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Managing Irrigation

The following overview provides insight into the use of moisture probes as part of an overall water management strategy beyond just knowing when to water.  At PCT all of our solutions are based on a very simple philosophy “water controls everything” and it is regarded as the number one yield-limiting factor. Water management is the fundamental principal of knowing if you’re applying too much or too little – everything else is secondary.

Using spatial data management techniques and understanding the relationship between Plant Available Water (PAW), Surface Water Management (SWM), plant growth and yield potential allows for the development of a superior nutritional program, variety and plant population plan.

Most fields have a range of different soil and terrain types that cause variability in field productivity.  Regardless of other yield limiting issues such as climate, compaction or input application timing, the two constant factors from which variability can be measured, understood and managed season after season are:

1. Soil :- spatial variation in water holding capacity matched with limitation and therefore creating plant available water, plus potential chemical imbalances

2. Terrain :- spatial variation in water movement and infiltration and their impact on plant available water and nutrient leaching.

The purpose of a moisture probe is to help the agronomist or grower understand crop water use and assist their irrigation decision-making. The ability to measure field variability of soil type and terrain change provides the most reliable insight available to understanding the relationship between yield, potential yield and crop water use.

Even though there is mention of the variables that will impact the growth and health of the crop, it’s not always practical or affordable to measure them. For example knowing the actual plant available water and nutritional requirement may make us sound intelligent, but realistically the crop itself tells you a lot about whats happening to it.

Using a moisture probe sensor helps to respond quickly to the crop health because its water use is being influenced. The moisture probe reports a lot of detailed information regarding whats happening in the root zone where it’s hard to see.

Measuring Soil Variability

Clay content and organic matter contribute significantly to the water holding capacity (WHC) of the soil and depending on factors such as salt content this will help determine the plant available water (PAW).
Mapping the soil condition is achieved using electromagnetic induction (EM).  The EM readings increase as clay, moisture and salts increase in the soil and the values represent an average of all three variables. This is described as ECa and a typical distribution of readings can be seen in Figure 1.

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Figure 1: Measurement results from DualEM soil survey.

The measured results are collated and presented in a statistical format to help understand the overall range of results and variability. The CV value shown in this example is the total field variability, which here is a approximately 25%.

Effectively 25% variability represents a wonderful opportunity to manage inputs variably but from the histogram there is evidence there are significant areas of the field around the 30 value. Even though this field is a pivot and lends itself to VRI, managing the majority of the field is paramount.

Measuring Terrain Variability

Understanding surface water movement during irrigation or rainfall events plays a significant role in the determining where to place a moisture probe. Probes should not be located in areas that are higher or lower than the plane of natural fit. In other words, do not locate probes in areas where water will pool or shed quicker than the majority of the field.
The impact of making irrigation decisions based on the readings from an incorrectly placed probe can be significant. If a probe has been located in a low lying area or field depression it will indicate a high PAW reading resulting in a delayed irrigation decision and potentially stress the remaining dry crop. This can be compounded if it is a low-lying area with high clay content and WHC. If the remaining majority soil type is a lighter high drainage soil then the impact of not watering could be devastating.

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Figure 2a – Slope – areas are selected based on the majority – not mean.

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Figure 2b – Landscape change – areas are selected based on the mean flow rate.

Managing for majority or base field area

Now that the soil variability has been measured and understood and the terrain elevation data is available we have all of the information required to ensure the moisture probe is placed in the optimal position. But first it is important to understand what you are trying to manage and the outcome you are aiming to achieve.

Many precision agriculture (PA) advocates promote the view of using PA tools to try and remove all variability from the equation and gain a uniform yield across the entire field. Whilst an exciting prospect, it is a costly approach that can result in a poor return on investment.
Instead of trying to manage all parts of the field, it is far more effective to manage the majority of the field. This is the largest percentage of land and therefore has the potential to deliver the largest return on investment if the maximum yield potential is achieved.

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Figure 3: Field management zones based on soil type. Soil test site for zone management included

All areas in the field need to be optimized but they can only be optimized when their individual responses to the ‘majority management practice’ is seen through other data layers such as yield maps.

Irrigation scheduling is critical to ensuring crop performance is not constrained by too little or too much water, both of which can have negative effects on final yield. Correct timing and duration of irrigation improves the overall efficiency of the crop and contributes significantly to productivity and profitability.

Relationship between spatial variation and yield.

Following are typical relationships  between the variables in the field and yield.

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Figure 4: PAW versus Yield Potential.

The graph shown in Figure 4 represents typical soil plant available water and yield potential relationships. Normally one of three instances will happen:

1) Blue line = yield increase as PAW increases
Normally a result of under watering or a very dry year where heavier (more WHC) soils, indicated by higher EM values increase in yield.  This is very typical in broadacre farming in dry regions such as Australia.

2) Green line = yields decrease due to too much water
This is the opposite effect to the blue line and is typically experienced in countries with abundant rainfall and supplementary irrigation.

3) Red line = spatially optimized conditions
This is what irrigation farmers are trying to achieve. It is important to remember we can’t control the weather so we must focus on a well defined irrigation management strategy.

Simply if a grower wants to achieve the best spatial result for a field then directing management and agronomic decisions to the majority area of the field is paramount. Therefore the moisture probe should be placed in an area that contains the majority soil type and does not have significant terrain influence.

This is achieved using EM surveys and derivatives of the RTK elevation data.  This is a starting point that can be refined over a number of years.

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Figure 5: Intersection of majority soil and neutral terrain

FAQ
Why do growers do this?
Primarily because where the probe is located is going to be managed and as a result will be the highest yielding region.  If it represents only a minority of the soil type then it most likely will cause sub optimal management of the larger part of the field and a reduction in yield for this zone. This often results in a blue or green line as shown in Figure 4.

Why not use yield data?
This is a good question and the answer is never simple.  Think of all the things that influence yield.  A yield map contains significant amounts of information such as current agronomic and management practices, an assumption that water is not being used properly (other wise why would you bother to change), along with many historical influenced such as compaction, crop type and weather conditions (i.e weather damage) etc. There are simply too many variables from one season to the next to use yield data as a reliable input for an irrigation management strategy.

How many probes should we use?
To properly answer this first requires an understanding of the management practice  If the probes are being placed to optimize a VR Irrigation solution then the return from managing the variability could warrant more than one. This will depend on the amount of variability in the field. The higher the CV the higher the potential return and therefore the investment in more probes and zone control could be beneficial. If the field is surface irrigated then the control of water is limited. Therefore one probe ideally placed and well managed should be adequate. Most countries already successfully using probes are using only one per field.

What about variable rate nutrients and seed?
Once a sound irrigation management strategy is in place then a suitable nutritional program can be implemented. The nutritional program is based on the potential yield zones, controlled by water application.