Defining practical limits for centre-pivot length and irrigation management on Lismore soils

Abstract

A study was conducted to investigate the dynamics of water infiltration into an unsaturated Lismore silt loam under centre‐pivot irrigation. This was done to quantify the relative impact of application intensity and application depth on the dynamics of the soil moisture profile and on the quantity and timing of drainage through the profile, particularly for longer centrepivots, where shorter but more intense applications of water typically occur. The potential for design and management strategies to improve water use efficiency on this common New Zealand agricultural soil type was assessed.

Four instrumented lysimeters at the Lincoln University Lysimeter Laboratory were subjected to a range of irrigation depths and application intensities between November 2010 and February 2012, to simulate different points beneath a 1,000 metre‐long centre‐pivot irrigator. Soil matric potential, soil water content, and drainage volume were measured at one minute intervals after each application of water.

Overall, there was little evidence to suggest that high application intensities had the expected negative impacts that were predicted. Greater application intensities resulted in a significantly faster infiltration of water to depths of 15 and 30 cm, but there were no observed trends in (1) the incidence of field capacity at any point in the soil profile, (2) the percentage change in soil water content at the 15 cm depth, or (3) the fraction of water that drained from the bottom of the soil profile. In fact, the majority of observed drainage occurred at lower application intensities of < 50 mm/hr, with the three largest observed drainage events occurring during 40, 20, and 10 mm/hr application intensity scenarios.

Instead, application depth, application duration, and the size of the pre‐irrigation soil moisture deficit (SMD) were shown to have complexly interrelated, and significant impacts on the measured parameters. Application depth was positively correlated with the fraction of water that drained from the bottom of the soil profile and with several indicators of preferential flow. Application durations of 20‐60 minutes corresponded with the greatest volume of drainage, possibly resulting from the activation of just a small number of macropores over a relatively long time period, leading to a large volume of preferential flow. Further, application depths greater than the SMD resulted in drainage in 100% of the experimental scenarios, application depths less than the SMD but greater than 60% of the SMD resulted in drainage 37% of the time, and application depths less than 60% of the SMD resulted in drainage only 10% of the time.

A formula for calculating an appropriate trigger level for a Lismore silt loam is proposed, and an optimum application depth of approximately 10 mm is suggested. Given the relatively small water holding capacity and high degree of spatially variability of this soil, this does not leave much margin for management error, and further highlights the challenges facing farmers irrigating on this common Canterbury soil type.