Water versus Land Productivity
Land productivity (yield) and water productivity are indicators for assessing the performance of supplemental irrigation. Higher water productivity is linked with higher yields. This parallel increase in yields and water productivity, however, does not continue linearly. At some high level of yield, greater amounts of irrigation water are required to achieve additional incremental yield increase. Water productivity of wheat (Figure 14.5) starts to decline as yield per unit of land increases above certain levels.
Relationship between crop water productivity and crop grain yield for durum wheat under SI in Syria. (Adapted from Zhang, H., and T. Oweis, Agric. Water Manage. 38, 195, 1999.)
It is clear that the amount of water required to achieve yield increases above 5 t/ha is much higher than that needed at lower yield levels. It would be more efficient to produce only 5 t/ha with lower water application than to achieve maximum yield with application of excessive amounts of water. The saved water would be used more efficiently if applied to new lands. This, of course, applies only when water, not land, is the limiting resource and is insufficient to fully irrigate all available land.
The association of high water productivity values with high yields has important implications for crop management in achieving efficient use of water resources in water-scarce areas (Oweis et al. 1998). Attaining higher yields with increased water productivity is economical only when the increased gains in crop yield are not offset by increased costs of other inputs. The curvilinear water productivity-yield relationship reflects the importance of attaining relatively high yields for efficient use of water. Policies for maximizing yield should be considered carefully before they are applied under water-scarce conditions. Guidelines for recommending irrigation schedules under normal water availability may need to be revised when applied in water-scarce areas.