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-120, N-180, N-240, and N-300) and three irrigation levels (I-f,I-0.8f, andI(0.6f)). The main aims were to characterize winter wheat productivity, drought response factorK(y), 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 ofET(a)(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(a)increased as the amount of irrigation increased. The average values ofI(f),I-0.8f, andI(0.6f)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 inET(a)among different nitrogen levels; for the three irrigation levels, the values ofET(a)in N-120, N-180, N-240, and N(300)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(-3)for (rain + irrigation) and 1.45 to 2.05 kg center dot m(-3)forET(a). TheK(y)linearly decreased with the increase in nitrogen amount, and the values of r were greater than 0.92. The values ofK(y)for winter wheat in N-120, N-180, N-240, and N(300)were 1.54, 1.41, 1.28, and 1.25, respectively. The mean value ofK(y)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 andWUE, optimal combination of N fertilizer of 180-240 kg center dot ha(-1)and irrigation quota of 36-45 mm per irrigation should be applied for winter wheat with drip fertigation in the North China Plain.