Variable-rate irrigation
Better understanding of soils and improved irrigation technology is allowing farmers to grow more using less water.
Variable-rate irrigation has been developed by Landcare Research scientists in conjunction with Massey University and engineers from Precision Irrigation, a division of WMC Technology.
Precision Irrigation are modifying existing centre-pivot and lateral-sprinkler irrigators for variable-rate irrigation allowing control of each individual sprinkler. Software automatically controls valves on the sprinklers so that water can be varied and optimised along the length of the irrigator. Uniform applications across paddocks – current standard practice – could become a thing of the past for farmers with variable soil types.
The effective application of this revolutionary new technology is based on work by Landcare Research to better understand soils and their capacity to hold and supply water.
In the case of Mid-Canterbury farmer Eric Watson, the benefits of precision irrigation are considerable.
‘One of our irrigators runs across both heavy and light soils. We had early-sown beans on the light soil that required 30 mm of water. On the heavy soil we had spring-sown pak choi, which required a minimum amount of water. By utilising variable rate irrigation our lateral delivered 30 mm of water on one part of the paddock and just 15 mm over the other part. This saved both water and money, and prevented overwatering of crops.’
Mr Watson’s farm, near Ashburton, has been the site of one of two summer fi eld trials.
‘We have a fairly low annual allocation of water and potentially were running out of water. We were looking for options for using water more efficiently and variable-rate irrigation seemed to offer the best solution,’ he says.
Landcare Research staff Sam Carrick and Carolyn Hedley undertook a combination of sensing and soil sampling to establish the different soil water holding capacities across Eric’s paddocks while an electromagnetic induction survey was used to map soil variability. Irrigation management zones were then defined on the map and ground-truthed by Sam and Carolyn. They then produced an available water-holding capacity map that will be used to guide variable irrigation scheduling. Their colleague Jagath Ekanayake installed a network of wireless sensors to monitor soil moisture real-time in each irrigation management zone. This information is transmitted to a website for researchers and farmer to view and use.
‘We were able to produce a soil available water map for Eric that shows that across the stony fl ats the water holding ability roughly doubles as you move from the river to the base of the terrace and it’s higher again as you move up the terrace,’ says Dr Carrick.
‘That map will be uploaded to the Precision Irrigation software to help Eric decide how to irrigate different parts of his paddocks at different rates.’
Earlier research has shown that variable-rate irrigation saves between 9 and 26% of irrigation water, with accompanying energy savings, as well as reducing runoff and drainage by up to 55%, which reduces the risk of nitrate leaching.
Dr Hedley estimates that these water and associated energy savings reduce costs by $60– $150/ha/year.
There are two components to variable-rate irrigation – first, a spatial soil and crop-based decision support tool for spatial irrigation scheduling, using real-time soil water status mapping. The second component is an accurate irrigation system with variable-rate control.
Centre-pivot and lateral-sprinkler irrigation systems are ideally suited for variable-rate modification. To modify an existing system each sprinkler is fitted with a valve that pulses on or off via a wireless node (www.precisionirrigation.co.nz). Each node controls four sprinklers and receives wireless inputs from a central controller to guide variable water delivery.
Spray systems make up about 70% of all irrigation systems in New Zealand and cover an area of about 600,000 hectares, up from 460,000 in 2004 (NZ Statistics). These systems often occur on highly variable soils, such as the sandy and stony soils of the Canterbury Plains, applying uniform rates of irrigation to large areas. This has led to inefficiencies. But with remote programming and monitoring through easy-to-use software, and an advanced system controlling every sprinkler on the irrigator, water is only applied at varying rates and only where necessary.
‘A big benefit of this system was that we had 5 hectares of overlaps in our irrigation system but now we can shut off the nozzles that go over these areas, therefore saving water and money and avoiding overwatering,’ says Mr Watson.