RAW is Readily Available Water, that is the water available to a plant’s roots stored in the soil within the plant’s root zone. It is often referred to as AW, Available Water. By knowing your soil types and the water requirements of your crops, in different climatic conditions, irrigation becomes accurate with none or very little wastage.
Using the table provided below, the RAW stored within the effective root zone for a soil can be calculated by measuring the thickness of each soil layer (in metres or parts of a metre) determining the soil texture of each layer, and then multiplying the thickness by the appropriate RAW Value.
In general, soft leafy crops will wilt very quickly and are very water-sensitive whereas trees and vines have better developed root systems and can exert strong tension on the soil.
This is an example of calculating root zone RAW for a lucerne crop grown in 0.3metres of sandy loam over 0.5m of light clay and 0.4m of medium clay. Once you had dug a soil pit at this site, you would work out the RAW as follows:
- Identify and measure the soil layers.
- Determine the soil texture of each layer.
- Identify the crop water tension and RAW value for each layer for lucerne. eg. Layer 1 Sandy Loam 65 mm/m, Layer 2 Light clay 55 mm/m, Layer 3 Medium clay 55 mm/m.
- Multiply the thickness of each layer by the RAW value (Layer 1 = 19.5mm, Layer 2 = 27.5mm, Layer 3 22mm)
- Add these values for each soil layer together to come up with the total soil profile RAW (69mm).
- Identify the effective root zone for this example soil (0.8m)
- Add up the RAW values for the crop’s effective root zone (which only includes Layers 1 and 2 in this case = 47mm)
For this lucerne crop example, 47 mm of water is readily available in the soil for the plants use. When the soil suction reads -60kPa, 47mm is required to refill the plants effective root zone.
Now that I have shown you how to calculate RAW values for a crop and soil, its time to put this technique into practice.
Take a soil profile on your property, either from a roadside cutting or from a soil pit that you have dug especially for this exercise. Go through the steps to calculate the RAW value for the soil and crop as outlined. What was the RAW value that you calculated? Was it what you expected? Can you see how to apply this technique to improve your irrigation efficiency?
The rate at which water can enter the soil is called the Infiltration Rate. The longer that water is applied, the slower this rate becomes.
The rate that water is applied must not exceed the soil’s infiltration rate with pressurised irrigation systems. With surface irrigation systems such as border dyking, the application at any one point must be long enough to allow enough water to enter the soil profile (based on the RAW values).
The consequences of exceeding the soil’s infiltration rate are soil damage and runoff causing soil erosion, loss of fertilisers and excessive waterlogging of root zones in lower lying areas.
This Table shows the average Infiltration rates for the six main soil texture classes. Note how the application rate (in mm per hour) varies significantly according to how well structured the soil is. If the soil structure changes in the subsoil, then the infiltration rate will change and the application rates will need to be adjusted for this.
Infiltration depends on soil texture, structure, porosity, bulk density as well as groundcover, slope and dispersion
In practical terms, you can judge how far moisture is infiltrating into the soil by digging a hole after a rainstorm event or by using a soil moisture probe. This is essentially a sharpened steel rod with a handle that you can push into the soil. The moist soil provides far less resistance than when you penetrate the dry soil beneath.
All irrigators have to decide many times during a growing season whether to water their plants immediately or wait a little longer. Water is generally a scarce resource and becoming more expensive, so there can be good practical reasons for delaying the next watering.
The main methods that farmers use to decide when to water are:
- “Hit or Miss”
- Using Instruments
- Using Evaporation figures.
If you water frequently, just to “make sure my plants get plenty”, you will usually apply much more water than is needed. In terms of the “Hit or Miss” system, you may “hit” the problem but you will certainly “miss” some money from your pocket! However, if you take the other extreme, many of us are reluctant to water any more than is absolutely necessary. By the time many plants are clearly wilted, they will “miss out” on water supplied at that point to try and recover them to full productivity. Another version of the “hit or miss” method is the “give my plants the same amount once a week, no matter what the weather has been” system. This is equally expensive and wasteful and we should be able to do better than this.
Some farmers can, after years of experience, recognise the subtle changes in plant colour and appearance resulting from mild moisture stress, and appreciate the effects of different weather conditions on water loss, by understanding their soils but these farmers are probably in the minority. By the time you have sufficient experience to notice these things, your eyesight starts to fail and you’re back at square one!!
However, careful observation of your crop should not be discounted, particularly if you back it up with a regular digital photo of the crop from the same place. I suggest this because digital photos are now almost free and otherwise, your memory may be a pretty hazy instrument.
Mild moisture stress in corn.
For further information refer to the article Irrigation Scheduling using instruments, and further exploration of the INFORMED FARMERS categories of Natural Resource Management - Soils and Water.
