Improving water productivity is urgently needed in water-scarce dry areas. This chapter discusses crop–water production functions, i.e. the relationships between yield and water supply and water productivity. Using data from Syria, the North China Plain and Oregon, USA, crop–water production functions are developed from which the productivity of the applied water can be derived. After an initial sharp increase, the productivity reaches its maximum at a given amount of supplied water to the plant and then decreases or remains at a relatively high level with further increasing water supply. This chapter demonstrates that deficit irrigation produces a higher overall grain yield with the same amount of water resources compared with full irrigation and, therefore, has a higher productivity. Deficit irrigation can be considered as a key strategy for increasing on-farm water productivity in water-scarce dry areas. The risk associated with deficit irrigation can be minimized through proper irrigation scheduling (when and how much to irrigate) and by avoiding water stress during the growth stages when the crop is especially sensitive to water stress
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