Is Subsurface Drip Irrigation an option for you?
Subsurface Drip Irrigation (SDI) consists of buried lines of polyethylene driplines with built-in emitters. Water is supplied to the driplines via submains and mainlines. SDI systems normally include a flushing submain at the end of the driplines.
Designed and installed correctly, SDI applies water very uniformly and efficiently, has no water losses above ground and no runoff. It can apply reduced water applications (little and often) and improve nutrient uptake if fertilizers are injected through the system.
While there’s limited hard data on the production benefits and profitability of SDI systems in NZ, it’s known to increase yields and crop quality while saving water, energy and fertilizer. Recently commissioned projects are looking at quantifying the benefits now.
SDI can be installed in unevenly shaped blocks and can be designed to work well on sloping ground. Installed at typically 100-300 mm below ground, driplines supply water directly to plant roots. There is no labour for shifting and it’s not affected by farming operations.
Given these advantages, why hasn’t SDI taken off in NZ?
The initial investment is high compared to other systems, although it has a life of 15-25 years. Being low pressure, it uses less energy than many other systems. The whole of life cost can be lower than some other methods. It’s not DIY and requires expert design and installation. Poorly done, it will not give the benefits described above. Once buried, it’s hard to fix mistakes.
There is still an element of it being unproven technology despite it having been widely used. Some systems failed due to poor design, poor management or poor-quality materials, causing root intrusion and emitter clogging.
Deeper driplines may not be suitable for seed germination during drier periods. A backup irrigation system may be needed to wet up the soil surface before planting. Topography, crop, soil, and paddock shape determine dripline configuration. Dripline spacing will depend on whether you intending to irrigate a row crop such as grapevines or require total coverage such as for pasture.
Soil texture and depth is critical to determining suitable dripline spacing and depth and selecting appropriate dripper flow rates and spacings. Water will move further upwards and sideways in clays or loams than it will in sandy or stony soils, generally. Driplines and emitters are installed closer together and have higher flow rates in light soils, increasing cost. In heavy soils, dripline and emitter spacings can be increased and emitter flow rates decreased lowering cost. We strongly recommend getting soil tests done to determine the depth and soil properties of the soil horizons. Holes should be dug, and samples taken to check soil textures.
Don’t guess when it comes to depths, spacings and flow rates. Unless you have evidence that a particular configuration will work, get the system designed and analysed by an expert. This is normally completed using specialist software that works out how the water will move through the soil, as shown in figure 1.
The quality of water going into the driplines must be well- filtered and possibly treated to remove physical, chemical, and biological impurities. The filtration system must be well-designed and reliable to prevent failures resulting in emitter clogging. If there is any doubt about water quality, get it tested. Get an experienced installer to put the system in and ensure that it is fully flushed out, tested, and commissioned.
Finally, learn how to manage it. Don’t let the soil dry out and flush the driplines regularly and it will serve you well for many years.
