PAW is about water storage capacity.
Drainage class is about whether water sits in the soil over significant periods of time. This is independent of PAW.
A poorly drained soil might have a high water table or a firm layer that water perches upon rather than draining away. It can have low or high PAW.
A well drained soil means that there are no soil limitations to water draining below the roots – and it might have low or high PAW.
Indicates how wet a soil is likely to be under high rainfall conditions. Well-drained soils allow water to drain through the profile in all periods of the year. Poorly drained soils either have a water table close to the surface or a compact subsurface layer that limits the rate that water can drain through the soil. Imperfectly drained soils are in an intermediate condition between well-drained and poorly drained. A poorly drained soil is susceptible to pugging and it will lose nitrogen to the atmosphere. A deep, poorly drained soil will cope better in a drought. Well-drained soils can sometimes be droughty, but are less likely to pug. Provided they are not prone to bypass flow, poorly drained soils will not leach to the same extent as well-drained soils.
Profile available water (PAW)
The amount of water potentially available to plant growth that can be stored in the soil to 100 cm depth. PAW takes into account variations in soil horizons and is expressed in units of millimetres of water, i.e. in the same way as rainfall. A PAW of 100 mm implies that 10% of the soil volume is water available to plants. Low PAW is <60 mm, moderate is between 60 and150 mm, and high is ≥150 mm.
Plants can only extract water where roots can grow. Thus where a root barrier occurs within 100 cm, the PAW reported will be the PAW to the root barrier. It is important to recognise that PAW is a potential value and not all the water is equally available. For example, as the soil dries out the water becomes more difficult to extract. As a general ‘rule of thumb’, plant growth will begin to slow down when 50% of PAW has been extracted. There are some crops that have shallow rooting depth, e.g. potatoes usually only root to a depth of 60 cm. In this case the PAW to 60 cm depth should be used.
NB. Answer provided by Manaaki Whenua Landcare Research and reproduced here with permission.