The main methods that are used for irrigation scheduling, or that have the potential for development in the near future, are summarized in Table 1. Irrigation scheduling is conventionally based either on ‘soil water measurement’, where the soil moisture status (whether in terms of water content or water potential) is measured directly to determine the need for irrigation, or on ‘soil water balance calculations’, where the soil moisture status is estimated by calculation using a water balance approach in which the change in soil moisture () over a period is given by the difference between the inputs (irrigation plus precipitation) and the losses (runoff plus drainage plus evapotranspiration). Soil moisture measurement techniques have been the subject of many texts and reviews (Smith and Mullins, 2000; Dane and Topp, 2002) and will not be addressed here. Similarly, the detailed methods for estimating evapotranspiration and calculation of crop water requirements for different crops and different climates, as required in the water balance calculation, have been reviewed in detail by Allen et al. (1999). Although the water balance approach is not very accurate, it has generally been found to be sufficiently robust under a wide range of conditions. Nevertheless it is subject to the serious problem that errors are cumulative over time. For this reason it is often necessary to recalibrate the calculated water balance at intervals by using actual soil measurements, or sometimes plant response measurements (as outlined below). Some of the main advantages and disadvantages of the different irrigation scheduling approaches are outlined in Table 1.
A potential problem with all soil-water based approaches is that many features of the plant's physiology respond directly to changes in water status in the plant tissues, whether in the roots or in other tissues, rather than to changes in the bulk soil water content (or potential). The actual tissue water potential at any time therefore depends both on the soil moisture status and on the rate of water flow through the plant and the corresponding hydraulic flow resistances between the bulk soil and the appropriate plant tissues. The plant response to a given amount of soil moisture therefore varies as a complex function of evaporative demand. As a result it has been suggested (Jones, 1990a) that greater precision in the application of irrigation can potentially be obtained by a third approach, the use of ‘plant "stress" sensing’. For this approach irrigation scheduling decisions are based on plant responses rather than on direct measurements of soil water status; some of the possible physiological measurements and responses that can be used are discussed in the following section.