5.5 What Happens to Rain On Your Paddocks?

Surface properties of the soil influence its ability to infiltrate water and resist loss by evaporation. This table aids in the evaluation of surface soil properties, providing a means to estimate infiltration and evaporation rates.

Symptom	Rating				Score
Symptom	1	2	3	4	Score
Water repellence	No repellence problems	Rarely repels	Frequently repels	Always repels	1
Clay content	Sand	Loamy sand	Loam	Clay	2
Slope	Flat ground	Slight slope	Moderate slope	Steep slope	2
Ground cover at break	50% of ground is covered	20% of ground is covered	Some straw visible	No grass or straw present	4
Aggregate stability (when dropped in still water)	Soil aggregates do not collapse	Aggregates break up a little	Aggregates collapse into small pieces	Aggregates collapse to a powder size	3
Run off/erosion	Never	Rare (1 year in 3)	A few minor channels most years	Most years large channels (> 50mm)	3
Surface crusting	No surface crust	Thin crust forms sometimes but not a problem for emergence	There are often emergence problems - plant has to push up a 'plate' of soil	Serious emergence problems in most	4
Pre-cultivation strength*	Can push in a 3mm ros with the palm of your hand - no pain	Can push in a 3mm rod with the palm of your hand - a bit painful	Cannot push in a 3mm rod with the palm of your hand		3
				TOTAL SCORE	22

*measure after good soaking rain

Note: A high score (low SPP) can indicate a poor soil, but high scores can also result if the soil is a clay or steeply sloped where even the best management will result in water wastage.

DETERMINE SSP RATING:

SSP rating = 1 - (Total / 85)

Example: 1 - (22/85) = 0.74

round down to SSP 0.7

The SSP rating indicates the fraction of rainfall water to enter the soil. A soil with SSP 1.0 will allow all rainfall to infiltrate, whereas a soil with SSP 0.6 infiltrates 60% of rainfall water.

The SSP rating also helps us to determine soil water loss by evaporation and is used in water balance calculations.

This soil loses topsoil water at the PAN evaporation rate divided by 0.7.

Improving infiltration and evaporation

Stubble retention on all soil types helps to:

protect soil from raindrop impact
reduce evaporation by cutting wind
improve structure by increasing organic matter content

After 5-10 years of direct drill and stubble retention at the SA Long Term trial Sites, the following were observed. These would give a potential change in SSP (figures in brackets):

Water use efficiency and yield increased over the life of the trial.
Runoff and erosion channels were reduced (3 to 2).
Surface sealing was reduced (4 to 2).
Aggregate stability improved (3 to 2).
Pre-cultivation strength of topsoil reduced (3 to 2).
Improved water holding capacity in top 10cm.

Stubble retention would change SSP for surface cover from 4 to 1. The soil now has a total score of 14 which calculates to 0.84 (round down to 0.8).

At the start of the trial the soil rated a surface soil property (SSP) of 0.7. The changes seen with direct drill and stubble retention would change the SSP rating to 0.8. The soil now infiltrates 10% more water and so, in a season where 200mm used to infiltrate, the crop now has access to 220mm. This extra 20mm potentially translates to an extra 0.4 tonnes grain per hectare ($60/ha at $150/tonne).

The increase in organic matter has also increased the water holding capacity of the soil. The crop now has a bigger bucket to access, allowing it to last longer between rains without water stress.

Addition of gypsum to sodic soils and clay to non-wetting sands will improve soil structure, increasing the SSP rating of the soil in a similar fashion and leading to improvements in soil infiltration and evaporation. (see Fact Sheet 3)

PAN EVAPORATION RATES

Click here or on the the chart thumbnail on the right for an Excel chart of PAN evaporation rates for selected places in South Australia.

Evaporation from the paddock

Bare soil loses topsoil water by evaporation at PAN rate (see Fact Sheet 2).
Stubble covered soil loses topsoil water by evaporation at a slightly lower rate.
Plant covered soil loses little water by evaporation, but loses topsoil water and deep profile water by transpiration at the PAN rate.

5.5 What Happens to Rain On Your Paddocks?

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