Comparison of different irrigation methods for onion by means of water and nitrogen response functions

Considering the cost of two common agriculture inputs, water and nitrogen as well as their effects on the environment, optimum use of them is necessary. In this way, the production functions are applying to achieve maximum profits and environmental protection. Therefore, spilt-plot experiments were conducted on onion (Allium cepa L.) in a completely randomised block design for two years (2014/15-2015/16) in an arid region of Iran. The treatments included three irrigation methods (subsurface drip irrigation (SDI), drip irrigation (DI) and furrow irrigation (FI)), four levels of irrigation water (50(I-1), 75(I-2), 100(I-3) and 120(I-4) % of crop water requirement) and four levels of nitrogen fertiliser (25(N-1), 50(N-2), 75(N-3), and 100(N-4) % of crop nitrogen requirement). Regression analyses were done by linear, Cobb-Douglas, Quadratic and Transcendental functional forms of onion production. Results showed that the quadratic fits the data best, for the given local condition. The onion reached the highest increase in yield rise by increasing a unit of I (irrigation water) and N (nitrogen fertiliser) in SDI and the lowest in FI. Moreover, the pressurised irrigation methods are found to be advantageous to the substitution of N by I. The marginal rate of technical substitution demonstrates that in comparison to FI, more amounts of I and N are required to achieve equal yield in the pressurised irrigation methods.