Use of a water deficit sensitivity index for partial irrigation scheduling of wheat and barley

Shortage of water is the most important limiting factor for crop production in the arid and semi-arid regions in Iran. More land can become productive by using partial irrigation at strategic times during the growing season. This may be accomplished if a proper index of crop sensitivity to water deficit at various growth stages is used. A theoretical procedure was applied to determine the savings in water and the economic benefit derived from partial irrigation, using a water stress sensitivity index for winter wheat (Triticum aestivum L.) and spring barley (Hordeum vulgare L.) in a dry region of Fars province in Iran. The results indicated that some water reduction is possible. In general, the suggested maximum allowable water reduction was unreasonably high for the sensitivity index (lambda(i)) proposed by Nairizi and Rydzewski. Their index is not applicable in arid and semi-arid areas. However, the field-derived lambda(i) of Aryan resulted in a reasonable water reduction close to the field-applied water reduction to achieve a corresponding relative yield. Therefore, appropriate values of lambda(i) for different climatic conditions should be used to calculate rational water reductions. The maximum allowable water reduction for spring barley was higher than that for winter wheat. The reduction increased as the benefit to cost ratio (B/C) was increased. Water reductions of 7 and 26% were allowed for winter wheat and spring barley, respectively, at a B/C ratio of 1.5. This corresponded to an 8 and 35% increase in cultivated area, respectively. These results need to be validated in more extensive field experimentation.