The objective of the present paper was to study the impact of climate change on grain yield, water balance, crop water productivity (CWP) and water requirements for the summer-sown maize in Faisalabad, Pakistan. Climate-change scenarios (Special Report on Emission Scenarios (SRES) A1B, A2 and B1) were derived from the general circulation model ECHAM 5 and the crop model CERES-Maize was used to simulate impacts of the applied climate scenarios. Calibration and validation of the crop models were carried out for the summer-sown maize in 2007 and for the spring-sown maize in 2008. Three predefined reduced irrigation scenarios were compared to traditional irrigation practices for the summer-sown maize. Under the current conditions, scenario S1 (one irrigation event skipped at the vegetative stage) showed a higher simulated yield than scenario S2 (one irrigation event skipped at the grain-filling stage) due to higher water drainage and nitrogen (N) leaching rates in scenario S2. Scenario S3 (irrigation events skipped at both crop establishment and the grain-filling stage) showed significantly higher grain yield because it had the lowest drainage and N leaching rates. In this irrigation scenario, 60 mm of water were saved compared to the other two scenarios, and much more water was saved compared to the traditional local regime.
In the predicted climatic scenarios and with reduced irrigation, the simulated maize yields and crop water productivities were affected differently. For the period from 2036 to 2065, a more significant yield decrease was shown in all emission and irrigation scenarios. A yield decrease was simulated by both, including and not including the direct effect of elevated atmospheric CO2 concentrations on photosynthesis. However, the simulated direct effect of elevated CO2 was to produce higher yield and CWP in all scenarios. The highest grain yields and crop water productivities were achieved in the reduced irrigation scenario S3 for all emission scenarios and climatic periods for the same reason as under the current conditions (N leaching). However, the yield differences between the climate scenarios were mainly due to the shortening of the simulated growing period. This was caused by increased temperatures compared to current conditions. A shortened growing cycle reduced the potential time for biomass accumulation and in the present case it was not balanced by the CO2 fertilizing effect (without a potential change in maize cultivars).
By simulating optimum yields (where automatic irrigation is determined by the model to receive optimum yield), under the current conditions it was found that 285 mm of irrigation would ensure the highest grain yield and CWP (30 mm more than under irrigation scenario S3). In this case, actual evapotranspiration reached 373 mm and less deep drainage and N leaching occurred. In the future climate scenarios, optimum yields and irrigation demands diminished depending on the emission scenario, but CWP increased slightly.
The present simulation study shows a clear decreasing yield trend for autumn maize under a warm climate for each type of (unchanged) irrigation management due to the shortening of the growing period. However, in the current climate, as well as in the future climate scenarios, maize yield levels could be improved by optimized (and reduced) irrigation compared to traditional irrigation due to reduced N leaching. Even in the scenario with the highest warming trend (A1B emission scenario for the period 2036–65), the current yield levels could be kept or even improved.