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Fact Sheet 1- What happens to rainfall on your paddocks?
Fact Sheet 2- Does summer weed control save soil water?
Fact Sheet 3- Calculating maximum plant available water of your rootzone
Fact Sheet 4- Increasing Plant Available Water
Fact Sheet 5- Properties of a healthy soil
Fact Sheet 6- Root Facts
Fact Sheet 7- Soil Pit Checklist
Fact Sheet 8- Soil Water Facts


Fact Sheet 3: Calculating maximum PAW of your rootzone

This is based upon the different textures of the soil within the rootzone and their ability to hold water.

 Why?

  • To understand better water balance in your soils.

  • To assist in estimating stored soil water throughout the season.

Rootzone PAW (max) = the amount of water in the soil which is available to the plant

  •  at PAW (max) the soil is at field capacity

  • at PAW (0) the soil is at wilting point

  • (there may still be water in the soil, but it is not available to the plant).

The water holding capacity of the soil is dependent upon texture.

 We must determine the texture of the rootzone soil.

Rootzone Texturing:

  • Texture each 10cm layer to 75cm, 
    or

  • Dig to 30cm only and assume that the deepest texture continues to the 75cm depth.

Calculating Maximum Plant Available Water of your Rootzone:

PAW (max)= (Water holding capacity of soil) x 
                  Depth (From Table
                 for each soil texture down to 75cm

 

 

Texture

Plant Available Water (PAW)*

Coarse sand

0.4

Fine sand

0.6

Loamy sand

0.8

Sandy loam

1.2

Light sandy clay loam

1.3

Loam

1.5

Sandy clay loam

1.5

Clay loam

1.8

Clay

1.5

Self mulching clay

2.0

*(mm water per cm soil)

Water Balance Tables

 ORIGINAL SECTION

 

NEW SECTION

A

 

 

 

 

SSP

PAW (max)

Month

Evaporation rate (mm water/day)

 

 

 

 

 

 

 

A

 

 

 

 

SSP

PAW (max)

Month

Evaporation rate (mm water/day)

 

 

 

 

 

 

 

 

 

 

B

 

 

Initial PAW

JULY RAINFALL (mm)

INPUTS

 

 

 

 

B

 

 

Initial PAW

JULY RAINFALL (mm)

INPUTS:

 

 

 

 

 

C

 

INFILTRATION

= Total Rainfall x SSP

OUTPUTS:

 

 

 

C

 

INFILTRATION

= Total Rainfall x SSP

OUTPUTS:

 

 

 

D

 

 

 

RUNOFF

= Total Rainfall Infiltration

EVAPORATION

= (Evap rate x No days) / (SSP x 5)

DRAINAGE

= Initial PAW + Infiltration Evaporation PAW (max)

= B + C D A

If less than 0, put 0

OUTCOME:

 

 

 

 

 

 

D

 

 

 

RUNOFF

= Total Rainfall Infiltration

EVAPORATION

= (Evap rate x No days) / (SSP x 5)

DRAINAGE

= Initial PAW + Infiltration Evaporation PAW (max)

= B + C D A

If less than 0, put 0

OUTCOME:

 

 

 

 

 

 

 

 

E

 

 

TOTAL PAW

= Initial PAW + Infiltration Evaporation

= B + C D

If greater than PAW (max), put 105

If less than 0, put 0

YIELD CONTRIBUTION

50mm water = 1 tonne grain

= E x 0.02 (tonne/ha)

INCOME

@ $150/tonne

= Yield x $150

 

 

 

 

 

 

E

 

 

TOTAL PAW

= Initial PAW + Infiltration Evaporation

= B + C D

If greater than PAW (max), put 105

If less than 0, put 0

YIELD CONTRIBUTION

50mm water = 1 tonne grain

= E x 0.02 (tonne/ha)

INCOME

@ $150/tonne

= Yield x $150

 

 

 

 

Post canopy establishment:

Once the plant canopy is fully established, the soil is protected from wind and rain, so infiltration improves (100% rainfall infiltrates) and evaporation drops to almost nothing.  As the crop continues to grow it pumps water from the soil by transpiration at the PAN rate for that period (transpiration loss = no day in month x PAN rate for that month).

Estimating Initial PAW

We assume that initial PAW in April is 0 unless there has been a significant rainfall within six weeks (see Fact Sheet 2- Does summer weed control save soil water?).

 

We can estimate soil PAW at any stage by keeping track of how much rainfall infiltrates and how much is lost by evaporation or transpiration (using local PAN evaporation rates: Fact Sheet 1- What happens to rainfall on your paddocks? ).

 

Fact Sheet 3: Calculating maximum PAW of your rootzone

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