Taking the guesswork out of irrigation scheduling
Author: Nick O’Halloran and Robert O’Connor (Agriculture Victoria) | Date: 26 Jul 2018
Take home messages
- Accurate irrigation scheduling is important for maximising crop yield and water productivity.
- The use of soil moisture and evapotranspiration monitoring takes the guesswork out of irrigation scheduling and helps assess irrigation performance.
- Evapotranspiration data, both past and forecasted, is freely available.
- Agriculture Victoria provides evapotranspiration data in a free weekly email service and on the Irrigating Agriculture website.
Background – irrigation scheduling
Rising water costs has driven interest in systems to improve irrigation scheduling and increase water productivity. Irrigation system upgrades, both in supply and on-farm, have also resulted in better water control and have enabled farmers to take advantage of irrigation scheduling tools.
Irrigation scheduling tools such as soil moisture and evapotranspiration monitoring help take the guesswork out of irrigating. Many farmers find them particularly useful on the shoulder periods of the irrigation season to determine when to start irrigating in spring and when to stop irrigating in autumn. These tools are also very valuable in the peak of summer when crop water demands are highest and growth rates are more sensitive to delayed irrigations.
Irrigation scheduling tools can help farmers to irrigate at a similar soil water deficit (soil dryness). If done well, crop losses from waterlogging and drought stress are minimised, and irrigation systems will operate more consistently. For example, with border check systems water cut-off times will be more consistent with similar runoff. This improves application efficiency, particularly if using timer based automation systems.
Irrigation scheduling tools can provide a record of irrigation events and how much water was applied. This is useful for benchmarking performance and making informed decisions about how to change irrigation scheduling practices.
No single irrigation scheduling system should be used in isolation. Combining objective irrigation scheduling tools such as soil moisture and evapotranspiration monitoring with more traditional methods of digging in the paddock or using a penetrometer to test soil hardness will give greater confidence all round.
Visual crop symptoms should not be used for irrigation scheduling. If your crop is wilting or visually water stressed, crop growth will already be compromised and yield lost.
Evapotranspiration is calculated from weather data (sunlight, wind, humidity and temperature) and is a reliable estimate of daily crop water requirements in millimetres per day (mm/day). By summing daily evapotranspiration and subtracting rainfall, when to irrigate next can be calculated. Past and forecast evapotranspiration data is freely available from a range of sources. Some of these sources provide user-friendly tools that take the hassle out of doing the calculations required to use evapotranspiration data.
When farming on the Riverine Plains, estimates of evapotranspiration are relatively consistent over large distances, so the use of evapotranspiration data from your nearest Bureau of Meteorology weather station is recommended. However, rainfall is much more variable over small distances, so it is better to use rainfall data collected from your own property when scheduling the next irrigation.
Evapotranspiration calculated from weather data gives an estimate of crop water requirements for a ‘standard crop’ or ‘reference crop’, so this is called reference evapotranspiration (ETo). Your crop is unlikely to be identical to the reference crop, and therefore, ETo is multiplied by a crop coefficient (Kc) to calculate daily water use of your crop. The crop coefficient will vary depending on crop type and stage of development (Figure 1). The internet based tool IrriSAT uses weekly satellite images to estimate the crop coefficient for your crop and automatically does all the necessary calculations.
Figure 1. Generalised diagram of how crop coefficient (Kc) changes with crop development (Allen et al, 2005).
Free sources of evapotranspiration data include:
For weekly email services email:
- Northern Victoria – email@example.com
- North East Victoria – firstname.lastname@example.org
Weekly ETo updates for Northern Victoria and other valuable irrigation information can be viewed online
Soil moisture monitoring
The two main types of commercially available soil moisture sensors are: suction based and volumetric based systems.
Suction based sensors measure how tight water is held in the soil. The measurement relates directly to how hard the plant needs to work to extract water and is therefore consistent across different soil types.
Volumetric soil moisture systems measure the total amount of water in the soil. To estimate how much of this water is ‘readily available’ to plants, the soil type needs to be known. In practice, volumetric moisture monitoring tools can be used to guide when to irrigate, but also how much water to apply.
Real-time monitoring of soil moisture data is possible with automatic data logging and transmission to your office or mobile device. Logged data provides a record of your irrigation practices, which can be used to assess your irrigation performance. Alternatively, lower cost manual-read options are available. There are numerous companies that will install soil moisture sensors and manage the data, with varying fee structures and total costs.
Complementary benefits of soil moisture and evapotranspiration monitoring
Soil moisture and evapotranspiration monitoring complement each other. To use evapotranspiration data for irrigation scheduling knowledge of how much water is available in the soil is also necessary. This varies depending on soil type and the crop type grown. Soil moisture monitoring can help by telling us what is happening in the ground; what depth the crop is drawing water from, how deep rain and irrigation water is penetrating, how dry the soil gets between irrigation events and how much water is available to the crop. However, soil moisture sensors only measure a small patch of soil at a single point in the paddock. Evapotranspiration tells us about the potential water use of the crop across the entire paddock, while soil moisture monitoring can confirm what is actually happening in the paddock. Installing soil moisture sensors in a representative location in the paddock is important.
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