**3. Irrigation Scheduling**

Irrigation scheduling is the process of determining how much water to apply and when to irrigate, and thus has a direct effect on WUE: the application of more water than that is necessary for optimal plant consumption reduces the irrigation WUE. Irrigation scheduling requires an understanding of the pattern of plant water use, which is affected by factors such as weather, growth stage and canopy wetness. The meteorologic component varies seasonally, daily and diurnally.

Irrigation may be scheduled based on the plant water status, which may be measured directly using a pressure bomb, or indirectly by monitoring the flow of the stem sap. Other indirect methods include the measurement of soil moisture content using probes and estimation of crop evapotranspiration (ET). A summary of the main irrigation scheduling approaches is presented in Jones [16]. In Australia, the most common tools used for irrigation scheduling are soil probes and tensiometers [17]. The major drawback of using these soil-moisture-based tools for scheduling is that they give point-based measurements, while the soil characteristics are known to vary spatially and temporally [18].

Farmers who do not use any scheduling tool often rely on their experiences to schedule their irrigations. However, previous studies have shown that such farmers who rely on the "rule-of-thumb" may be losing water [19]. Emerging methods of measuring or estimating crop water status for irrigation scheduling purposes are discussed later in this paper.

With the advancement in technology including the internet, a number of computer-based irrigation scheduling systems have been developed to help farmers in their decision-making process. Typical examples of these used in Australia include WaterSense, WaterTrack Rapid and IrriSatSMS [20]. However, despite the proven benefits of improved WUE using these technologies, their adoption is still limited because of reasons ranging from complexity to cost [17]. In the recent years, cheaper and more versatile sensors have become available (for instance Figure 2).

**Figure 2.** An example of a device with multiple sensors (**a**), with the ability to measure soil properties (**b**) (moisture, conductivity and temperature), air temperature and relative humidity. The device has a 2G/3G connectivity, and its web platform has an open Application Programming Interface (API) for ease of integration with other systems.

It is clear from the above discussion that scheduling of irrigation is easier if the irrigation system is automated, and with features such as accurate metering and sensors.
