**2. Concept and strategies of deficit irrigation**

amount of water to apply are essential to attain sustainable management and environmentally sound water management, since this natural resource is increasingly scarce and expensive. Projected global warming will enhance this problem as climate change scenarios forecast reductions in the total amount of precipitation and changes in its seasonal distribution, up surging the problem of water scarcity for agricultural use [1]. Agricultural water management comprehends different features related to irrigation, for instance, water productivity index (WP), that is, ratio yield/marketable product or yield/net income, to water used by the crop [2]. Optimization of irrigation strategy is necessary to increase WP and minimize yearly fluctuations of crop production. Irrigation is also essential to ensure the productivity increase and therefore meet the rising food needs in a world with an ever larger population, which is expected to augment by 30% in 2050 [3]. Overall, food production from the irrigated agriculture accounts for 40% of the total output, using only 17% of the land area devoted to food production [4]. The agriculture uses correspond to more than two-thirds of the total of freshwater uses [5, 6]. In many parts of the world, irrigation water has been over-exploited and over-used and freshwater shortage is becoming critical mainly in the arid and semiarid areas, such as some of the Mediterranean region. Freshwater allocation between agriculture and other economic sectors is a source of conflict, claiming to a constant need to improve WP of crops. Thus, precise irrigation scheduling, combining plant and/or soil water stress indicators, is one of the tools that can help growers to achieve this goal [7, 8]. The combination of

52 Irrigation in Agroecosystems

these indicators with modeling has been defended by several authors [9].

relations, and plant stress sensitivity according to their phenological stages.

In the last decades, extensive research in fruit crops has shown that they respond positively to conditions of mild water deficit imposed by deficit irrigation (DI) strategies [10]. Under this agronomic practice, the amount of water applied is reduced to a value below maximal crop irrigation requirements allowing the development of a mild water deficit with minimal effects on yield [11, 12]. In fact, several studies have demonstrated that DI is particularly suitable for regions where water is scarce, and improving WP is a critical goal [13, 14]. The increase of WP when DI is applied to woody crops is explained by: (i) DI efficiently reduces plant transpiration (T) by stomatal closure in fruit trees and vines as tall, rough canopies are well coupled to the atmosphere [15]; (ii) in most woody crops, net incomes are not linearly linked to biomass accumulation, but to fruit yield and fruit quality [4] and DI normally enhances the quality of fruits and derived products [16–18], eventually increasing the net income of the grower; and (iii) DI increases WP by the control of excessive growth that reduces pruning frequency and intensity. In fact, the control of plant vigor has a particular importance in orchards with high-plant densities, also called super intensive orchards [19, 20], thus DI may increase their productive life through decreasing the competition between trees for solar light [21]. Scheduling DI in commercial orchards usually requires knowledge of the soil water capacity, the actual plant water requirements, plant water

Fruit orchards and vineyards constitute an integral and significant part of the Mediterranean environment and culture, with a great economic, ecological, and social support in different countries [22]. Therefore, it is easy to understand that the study of the response of fruit trees and vineyards to deficit irrigation is of key importance for the agriculture and the economy of the Mediterranean countries. Based on our own experimental results and also on information from the literature, the aim of the present chapter is to provide criteria to enable According to Fereres and Soriano [4], the term DI should be defined in terms of the level of water supply in relation to maximum crop evapotranspiration (ETc) and the terms deficit or supplemental irrigation are not interchangeable, because in the latter, a maximum yield is not sought. It is widely known that conditions that limit water use usually decrease crop evapotranspiration (ETc) and crop growth by the limitation of its main component, transpiration (T), and therefore carbon assimilation. Thus, it is of remarkable concern to be aware of the maximum reduction of ETc with the minimum impact on the economic return of production and quality on mature fruit trees, as compared to those obtained when ETc is fully replaced. In young fruit trees, it is not desirable to practice water deficit irrigation once in this stage of development, the main objective is to maximize vegetative growth leading to reach the mature phase as faster as possible to attain full production [23]. The correct application of DI requires precise knowledge on the crop response to water stress at different phenological stages, to identify the periods when fruit trees are less sensitive [24] and in order to define the level of DI to be applied.

This work focuses on the main strategies of DI since they have been studied and applied in olive, peach orchards, and grapevines. They can be depicted as: (i) sustained deficit irrigation (SDI), with a deficit throughout the season; (ii) regulated deficit irrigation (RDI), with periods when the irrigation can be stopped or reduced to a minimum level, based on physiological aspects of the response of plants to water deficit, and (iii) partial rootzone drying (PRD), see Section 2.3 for definition. All these practices aim at maximizing the efficiency of water use and WP [25, 26] with minimum impact on yield, which can be attained if precision tools are used to manage DI [27, 28].
