Guideline Index

Chapter 7: Managing Limiting Soil Factors

7.4 Waterlogging

Waterlogging occurs when most or all of the macropores become filled with water rather than air. It occurs more easily in soils that have a greater proportion of micropores than macropores, because the macropores promote free drainage while the micropores tend to hold on to water. Both compaction; which presses the soil particles and aggregates closer together, and dispersion; which fills the pore spaces with clay particles, tend to promote waterlogging.

7.4.1 Causes of waterlogging

Waterlogging is a common problem on many soil types, particularly in predominantly clay soils. Waterlogging may be due to periods of heavy rainfall, dispersion, compaction, poor irrigation management, rising watertables, or a combination of poor surface drainage (across the paddock) and poor subsurface drainage (down the soil profile). Figure 7.4 shows a well-aerated soil and a waterlogged soil.

Figure 7.4   Well-aerated soil (left) and waterlogged soil (right).
Figure 7.4 Well-aerated soil (left) and waterlogged soil (right).

In rain-fed WA dairy soils, waterlogging occurs in flat areas on soils comprising 0.5 to 2.0 m sand to sandy loam soils over a very impervious clay layer. This forms a surface aquifer and the pasture root zone is no deeper than 20 cm.

7.4.2 Effects of waterlogging

Soil strength decreases as the soil gets progressively wetter, resulting in a greatly increased potential for damage to the soil structure. Waterlogging prevents air and many nutrients from reaching the roots, thus seriously inhibiting plant growth. Also unfavourable gases and compounds are produced in the root zone due to the anaerobic (no oxygen) conditions favouring anaerobic soil micro-organisms at the expense of aerobic micro-organisms – See Chapter 5 . A build-up of salt is also more likely under poorly drained irrigation pastures.

Where soil drainage is poor, numerous effects are seen. The severity of these effects will depend on things such as the duration of waterlogging and the soil texture. In light-textured soils the effects will not be as detrimental.

Waterlogging effects include:

  • Plants are stunted and yellow as nitrogen is lost due to denitrification which is caused by a lack of aeration – see Figure 7.5.
  • Improved pasture plants are replaced by tolerant weeds (docks, smartweed, rushes, sedges, couch, etc.).
  • Pastures become fouled with mud and utilisation is reduced.
  • Pasture growth rates decline.
  • Soils become pugged and water ponds on the surface.
  • Responses to applied fertiliser are poor.
  • Nutrient balance in the pasture is upset with lower nitrogen, potassium, magnesium and chlorine in the pasture.
  • Change in soil biology from aerobic to anaerobic soil organisms – See Chapter 5.
Figure 7.5   Sorghum plants waterlogged for an extended duration showing poor growth and discolouration. (Photograph supplied by David Hall).
Figure 7.5 Sorghum plants waterlogged for an extended duration showing poor growth and discolouration. (Photograph supplied by David Hall).

7.4.3 Managing waterlogged soils

When waterlogging has occurred, prevention of further damage to the pasture and soil structure is the first priority. This can only be done by keeping vehicles and animals off waterlogged areas. A grazing management technique called ‘on-off grazing’ can significantly reduce pugging damage and increase pasture utilisation. With the ‘on-off grazing’ technique, stock are only allowed to graze the paddock for a short period (2 to 4 hours) and are then held in a stand-off area, such as a feedpad, a laneway, an old sand quarry, or the dairy shed yard.

Long-term management involves removing the cause of the waterlogging. Waterlogging may be caused by a soil chemical or physical problem, such as dispersion (see Section 7.2.2) or compaction (see Section 7.3), or by a high watertable or poor irrigation management.

In districts where waterlogging is caused by a high watertable, management strategies aimed at controlling the level of the watertable will be required, and these are outlined in Section 7.5.7, ‘How can we best manage salinity’, and Section 7.4.5, ‘Reducing waterlogging through drainage’.

If the problem is related to poor irrigation management, see Section 7.4.4, ‘Reducing waterlogging through irrigation management’.

7.4.4 Reducing waterlogging through irrigation management

Waterlogging is one of the most limiting factors affecting flood-irrigated pasture production in southern Australia. Unfortunately, white clover is the most susceptible pasture species to waterlogging.

In a flood irrigation situation, waterlogging can occur because it takes too long to get water:

7.4.4.1 Water On

A compromise needs to be found between irrigating quickly to reduce waterlogging, and allowing enough time for adequate soakage of water into the bay. With a well-designed bay on suitable soils, the required amount of water should be applied in 2 to 4 hours. However, up to 6 hours is generally accepted as reasonable. The time needed to apply the required amount of water can be varied by adjusting the flow rate. On lighter, more permeable soils, a shorter irrigation time is appropriate to reduce water losses beyond the pasture root zone. If the area at the bottom of the bays is not producing well due to waterlogging, or there is no drainage reuse system, watering ‘short’ can be a water saving compromise – See Victorian Resources Online.

Four factors commonly cause slow watering and can lead to a waterlogging problem. These are:

  • Inadequately sized (small) farm channels and channel structures.
  • Small bay outlets.
  • Weeds in channels restricting the water flow.
  • Irrigated ground that is high.

Any of these factors will restrict the flow of water, cause the wheel to slow down and slow down irrigation.

