Crop water production functions for winter wheat with drip fertigation in the north China plain

During four consecutive growing seasons (2014-2018), field experiments were conducted in the North China to determine winter wheat production function. The field experiments were carried out using winter wheat subjected to four N levels (N, N, N, and N) and three irrigation levels (I, If, and I). The main aims were to characterize winter wheat productivity, drought response factor K, and the winter wheat grain yield production functions in relation to water supply under the different N fertilizer levels. The amount of water supply (rain + irrigation) were 326-434, 333-441, 384-492, and 332-440 mm in 2014-2015, 2015-2016, 2016-2017, and 2017-2018 growing seasons, respectively. Similarly, the values of ET (including the contribution from soil water storage) were 413-466, 384-468, 401-466, and 417-467 mm in 2014-2015, 2015-2016, 2016-2017, and 2017-2018, respectively. ET increased as the amount of irrigation increased. The average values of I, If, and I over the four growing seasons were 459-465, 432-446, and 404-413 mm, respectively. For the same amount of irrigation, there was only small difference in ET among different nitrogen levels; for the three irrigation levels, the values of ET in N, N, N, and N ranged from 384 to 466, 384 to 466, 385 to 467, and 407 to 468 mm, respectively. Water productivity values ranged from 1.69 to 2.50 kg m for (rain + irrigation) and 1.45 to 2.05 kg·m for ET. The K linearly decreased with the increase in nitrogen amount, and the values of r were greater than 0.92. The values of K for winter wheat in N, N, N, and N were 1.54, 1.41, 1.28, and 1.25, respectively. The mean value of K for winter wheat over the three irrigation levels and the four nitrogen levels was 1.37 (r = 0.95). In summary, to gain higher grain yield and WUE, optimal combination of N fertilizer of 180-240 kg·ha and irrigation quota of 36-45 mm per irrigation should be applied for winter wheat with drip fertigation in the North China Plain.