Crop yield response to water - Cra

emergence to flowering is about 65 to 70 days; the start of canopy senescence is about 105days. Short-season cultivars for more northern latitudes flower 10 to 15 days earlier. Maximumcanopy cover (CC x ) is achieved at flowering if plant density is 70 000 plant/ha or less; and undergood conditions full canopy cover is achieved sooner if the density is substantially higher,because the leaf area per unit land area (leaf area index, LAI) needed to close the canopy isreached earlier, before flowering.The emergence of all the silks on an ear and their pollination may take 7 or 8 days whentemperature and water regimes are favourable. Because of heterogeneity of a field plantpopulation, the overall pollination time for a field may last at least twice as long as that for asingle ear.Kernel weight increases after pollination (Duncan, 1975) follows the classical time course curvefor cereal grains. The grain maturation process is associated with declines in kernel watercontent and full maturity is generally considered to be at the time when a ‘black layer’ formsat the base of the germ of the kernel (Daynard, 1972). At full maturity and under favourableconditions, the grain of modern cultivars comprises around 50 percent of the above-groundbiomass produced by the crop, that is, the harvest index is close to 0.50 (or 50 percent). Forunimproved cultivars or land races, the harvest index may be as low as 0.3. The AquaCropsimulation recommends that HI be set to increase until the time of full maturity, with the lattertaken to be the time when green canopy cover declines to 10 percent of the maximum canopycover reached. As for many other crops (Evans, 1993), maize grain yield is often correlatedwith green leaf area duration (Wolfe et al., 1988a). Reduced green leaf area duration may bethe result of the leaf growth rate being reduced and/or premature leaf senescence, caused bywater stress or the nitrogen or other nutrients deficiency, as discussed below.Irrigation, Water Use and ProductivityIn the United States corn belt, where rainfall amount and distribution are usually favourableand the soil is deep with a high water-holding capacity, maize is grown without irrigation oronly with supplemental irrigation. In the more arid areas of the United States, maize is irrigated.In northern China, where rainfall coincides with the maize-growing season, the crop can berainfed or grown with supplemental irrigation. The common application methods includefurrow and centre pivot irrigation. Seasonal maize water use varies according to evaporativedemand of the atmosphere, and hence according to climate, time of season when the cropis grown, life cycle length of the crop, and water availability. For well-watered situations,seasonal ET ranges from less than 500 to more than 800 mm, the typical seasonal ET of acultivar of medium-season length grown in a temperate climate at latitude of 35º to 40º beingaround 650 mm.At midseason, when there is complete canopy cover of the ground and water and mineralnutrients are not limiting, maize transpires at a rate slightly higher than the reference ET(ET o ). Detailed data (Steduto and Hsiao, 1998) show the crop coefficient (K c ) is slightly lessthan the values used earlier, only in the range of 1.07 to 1.12. This presumably is the result oflower stomatal conductance of maize leaves relative to most broad-leaf crop species. As a C 4crop, maize water use efficiency is high (de Wit, 1958; Steduto et al., 2007), mostly becauseof the high rate of photosynthesis, with only a minor contribution from the slightly moremaize 117