Using Wetting Front Detectors (WFD) as a Cheap and Easy Way to Manage Water and Nutrients in Macadamias – Dr. Theunis Smit

When it comes to managing water and nutrients in macadamias, we are often left with two options. We either have to spend a fortune on high-end equipment to help us manage our resources or we simply neglect to monitor water and nutrient use. Fortunately, some basic and inexpensive tools are available to help growers manage these critical components of the production system. Wetting front detectors (WFD) were developed in response to low adoption of existing irrigation tools. Prof. Richard Stirzaker describes a WFD as a switch, which alerts the irrigator that a front of a given strength has passed a given depth in the soil. The WFD comprises a specially shaped funnel, a filter and a mechanical float mechanism (Figure 1).

FIGURE 1 FullStop Wetting Front Detector (WFD). The funnel part is buried in the soil with the black tube protruding above the soil surface. When a wetting front reaches the detector, a red indicator pops up. Detectors are usually placed in pairs, about one third and two thirds down the active root zone.

When rain falls or the soil is irrigated, water moves downwards through the rootzone.  The infiltrating water converges inside the funnel and the soil at the base becomes so wet that water seeps out of it, passes through a filter and is collected in a reservoir. This water activates a float, which in turn operates an indicator flag above the soil surface.  There are no wires, no electronics and no batteries.

According to Prof. Stirzaker, If the soil is dry before irrigation, the wetting front will not penetrate deeply because the dry soil absorbs most of the water. A long irrigation would be needed to activate a detector. However, if the soil is relatively wet before irrigation, it cannot store much more water, so the wetting front penetrates deeply (Stirzaker 2003, Stirzaker and Hutchinson 2005).

He further notes that knowing how deep a wetting front moves into the soil is critical for irrigation management. If a crop is given frequent but light sprinklings of water, the wetting front will not go deep and the WFD will not be activated. Much of the water will evaporate from the soil surface. If too much water is applied at one time, the wetting front will go deep into the soil, perhaps below the rooting depth of the crop, wasting water, nutrients and energy.

Furthermore, the WFD retains a sample of water which can be extracted via a tube using a syringe. This can be analysed for its salt or nitrate concentration using simple tools such as nitrate test strips and an Electrical conductivity (EC) sensor. Monitoring EC or nitrate levels can tell you more about irrigation management than measuring water content itself. For example, nitrate levels will drop sharply if over-irrigation occurs (Stirzaker and Wilkie 2002). Depending on the quality of the irrigation water, EC levels will gradually rise during periods of under-irrigation (Stirzaker et al 2004, Stirzaker and Thomson 2004).

Of particular interest to most growers will be watching the videos of Prof. Stirzaker explaining these and other useful irrigation and nutrient management tools in his own garden:

Installing a wetting front detector –

The wetting front explained –

Detecting the wetting front –

Nutrient leaching explained –

These tools and many other, reasonably priced and practical tools, are explained in great detail on the Virtual Irrigation Academy (VIA) website ( VIA is a global community that aids farmers and communities to learn how manage water and nutrients to grow more food.

Why are these tools so useful in macadamia production systems?

Macadamias have shallow root systems and are particularly sensitive to overwatering. The sensitivity to overwatering is not only linked to the increased risk of root rot (Phytophthora), but also to the fact that nutrients are leached passed the rootzone. Using simple tools such as a WFD, macadamia growers would be able to monitor that both water and nutrients are not going past the rootzone, which will not only help them use resources more efficiently but could potentially increase both yield and quality. Furthermore, these tools are reasonably priced, practical and user friendly and could therefore be used by an array of macadamia grower.

For more information regarding these and other tools, please feel free to contact the folks on the VIA platform, or to contact myself (


Stirzaker RJ and Hutchinson PA (2005). Irrigation controlled by a Wetting Front Detector: field evaluation under sprinkler irrigation. Australian Journal of Soil Research (in press)

Stirzaker R, Stevens J, Annandale J, Maeko T, Steyn J,Mpandeli S, Maurobane W, Nkgapele J & Jovanovic N(2004). Building Capacity in Irrigation Management with Wetting Front Detectors. Report to the Water Research Commission No. TT 230/04.

Stirzaker R and Thompson T (2004). FullStop at Angas Bremer: A report on the 2002-3 data to the Angas Bremer Water Management Committee Stirzaker RJ (2003).

When to turn the water off: scheduling micro-irrigation with a wetting front detector. Irrigation Science 22, 177-185.

Stirzaker RJ and Wilkie J (2002). Four lessons from a wetting front detector. Irrigation Australia 2002 conference, 21-23 May, Sydney.

Irrigation and Water Management During Flowering

By Theunis Smit, White River

Macadamias, and many other fruit trees tend to flower in spring, a season which is characterized by moderate temperatures, varying degrees of wind and more often than not a significant lack in rainfall.  Not only is rain scarce during spring, it is also the driest part of the year considering that rain during winter is either limited or completely lacking. Fortunately, most farmers have some water available for irrigation, the real problem is, however, how to manage the limited amount of water that you have during this critical time?

Before answering the million-dollar question, we need to consider some biological traits of macadamias that will dictate how water should be managed. Firstly, and most importantly, macadamias have a shallow root system, with a large portion of the roots being in the top 30-40 cm of the soil. Secondly, research has shown that macadamias are very efficient at taking water from the soil when it is available. Lastly, macadamias and many other crops get stressed when they are over irrigated, mainly due to the lack of oxygen in the soil. 

That being considered, during flowering the tree and its environment requires more water than most of the other parts of the production cycle. So how do you manage irrigation during the flowering period, considering that we especially don’t want trees to stress during flowering?

  1. If water is available, provide the trees with one long cycle of irrigation to increase the overall soil moisture content – Use irrigation scheduling probes to make sure the soil profile is filled.
  2. Following the long irrigation cycle, irrigate short cycles of irrigation more frequently. For example, if the plan is to provide the trees with 400 liters of water during the week, it is better to apply 100 liters four times per week than applying 400 liters in one go – This is essential when considering that macadamias have an extensive, but shallow root system.
  3. Under no circumstances should irrigation be increased to unreasonable amounts (i.e. 50 – 60 mm per week) when it is hot and dry – During hot and dry conditions, trees will actually “shut down” and no water will be used by the tree. Unreasonable amounts of irrigation would therefore lead to wet soils and subsequently a reduced amount of oxygen in the soil which will undoubtably lead to stress. Furthermore, irrigating long cycles and applying large volumes of water during flowering will invariably lead to increased nutrient leaching, which will lead to reduced nutrient availability and reduced tree performance.

In summary, macadamia growers are advised to frequently irrigate trees during the flowering period and use irrigation scheduling tools to carefully monitor soil moisture content. Growers are also advised to invest in mulching material if water is limited, as the addition of the mulch will reduce soil evaporation and make the little bit of water that you have last a bit longer.