf and the yield response factor (Ky), which represents the slope of relative Etpf, was calculated for onions at Farmington, NM an d found to be 1.52, compared to 1.5 obtained by [Doorenbos, J., Kassam, A.H ., 1986. FAO Irrig. Drain., Paper 33, Pome, Italy] for onions stressed at t he 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 w ater 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 exces s 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 contin ue. The result was that part of the applied water went to deep drainage rat her than to evapotranspiration. The wpf was corrected for the amount of irr igation water lost as deep drainage and expressed as evapotranspiration ver sus yield (Etpf) by using reference evapotranspiration measured at Las Cruc es and season crop coefficients for selected yield levels measured at Farmi ngton, NM. Maximum onion yield at Las Cruces under the drip irrigation syst em was 20% higher than measured at Farmington using the sprinkler system. T he results indicate that high onion yield are achievable using a drip syste m 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 achi eve maximum yield. (C) 2000 Elsevier Science B,V. All rights reserved.