Peer review

Rice production in Asia needs to increase to feed a growing population whereas water for irrigation is getting scarcer. Major challenges are to (i) save water; (ii) increase water productivity and (iii) produce more rice with less water. This study analyzes the ways in which water-saving irrigation can help to meet these challenges at the field level. The analyses are conducted using experimental data collected mostly in central–northern India and the Philippines. Water input can be reduced by reducing ponded water depths to soil saturation or by alternate wetting/drying. Water savings under saturated soil conditions were on average 23% (±14%) with yield reductions of only 6% (±6%). Yields were reduced by 10–40% when soil water potentials in the root zone were allowed to reach −100 to −300 mbar. In clayey soils, intermittent drying may lead to shrinkage and cracking, thereby risking increased soil water loss, increased water requirements and decreased water productivity. Water productivity in continuous flooded rice was typically 0.2–0.4 g grain per kg water in India and 0.3–1.1 g grain per kg water in the Philippines. Water-saving irrigation increases water productivity, up to a maximum of about 1.9 g grain per kg water, but decreases yield. It therefore does not produce more rice with less water on the same field. Field-level water productivity and yield can only be increased concomitantly by improving total factor productivity or by raising the yield potential. Total rice production can be increased by using water saved in one location to irrigate new land in another. If this is not done, a strategy of saving water at the field level potentially threatens total rice production at large.

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