5.7 Calculating maximum Plant Available Water 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 1) for each soil texture down to 75cm

Diagram

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)

Source: Better Soils Mean Better Business Handbook (Figure 17, P 143)

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).

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)

5.7 Calculating maximum Plant Available Water of your rootzone

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