Leaking bay outlets may also be the cause of a waterlogging problem at the top of an irrigation bay. Refer to the Target 10 Irrigation and Drainage Reference Manual. See Victorian Resources Online to find out how to address these problems.

7.4.4.2 Water Off

Water should not be left lying on the bay. It should be drained away as quickly as practicable to prevent ponding and waterlogging. Ideally, there should be no water lying on the bay 24 hours after irrigation has commenced.

Drainage off the bay will be affected by:

  • Bay slope.
  • Soil type.
  • Condition of the drain at the bottom of the bay.
  • Length of bay.
  • Evenness of slope.
  • Presence of spinner cuts.
  • Pasture height, type and density.

Water will stay longer on longer bays, on bays that are flat or are on heavy soil types, or where the drains at the bottom of the bay are in poor condition.

Longer bays (longer than 400 m) do not drain well, particularly in winter when evaporation rates are low. The water has to move a longer distance to reach the drain, and the bay stays wetter longer.

Spinner cuts running down the bay will improve drainage. The spacing of the spinner cuts across the bay will depend on the severity of the drainage problem. A spacing of 15 to 20 m between spinner cuts is often used.

It is important that there is a well-defined drain at the bottom of the bay. This drain should connect with the rest of the farm drainage system so that it carries runoff to a reuse system, to a regional or community drain, or to a natural watercourse.

There are a large number of factors to consider when trying to overcome an irrigation waterlogging problem. Developing a whole-farm plan provides a means to ensure that an irrigation layout is well planned and well designed. A good irrigation layout is essential to minimise waterlogging problems and optimise pasture production.

7.4.5 Reducing waterlogging through drainage

Overcoming waterlogging through drainage may help preserve soil structure.

In high-rainfall dairy pastures, increases in pasture utilisation of 40% to 60% have been measured on drained (subsurface drainage) compared to undrained paddocks.

 

Improved pasture yields and pasture composition have also been measured on drained paddocks. Soil salinity levels are also often lowered by subsurface drains because the drained water removes some of the salt.

Drainage systems need to be planned, constructed and maintained effectively to have a long-term, positive effect on both the on-farm and off-farm environments.

Because the drainage water often has to flow onto neighbouring properties, drainage works are best done in cooperation with the neighbouring landholders and in conjunction with the relevant water authorities.

Drainage of the soil can reduce waterlogging effectively but at a cost. Other strategies, such as ‘on-off grazing’ (see Section 7.4.3) and agistment, are used on some farms.

7.4.5.1 Surface drainage

Improvement in surface drainage should be investigated first, as it is the simplest and cheapest option. Surface drainage involves maintaining existing drains and installing additional drains that are adequately sized and positioned, usually placed along fence lines or through depressions. If possible, emphasis should be placed on preventing water from the upper paddocks flowing over onto the lower paddocks.

However, many farmers mistakenly believe their waterlogging problems are due to surface water alone, when they are actually often due to subsurface water or to a combination of surface and subsurface water. In these cases, a combination of surface and subsurface drainage may be required.

7.4.5.2 Subsurface drainage

Poor subsurface water movement is caused by an impediment to the water moving down the soil profile. Possible impediments include a heavy soil texture, compacted layers, and natural or induced hard pans in the profile (which can create a raised watertable). Poor subsurface water movement can also be caused by subsurface water moving downhill from upper slopes or by springs.

In irrigation areas, groundwater pumping is a common form of subsurface drainage.

In high-rainfall areas of Australia, the main forms of subsurface drainage are subsurface pipe drains or mole drains or a combination of both. The type of drain installed depends on soil characteristics, rate of drainage required, and topography.

Note: In irrigation areas, your irrigation supply authority should be contacted before installing subsurface drainage. In some regions, permission is required from the local irrigation supply authority to discharge water from subsurface drainage systems off the farm.

Subsurface pipe drains

Free-draining topsoils with an impermeable layer at a depth of more than 0.7 metres, or deep, free-draining soils subject to rising watertables, require pipe drains. Pipe drains are constructed by placing a slotted PVC or corrugated plastic pipe in a trench and then surrounding the pipe with a permeable backfill, such as stone or gravel – see Figure 7.6.

Figure 7.6   Subsurface pipe drain
Figure 7.6 Subsurface pipe drain

Subsurface pipe drains are expensive to install but are very effective and economically viable in the correct situations. They can last for many years, provided they are correctly installed and consistently maintained.

Mole drains

Clay and clay loam soils with poor natural drainage and with clay less than 40 cm from the surface are generally suitable for mole drainage – see Figure 7.7. A mole drain can be made simply by pulling a metal object (i.e. a ripper blade with cylindrical foot, or mole plough) through the soil, leaving an open channel. Mole drains cost less than tile drains but require more maintenance .

Figure 7.7  Mole drain
Figure 7.7 Mole drain

The clay through which the mole is pulled must be plastic when wet (retains shape of mole) and stable (not prone to cracking, dispersion or slaking). If installed correctly in appropriate soil, mole drains may function adequately for 3 to 7 years. For more information on mole drainage see http://www.agric.wa.gov.au/objtwr/imported_assets/content/lwe/water/drain/bulletin4610.pdf