Diagnosing waterlogging in south-west Western Australia

Page last updated: Monday, 17 May 2021 - 12:51pm

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Waterlogging in the higher rainfall areas (more than 450 mm annual rainfall) of south-west Western Australian crops and pastures is a common cause of reduced plant growth in winter, especially on duplex soils.

Identifying waterlogging as the cause of crop yellowing or stunted growth allows managers to change in-season agronomy, and to avoid or treat the problem in following years.

This page provides information on the signs of waterlogging and practical ways to confirm the diagnosis.

This information is for the higher rainfall (usually more than 450 mm) areas of the lower south-west of Western Australia.

Steps to diagnose waterlogging

  1. Observe the signs of waterlogging in soils and plants.
  2. Note the soil type and landscape position.
  3. Check soil in the root zone for water content.
  4. Note yield changes against marked areas.

The best way to identify waterlogged areas is to look. In winter, if water is present at the surface (i.e. glistening in small depressions), the area is likely to be waterlogged. If waterlogging is suspected and not visible, dig a couple of holes to about 30 cm and check if water flows into them; if it does, the soil is waterlogged.

Observe the signs of waterlogging

Waterlogging is most likely in winter after periods of heavy rainfall. Indications of waterlogging are:

  • water ponding on the soil surface
  • the surface staying wet and glistening
  • dark, wet, or slimy topsoil, often with algae on the surface
  • boggy soils (you just lost the tractor)
  • yellowing crops and pastures, and 
  • weeds, such as toad rush, cotula, dock, and Yorkshire fog grass.

Note the soil type and landscape position

Duplex soils are particularly susceptible to waterlogging, especially shallow duplexes on gentle slopes, change of slope, or level areas. Waterlogging can also occur on flat valley floors and level plains. Sand over clay duplexes can have short-term root zone waterlogging without showing obvious signs.

Check soil in the root zone for water content

  • Dig holes to about 30 cm deep to determine whether any free water flows into it; if it does, the soil is waterlogged and plant growth can be affected. See waterlogging: the science for more information on assessing the severity of waterlogging.
  • Duplex soil can have very rapid onset and drainage of waterlogging. Daily monitoring may be needed to identify periods of waterlogging.
  • Grey or greenish-coloured subsoil can indicate long-term waterlogged conditions.
  • The degree of waterlogging can be very different over short distances (metres) and short periods (days).

Note yield changes against marked areas

Waterlogging that reduces crop or pasture yield is worth managing:

  • Mark out the affected areas, either with posts laid on the ground or on an accurate map, and 
  • At harvest time, observe where the crops are poor and check this against earlier observations.

Severity of waterlogging

The severity of waterlogging can be indicated by a Sum of Excess Water (SEW) value. SEW integrates waterlogging that occurs during the winter growing season in the primary root zone of the top 30 cm soil layer (SEW30) (Setter & Waters 2003).

SEW30 is the sum of all daily values in centimetres (cm) of watertables closer than 30 cm to the soil surface (in units of centimetre days [cm.days]). For example: 10 days rising to 10cm below the surface (20 cm above the 30 cm depth measured) is 10 x 20 = 200 cm.days. See Table 1 for waterlogging classes.

Table 1 Waterlogging classes using the SEW30 index
Waterlogging class
(in terms of drainage)
SEW30 index
(cm.days in an average growing season)
Well drained <30
Moderately well drained 30–100
Moderately drained 100–250
Imperfectly drained 250–500
Moderately poorly drained 500–1200
Poorly drained 1200–2500
Very poorly drained >2500

Table 1 is derived from Bulletin 4343 Soil Guide: a handbook for understanding and managing agricultural soils.

Now you have diagnosed waterlogging, what next?

There are several ways of preventing or reducing waterlogging; see the managing waterlogging page.