Code of Practice - Irrigated Public Open Space - SA Water

5.8.2 How much water is stored in the soil?
Total Available Water (TAW)
The amount of water stored in the soil and available to the
plant is referred to as the Total Available Water (TAW).
When the soil reservoir is full it is said to be at field
capacity. Any further water applied to the soil will either
run-off or be lost below the root zone by drainage. Plants
will extract water from the soil as required until the
remaining water in the soil is no longer able to be taken in
by the plant. This is called ‘wilting point’. If water is not
applied, either through natural rainfall or irrigation, the
plant will become stressed to the point where it will die.
The total available water in the root zone is the difference
between the water content at field capacity and wilting point.
The TAW is determined by the soil type and its water
holding capacity and the Root Zone Depth (Zr) of the plant.
The calculation is:
TAW = WHC x Zr
Where:
TAW - Total Available Water (mm)
WHC - Water Holding Capacity of the soil (mm/mm)
Zr root - Zone Depth (mm)
Soil Water Holding Capacity (WHC)
Water is stored in the soil between the pore spaces or the
soil particles. Different soils have different sized pore spaces
and there is a significant variation in the water holding
capacity of different soils. By taking a soil sample and
determining the soil type, the water holding capacity of the
soil can be determined from the following table.
5.0 TURF & IRRIGATION MANAGEMENT PRINCIPLES
24
Table No 7 – Typical Water Holding Capacity and
Infiltration Rate of Soils
SoilType Soil Water Soil Water Soil
Holding Holding Infiltration
Capacity (WHC) Capacity (WHC) Rate
mm/metre mm/metre (mm/hr)
Sand 60 0.06 >20
Fine sand 90 0.09 15 – 20
Sandy loam 110 0.11 10 – 18
Loam 150 0.15 10 – 15
Silt loam 160 0.16 8 – 12
Clay loam 180 0.18 5 – 10
Clay 150 0.15 < 5
Plant Root Zone Depth (Zr)
The extent to which the plant roots grow into the soil
determines the depth and volume of water in the soil the
plant can access. Generally open soils such as sand enable
plants to develop root systems to a greater depth than
heavier clay soils. The depth of roots in turf can be
determined by taking a soil sample and measuring the
depth of roots in the profile.
Having determined the soil water holding capacity and the
plant root zone depth, the total available water can be
calculated as follows;
Where Kikuyu is grown in sandy loam with a root zone
depth of 150mm:
TAW = WHC x Zr
= 0.11 x 150 = 16.5 mm