Irrigation management

Figure 1.6: Relationship between evapotranspiration and dry matter production.

Improvement of water use efficiency can be achieved by reduced soil evaporation or higher transpiration efficiency. The impact of soil management is on evaporation and other loss components of field water balance

Potential water savings would be even higher if the technology switch were combined with more precise irrigation scheduling and a partial shift from lower-value, water- intensive crops to higher-value, more water-efficient crops (Cooley et al., 2008).

Measurement based irrigation scheduling is generally based on soil parameters such as water content or pressure head. While plant based irrigation scheduling methods would have the advantage to directly respond to a crop water stress parameter, they are still limited by practical problems such as automatization (Jones, 2004b).

Irrigation management increasingly focuses on more effective and rational uses of limited water supplies with increasing water use efficiency (Marouelli et al., 2004; Payero et al., 2009). Improved efficiency can be obtained by reducing drainage, runoff and evaporation losses by using measurement or model assisted irrigation scheduling (Pereira et al., 2002). Also supplemental irrigation at critical growth stages has substantially improved irrigation efficiency (Oweis et al., 1999).

A proper timing of supplemental irrigation is critical for maximizing yield and water use efficiency. Manipulation of pre- and post-flowering water use in crops can be used to increase harvest index and by using methods of controlled irrigation the optimized water use by stomata can lead to an increase in water use efficiency, without a significant decrease in production and eventually with beneficial effects on quality (Chaves and Oliveira, 2004). Examples of some marked increase in water use efficiency by supplemental irrigation are given by Denget al. (2002), Oweiset al. (2004) and Xueet al.

(2006).

Several studies showed that optimizing irrigation not necessarily needs to provide full crop water requirements (English and Raja, 1996; Kirda, 2002). Water use efficiency can be increased if irrigation water is reduced and crop water deficit is intentionally induced (Zwart and Bastiaanssen, 2004). Studies on the effects of limited irrigation on crop yield and water use efficiency show that crop yield can be largely maintained and product quality can, in some cases, be improved while substantially reducing irrigation volume (Kang et al., 1992; Zhang and Oweis, 1999; Zhang et al., 1999). For example, Pandaet

al. (2004) evaluated the effect of different irrigation methods on root zone soil moisture, growth, yield parameters, and water use efficiency of corn and concluded that under water scarcity conditions irrigation should be scheduled at 45 % of the maximum allowable depletion of available soil water to obtain high yield and high water use efficiency. When irrigation is above the optimum, an excessive shoot growth can occur at the expense of roots and fruits (Zhang, 2004).

Thus, recent efforts in optimizing irrigation have studied practices that intentionally induce slight water deficits to plants such as regulated deficit irrigation and partial root zone drying. When water deficits start to build up, leaf stomatal conductance usually decreases faster than carbon assimilation, leading to increased transpiration efficiency (Chaves et al., 2004).

Regulated deficit irrigation involves the application of irrigation water below the evapotranspiration requirements of crop. It tends to reduce or eliminate drainage and helps to improve water use efficiency (Fereres and Soriano, 2007). The basic principle of regulated deficit irrigation is that water is withheld or reduced during a period when vegetative growth is normally high and fruit growth is low. A normal irrigation regime is resumed during the later period of rapid fruit growth. Successful application of regulated deficit irrigation requires careful attention to the timing of the water deficit period and to the degree of stress that is allowed to develop (Loveys et al., 2004; Geerts and Raes, 2009). This tactic helps to reduce vegetative growth with little effect on fruit development. In fruit crops like peach, apple and pear balance between vegetative and reproductive development is critical as excessive vegetative vigour may result in mutual shading with consequences of long-term fruitfulness. Knowledge about the phenology of vegetative and reproductive development of fruit crops can be used for saving water through regulated deficit irrigation (Chalmers et al., 1981; Chalmers et al., 1986).

Application of regulated deficit irrigation has doubled water use efficiency when compared with standard irrigation practice (Goodwin and Boland, 2002). These improvements are due to improved water use by reducing unproductive losses, reduction

in vegetative canopy size, and also due to reduced leaf stomatal conductance during the regulated deficit irrigation period (Boland et al., 1993). Effect of timing, method and scheduling of irrigation practices is summarized in Table 1.9 to demonstrate the importance of irrigation management.

Table 1.9: Effect of irrigation on water use efficiency Practice Increase in water

use efficiency (%) Reference

Irrigation scheduling 5-38 Karam (1993); Fare et al. (1993); Tyler et al. (1996); Ismail et al. (2008)

Method of irrigation 7-48 Liu et al. (2003);Aujla et al. (2005) ; Li et al. (2007) ; Cooley et al. (2008) ; Li et al.

(2010)

Timing of irrigation 25-57 Guinn et al. (1981); Hu et al. (2002); Buttar et al. (2007)

An irrigation practice that focuses on increasing water use efficiency by controlling stomatal opening is partial root zone drying. Stomatal closure is a common response to root zone stresses including soil drying, soil flooding and soil compaction. Beside hydraulic signals, this response is governed by increased levels of the plant hormone abscisic acid in plant roots and transmitted to leaves especially under dry soil conditions (Loveys et al., 2004). The knowledge about the ability of the particular plant genotypes to sense the onset of changes in moisture availability and fine-tune its water status in response to the environment has lead to the development of partial root-zone drying technique (Wilkinson, 2004). In this irrigation method, each side of the root system is irrigated during alternate periods and the maintenance of the plant water status is ensured by the wet part of the root system, whereas the decrease in water use derives from the closure of stomata promoted by dehydrating roots (Davies et al., 2000). It is recognized that stomatal closure and growth inhibition are likely to be responding simultaneously to different stimuli, some of which may operate through common signal transduction systems (Webb and Hetherington, 1997; Shinozaki and Yamaguchi-Shinozaki, 2000).

Physiological data from studies on grapevines under partial root zone drying point to subtle differences between partial root zone drying and deficit irrigation, where the same

amount of water is distributed by the two sides of the root system (Souza et al., 2003;

Santos et al., 2003). These differences include some reduction of stomatal aperture in partial root zone drying, a depression of vegetative growth, and an increase in cluster exposure to solar radiation, with some potential to improve fruit quality. There is also evidence that partial root zone drying can increase fruit quality in tomato, presumably as a result of differential effects on vegetative and reproductive production (Davies et al., 2000). The root system is also significantly altered in response to partial dehydration, not only in respect to total extension and biomass but also in architecture. Root system tends to grow deeper under partial root zone drying enabling roots to extract water from greater soil depths and provide higher plant water uptake (Dry et al., 2000). It is likely that this alteration in the root characteristics and in the source/sink balance plays an important role in plant performance under partial root zone drying. The technique had been found effective in improving water use efficiency for a wide range of crops in different environments (Kirda et al., 2007, Sadras, 2009) and its large scale implementation had been successful for vineyards (Loveys and Ping, 2002; Souza et al., 2003; Santos et al., 2003) .

Future developments in irrigation technology, better scheduling of timing and amount of water applied as well as new application methods are likely to contribute essentially to improved agricultural water use. Modern irrigation methods like supplemental irrigation, regulated deficit irrigation and partial root zone drying exploit physiological mechanisms to improve instantaneous water use efficiency at the leaf, make use of knowledge on sensitive phonological states of the crop to increase water use efficiency in relation to yield and provide a more effective water use by reducing losses and enhancing root water uptake. Site specific application of proper and efficient irrigation methods can therefore help to improve the overall agricultural water management and save water for other competitive demands (Playan and Mateos, 2006).