Using Your Soil Data To Calculate The Readily Available Water (RAW).

Knowing how much water your soils can hold will make irrigating more productive, and irrigation planning much easier. Where there is lots of clay, the holes (pores) in the soil are smaller and the water is held very tightly. It is more easily extracted from the larger pores found between sand particles. The Readily Available Water (RAW) is the amount of water stored in the soil pores which can be easily used by plants.

We express readily available water as millimetres per centimetre of soil depth (mm/cm).

As mentioned, plants cannot extract all of the water held in the soil. We know that plants extract water using their roots, so only water in the soil where roots occur (the rootzone) is available for plants.

Calculating RAW:
Readily available water is calculated for each soil layer at each site using soil data from our soil survey and values from the table below.

Grade	8-40kPa	8-60kPa	8-200kPa	8-1500kPa
Sand (S)	0.36	0.38	0.40	0.62
Loamy Sand (LS)	0.52	0.55	0.58	0.87
Clayey Sand (CS)*	0.55	0.60	0.64	1.00
Sandy Loam (SL)	0.59	0.65	0.70	1.15
Light Sandy Clay Loam (LSCL)	0.65	0.74	1.03	1.37
Loam (L)	0.69	0.84	1.00	2.43
Sandy Clay Loam (SCL)	0.61	0.71	1.01	1.43
Clay Loam (CL)	0.53	0.65	0.73	1.48
Clays (SC,LC,LMC,MC)	0.46	0.57	0.66	1.49
Heavy Clay (HC)**	0.25	0.41	0.49	1.20

* Interpolated value. ** Samples from Kununurra W.A.

In the table above the water deficit (mm) per cm of soil depth indicates the amount of water required to bring the soil back to field capacity at a particular reading on a tensiometer.

The following exercise is for a fictional property but it will show you what to do with your information.

In order to bring a sandy loam from a refill point (40 kPa) to field capacity (8 kPa):

According to the table above 0.59 mm depth of water is needed for every centimetre depth of a sandy loam soil type.
If the layer is 50 cm deep, 29.5 mm (50 cm x 0.59 mm/cm) would be needed to bring that particular soil layer back to field capacity.

In order to bring a sandy loam from dry (60 kPa) to field capacity (8 kPa):

According to the table above 0.65 mm depth of water is needed for every centimetre depth of a sandy loam soil type.
As the layer is 50cm deep, 32.5mm (50cm x 0.65 mm/cm) of water is needed to bring that particular layer back to field capacity.

Thus, the drier the soil before irrigation, the more water is needed to bring that soil profile back to field capacity. On the other hand soils are like big sponges, which can only soak up a limited amount of water - when fully wet little benefit is gained in applying more water, as this leads inevitably to waterlogging, drainage problems and loss of valuable fertilisers.

Example RAW Calculation

Layer Depth	25	cm
Deficit	0.61	mm/cm
Raw 1	15.25	mm
Layer Depth	50	cm
Deficit	0.53	mm/cm
Raw 2	21.2	mm
Raw 1 + Raw 2	15.25 + 21.2	mm
Total	36.45	mm

(Source: Swan Hill Irrigation Management Course, Book 1, Chapter 1,D.)