Reducing water use and nitrogen loss under irrigated cropping

Soil physical properties, such as profile available water, are key influences on irrigation management, while others influence environmental outcomes, such as soil drainage and nutrient loss. Soils can be spatially variable, with varying capacity for water storage.

Improved irrigation management enabled by precision agriculture technologies on variable soils, such as first defining, then using management zones with variable rate application of irrigation water or fertiliser coupled with good farm practices, can help reduce farm inputs, improve profit, and minimise nutrient and water losses from farms.

Several case-studies in the ‘Maximising the value of irrigation’ research programme (now completed) have focused on variable soils on arable farms, soil physical properties, and how proximal sensing can improve sub-paddock management. A range of fixed and variable rate irrigation scenario options in the OVERSEER Nutrient Budget model were also evaluated for several case studies.

Electromagnetic (EM) sensor survey methods were developed with ground truthing to produce the sub-paddock management zones. Ground truthing determined soil type differences and a range of soil physical properties were then measured. Soil water retention properties were compared, modelled, and linked to the national soil information system, S-map.

At one case study site, three management zones were determined based on soil differences (Fig. 1, with the pivot irrigator footprint shown). Management zones 1, 2, and 3, were well drained, moderately to well drained, and poorly drained, respectively. The poorly drained soil had a duripan subsoil horizon – restricting water movement. The well-drained and poorly drained soils show distinct visual differences (Fig. 2), which were also apparent when soil moisture was monitored.

Figure 1 Three management zones overlaid by the pivot irrigator footprint. Bright orange represents poorly drained soil in management zone 3. Image — Figure 1 Three management zones overlaid by the pivot irrigator footprint. Bright orange represents poorly drained soil in management zone 3.

Figure 2 (a) Well-drained soil (Brown Soil; Takapau sandy loam) from management zone 1, (b) poorly drained soil (Pallic Soil; Poporangi sandy loam) from management zone 3. Note very dense duripan that restricts drainage (shown by arrow). Image — Figure 2 (a) Well-drained soil (Brown Soil; Takapau sandy loam) from management zone 1, (b) poorly drained soil (Pallic Soil; Poporangi sandy loam) from management zone 3. Note very dense duripan that restricts drainage (shown by arrow).

Soil moisture in each management zone was measured on a weekly basis throughout the irrigation season. Results from this case study show the barley yield in the poorly drained zone 3 was 2.6 t/ha, much less than the 9.7 t/ha average yield from the other zones. Soil moisture monitoring showed zone 3 was more often at saturation than the other zones, with drainage or runoff more likely to occur.

This case study shows the benefits of mapping variable soils while considering their different water storage properties for potentially applying changes in irrigation management. The use of a Variable Rate Irrigation (VRI) system could be an option to consider, with potential savings in cost of water and yield penalty of about $3,000/year at this site. Benefits of adopting a VRI system should be weighed against implementation costs.

Potential benefits to the environment, from a nitrogen-leaching perspective, were assessed using the OVERSEER Nutrient Budget model. Predicted nitrogen leaching losses (kg N/ha/y; described as N loss to water) were determined for irrigation scenarios (combination of fixed application depth, fixed return period, or variable depth or return period) and reported on a whole-farm basis, for a whole year.

The modelled N losses to water for this whole-farm case study ranged from 37 to 71 kg N/ha/yr in fixed or variable irrigation treatments. Where irrigation application depth and return period were both variable, the N loss to water was much less (38–39 kg N/ha/yr) than when application depth and return periods were both fixed (63–71 kg N/ha/yr).

Other case studies in the research programme have been investigated using OVERSEER. Adoption of strategies with a variable irrigation component produced the lowest N losses, irrigation water applied and drainage, when assessed on a whole farm basis, and when compared with fixed irrigation management options. Adoption of strategies with a variable irrigation component reduced nitrogen loss to water by 46–72%, and reduced irrigation water applied by 66–91%, depending on the case study.

Near real-time soil moisture monitoring and visualisation methods were also developed for use by farmers, highlighting the benefits of this type of technology. For irrigation scheduling, graphs with control lines were generated using near real-time spatial soil moisture status and management zones. Smart phone apps and visualisation tools were developed (Fig. 3) and were popular with the farmers. These tools provided information not only about when to start and stop irrigation but also the amount of irrigation to apply.

Figure 3 Soil moisture smart phone app with map and irrigation scheduling tool for farmers, showing level of water storage in soil profile.

The technologies and tools developed in this research programme will help decision-making to potentially increase crop yield, better manage variable soils and water resources for irrigation, and reduce environmental impact such as drainage, and nutrient loss. These tools could be deployed more widely.