At present and more so in the future, irrigated agriculture will take place under water scarcity. Insufficient water supply for irrigation will be the norm rather than the exception, and irrigation management will shift from emphasizing production per unit area towards maximizing the production per unit of water consumed, the water productivity. To cope with scarce supplies, deficit irrigation, defined as the application of water below full crop-water requirements (evapotranspiration), is an important tool to achieve the goal of reducing irrigation water use. While deficit irrigation is widely practised over millions of hectares for a number of reasons—from inadequate network design to excessive irrigation expansion relative to catchment supplies—it has not received sufficient attention in research. Its use in reducing water consumption for biomass production, and for irrigation of annual and perennial crops is reviewed here. There is potential for improving water productivity in many field crops and there is sufficient information for defining the best deficit irrigation strategy for many situations. One conclusion is that the level of irrigation supply under deficit irrigation should be relatively high in most cases, one that permits achieving 60–100% of full evapotranspiration. Several cases on the successful use of regulated deficit irrigation (RDI) in fruit trees and vines are reviewed, showing that RDI not only increases water productivity, but also farmers’ profits. Research linking the physiological basis of these responses to the design of RDI strategies is likely to have a significant impact in increasing its adoption in water-limited areas.

Tree density in newly planted olive orchards has been increasing steadily as part of the intensification of olive oil production. Because of the importance that harvest costs have on the profitability of this crop, the mechanisation of harvesting operations has been an important goal. Hedgerow plantations at very high densities have been proposed as a means of reducing harvest costs by using an over-the-tree harvester. We conducted an 8-year experiment in Cordoba, Southern Spain, to evaluate the performance of olive orchards at four planting densities that included: a) the conventional density under irrigated conditions of 204 trees ha^–1 (7 m 7 m; D1); b) 408 trees ha^–1 (7 m 3.5 m; D2); 816 trees ha^–1 (3.5 m 3.5 m; D3); and, almost 1,904 trees ha^–1 (3.5 m 1.5 m; D4), the recommended density for the hedgerow system. Yields for the first 3 years of production were highest in D4, reaching almost 20 t fruit ha^–1 in year-3. Thereafter, the need for topping the D4 canopy to 2.5 m, to adapt it to the harvester, reduced D4 yields drastically. The relationships between intercepted radiation, vegetative growth, and reproductive development at the branch level, were explored in D4 canopies in 2005 to understand the effect of shading on yield-determining processes. At the end of the sixth producing year, the cumulative fruit production of D4 was 60,096 kg ha^–1, while the yields of D1, D2, and D3 were 32,513; 60,125 and 76,149 kg ha^–1, respectively. Fruit oil content in D4 was less than in all the other densities, and cumulative oil yields for the first six producing years were 6,829; 12,853; 14,973 and 10,113 kg ha^–1 for D1, D2, D3 and D4, respectively. It is concluded that the hedgerow system, in its current form and management, is less productive and probably less sustainable than orchards planted at densities such as that of D2.