Why waterlogging needs to be managed
Waterlogging can lead to:
- crop and pasture production loss
- poor trafficability
- loss of nutrients
- soil structure decline, and
- recharge to saline watertables.
Waterlogging plus salinity is much worse
Waterlogging reduces the ability of roots to screen out salt at the root surface. This results in large increases of salt uptake, concentration of salt in shoots, and reduced plant growth or death. The combination of waterlogging and saline soils is much more damaging than either alone.
Management options for waterlogging
Suitable options depend on the severity, position in the landscape, and land use. You can adapt crop or pasture management to the waterlogged conditions, reduce the waterlogging on-site, and reduce surface water flow to the susceptible site. We recommend a combination of these options as part of a whole-farm water management plan.
- Do nothing: just avoid waterlogged areas when seeding, spraying, and harvesting; not normally recommended.
- Use more tolerant crops or pastures, and suitable agronomy.
- Alter nitrogen management to suit the waterlogged situation.
- Use shallow surface drainage on flat waterlogged or inundated areas.
- Use surface water management upslope, to divert surface water flows.
- Use raised beds on highly susceptible flats, with shallow surface drainage, and
- In high rainfall areas and irrgation areas with clay soils, use mole drains.
Use tolerant crop species and adjust agronomy
Some grains are more tolerant of waterlogging than others:
- Most grain legumes and canola are more susceptible to waterlogging than cereals and faba beans.
- Grain legumes, in decreasing order of tolerance, are: faba bean, yellow lupin, grass pea, narrow-leafed lupin, chickpea, lentil, field pea.
- Oats tend to recover better from waterlogging than wheat and barley.
- Wheat and barley varieties have a large genetic range of waterlogging tolerance.
Options to reduce crop damage from waterlogging:
- Seed crops early.
- Use long-season varieties.
- Plant waterlogging-susceptible paddocks first and early, as crop damage is particularly severe if plants are waterlogged between germination and emergence,
- Resow the crop if waterlogging delays emergence and reduces cereal plant density to fewer than 50 plants per square metre, if accessible and if waterlogging has ended, and
- Increase sowing rates in areas susceptible to waterlogging as waterlogging depresses tillering (and the number of seed heads); higher sowing rates increase the number of heads and improve crop competition with weeds that take advantage of stressed crops.
Weed density affects a crop's ability to recover from waterlogging. Weeds compete for water and the small amount of remaining nitrogen. Waterlogged parts of a paddock are often weedy.
Spray the weedy areas with a post-emergent herbicide – if herbicide resistance is not a problem – when the paddock is dry enough to allow access, and provided the crop is at an appropriate growth stage. Aerial spraying is an alternative when ground-based sprays cannot be used.
Control root diseases
Diseases, particularly take-all, of wheat and barley are often more severe in waterlogged crops because the pathogens tolerate waterlogging and low oxygen levels better than the crops.
Reduce the severity of take-all in well-drained and waterlogged areas by eliminating grasses from the preceding crops or pastures. Leaf diseases are likely to be more severe in waterlogged crops because the crop is already stressed. Spraying may be an option after the site has dried, but only in crops with a high yield potential. See Crop diseases: forecasts and management for detailed recommendations.
Alter nitrogen management
Crops tolerate waterlogging better if they had good nitrogen status before waterlogging occurred. Waterlogging usually results in nitrogen being leached beyond the root zone, and crops may be nitrogen deficient after waterlogging.
- On moderately waterlogged sites, apply nitrogen after waterlogging, when the crop is actively growing where basal nitrogen applications were 0–50 kilograms per hectare.
- On severely waterlogged sites, the benefits of nitrogen application after waterlogging are questionable – seek agronomic advice.
Applying nitrogen at the end of a waterlogging period avoids loss of nitrogen by leaching or denitrification. Use aerial applications of nitrogen fertiliser if the site is too wet for vehicles.
Use shallow surface drainage on waterlogged areas
Use shallow relief drains to remove surface ponding and run-off water from areas affected by heavy rainfall, inundation or flooding. These drains will not reduce existing waterlogging, but do reduce the likelihood of future waterlogging. For more information, see the shallow relief drains page. All drainage should be part of a whole farm water management system.
Use surface water management upslope of waterlogging
Prevention is better than cure: prevent water from flowing downslope where it can lead to erosion, waterlogging, and inundation. Options are provided on the Surface water management page.
Use raised beds on waterlogged flats
Raised beds allow drainage from the beds into the corresponding channels. This provides deeper unsaturated soil for plant growth in the beds. See the Design, layout and installation of raised beds page.