Fact Sheet 2: Does summer weed control save soil water?

Summer rainfall will only be stored for seeding if: The paddock is weed free. Significant rain falls within six weeks of seeding. Rainfall events in the six week period are enough to fill the wicking zone.

The following information will help you understand what happens to summer rainfall and whether it is cost effective to control summer weeds for moisture conservation.

The soil can be divided into three zones for water storage and loss.

1: Surface Soil 

• Water is lost by evaporation at a fast rate. 

• Approximately 8mm water/day. 

• Evaporation rate is driven by wind speed and temperature. 

• Stubble cover will reduce the rate of evaporation. 

• Weed cover will reduce surface evaporation, but increase water loss from the rest of the soil profile. 

• Tillage will increase the soil surface area and more water will be lost from the surface layer. 

• Surface crusts will reduce infiltration, so less of the summer rainfall will enter the soil. 

2: Wicking Zone 

• The zone immediately below the soil surface. 

• As the surface dries out, water will be drawn up from this zone and lost to the surface by evaporation. Therefore, the rate of loss is also approximately 8mm water/day. 

• Shallow rooted weeds will draw moisture from this zone at approximately 8mm water/day. 

• The volume of water loss and size of the wicking zone depends on soil structure. Tillage can break the wick of water travelling upwards, reducing further losses by evaporation. 

3: Deep Soil Water Store 

• In weed free paddocks, water moves at a slow rate (approximately 0.5mm water/day) from this zone to the wicking zone. 

• Deep rooted weeds will draw water from this zone at a rate up to 8mm water/day. 

• Barriers. such as boron or thin hard pans, will prevent roots reading water in this zone.

Figure 1: Rates of water loss with different soil surface cover.

Rule of thumb 

Only summer rain, falling within six weeks of sowing, is likely to be stored in the soil for the crop at sowing. All water to infiltrate before this point is lost.

Rule of thumb 

Rainfall events less than 50mm are unlikely to remain in the soil until seeding, unless they occur in the week of seeding. 

Is water loss the same for all soils?

• Water loss is determined by soil structure and surface soil cover (see Figure 1 for cover).

• Sandy soils have bigger pore spaces and lose less water by wicking than clay soils. See Figure 2.

• Poorly structured soils have smaller pore spaces and will also lose more water. See Figure 3.

• A poorly structured soil can lose up to 40mm water from deep in the profile before the wick breaks.

• A well structured soil will wick from the top few centimetres and only lose 5 - 10mm before the wick breaks.

• 50mm of stored water equates to 1.0t/ha increase yield potential if the water is stored for plant growth.

• Wicking zone depth increases with clay content and declines with soil structure. See Figure 2.

Improve structure to increase rainfall capture and holding.

How? 

• Retain stubble for physical protection. 

• Increase organic matter retention. 

• Reduce tillage and traffic. 

• Incorporate gypsum in crusting soils.

Rule of thumb 

• 10mm rainfall will infiltrate approximately 10cm into the soil.

• For rainfall greater than 50mm, a larger proportion of the rainfall moves beyond the wicking zone and avoids the fast rate of evaporation.

Figure 2: What happens to a 50mm rainfall event on different soil types with different surface properties, assuming the soil is at wilting point?

Figure 3

How much infiltration is needed for moisture storage at depth?

The size of the wicking zone increases as soil structure decreases. the larger the wicking zone, the more rainfall that must fall before water is stored.

To estimate stored soil moisture we need to know: How much rainfall enters the soil. This is determined by surface soil properties (see Fact Sheet 1- What happens to rainfall on your paddocks?).

Note:
The numbers in Table 1 assume that the soil has dried out to wilting point over summer. · Some clay based soils dry out to below wilting point and, therefore, far more water is required to fill the upper region. Farmers with highly clay soils should err on the side of caution when estimating summer rainfall storage.

Summer rainfall storage - yield and returns 

Storing an extra 5mm will increase income potential enough to break-even on summer weed control.

Fact Sheet 2: Does summer weed control save soil water?

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