Irrigation scheduling has conventionally aimed to achieve an optimum water supply for productivity, with soil water content being maintained close to field capacity. In many ways irrigation scheduling can be regarded as a mature research field which has moved from innovative science into the realms of use, or at most the refinement, of existing practical applications. Nevertheless, in recent years there has been a wide range of proposed novel approaches to irrigation scheduling which have not yet been widely adopted; many of these are based on sensing the plant response to water deficits rather than sensing the soil moisture status directly (Jones, 1990a).
The increasing worldwide shortages of water and costs of irrigation are leading to an emphasis on developing methods of irrigation that minimize water use (maximize the water use efficiency). The advent of precision irrigation methods such as trickle irrigation has played a major role in reducing the water required in agricultural and horticultural crops, but has highlighted the need for new methods of accurate irrigation scheduling and control. In recent years it has become clear that maintenance of a slight plant water deficit can improve the partitioning of carbohydrate to reproductive structures such as fruit and also control excessive vegetative growth (Chalmers et al., 1981), giving rise to what has been termed by Chalmers et al. (1986) as ‘regulated deficit irrigation’ (RDI). Achievement of successful RDI depends on accurate soil moisture or plant ‘stress’ sensing, and requires an ability to irrigate ‘little and often’ on demand. A disadvantage of RDI is that it requires water status to be maintained accurately within a rather narrow tolerance; any excess application loses the advantage of the regulated deficit and can cost more in terms of water used, while any under-application can lead to severe yield or quality losses. An alternative recent innovation to achieve the same measure of growth control has been the development of partial root-zone drying (PRD), where irrigation is supplied alternately to different parts of the root system (Dry and Loveys, 1998; Stoll et al., 2000b). A potential advantage of this method is that precise irrigation control is probably less critical for success than it is for RDI, as plants can always obtain adequate water from the well-watered side of the root system and the drying side primarily provides a signal to modify growth and stomatal aperture (Stoll et al., 2000a).
The range of crops to which RDI and PRD methods have been applied is increasing all the time, but their greatest successes have been in high-value horticultural and fruit crops, usually those where the harvested part of the plant is its reproductive organ. Applications of such techniques to extensive arable crops are in their infancy, although there are some exciting preliminary reports (Kang et al., 2000, 2003). At present it is much less clear whether PRD or RDI would be so valuable for vegetative crops, although appropriate application can be used to restrict growth, as is required for high quality in some ornamental crop species (RS Harrison-Murray, personal communication).
The choice of irrigation scheduling method depends to a large degree on the objectives of the irrigator and the irrigation system available. The more sophisticated scheduling methods generally require higher-precision application systems; nevertheless even less sophisticated systems such as flood irrigation scheduling can benefit from improvements in irrigation scheduling as outlined here. The pressures to improve irrigation use efficiency and to use irrigation for precise control of vegetative growth, as in RDI, both imply a requirement for increased precision in irrigation control, maintaining the soil moisture status within fine bands to achieve specific objectives in crop management. Such objectives can only be met by precision irrigation systems such as trickle irrigation that can apply precise amounts of water at frequent intervals (often several times per day). Effective operation of such systems equally requires a sensing system that determines irrigation need in real time or at least at frequent intervals; this rules out large-scale manual monitoring programmes for such purposes and indicates a need for automated monitoring systems.