Projecting the impact of climate change and elevated CO concentration on rice irrigation water requirement in China.

Climate change and elevated CO concentrations significantly affect rice growth and water consumption. Understanding the specific impacts of climate change and elevated CO concentrations on rice physiological phenology, crop water demand (ET), and irrigation water requirement (IR) is of great significance for the sustainable utilization of water resources and food security. This is particularly true in China, the world's largest rice producer. In this study, with the help of two rice phenological models, the modified Penman-Monteith equation, and the paddy water balance model, we project the changes in rice phenological period, ET, and IR in four main rice-producing regions of China in the period 2015-2100 based on the 11 GCM outputs. The results show that the rice growing period is shortened in most rice-producing regions, except for the parts of the middle and lower reaches of the Yangtze River. Meanwhile, the trend of ET and IR of rice varies slightly among regions in the future scenario, with almost all regions decreasing yearly except for the middle and lower reaches of the Yangtze River, where the trend is increasing. The progressively increasing atmospheric CO concentration has a "fertilization effect" on the crop, which can reduce the water requirements of rice. In the SSP585 scenario, the " CO fertilization effect" can reduce up to 8.87 × 10 m of ET and 6.94 × 10 m of IR in the middle and lower reaches of the Yangtze River in the period of 2090s. This study provides beneficial references to understand the response of rice ET and IR to future climate change and CO concentration elevation in China and highlights that the simulation in terms of crop irrigation must account for the "CO fertilization effect".