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Abstract
Rice-wheat (RW) production system, which covers over 13.5 million ha in the Indo-Gangetic Plains of south Asia, is vital for food and nutritional security and livelihood of millions of poor people in this part of the region. Availability of irrigation water under projected climate change scenarios is a great concern, and demonstration of the impact of different irrigation regimes on rice, wheat, and system yields is essential to adopt suitable water saving technologies to minimize risk. This study tested the ability of the agricultural production systems simulator (APSIM) model to simulate the effects of different irrigation regimes on yield, irrigation water requirement, and irrigation water productivity (WP^sub i^) of rice, wheat, and RW system in upper-gangetic plains of India. The long-term simulated rice yield showed a steadily declining trend at an average rate of 120 kg ha^sup -1^yr^sup -1^ (R ^sup 2^ = 0.94, p < 0.05), while long-term simulated wheat yields showed a lower declining trend at an average rate of 48 kg ha^sup -1^yr^sup -1^ (R ^sup 2^ = 0.48, p < 0.05). The highest WP^sub i^ of 8.31 kg ha^sup -1^mm^sup -1^ was observed under RW system with the rice irrigation (IR) regime of 8 days alternate wetting and drying (AWD) and five irrigations for wheat with a yield penalty of 25.5 %. The next highest WP^sub i^ was observed in the treatment with a 5-day AWD regime in rice and five irrigations for wheat, with a yield penalty of 20.1 %. Thus, we can suggest that a 5-day AWD irrigation regime for rice combined with five irrigations during wheat could be the best option under water limiting situations.