When farmland is flooded by seawater

Storm surges and king tides

Published on Tuesday, January 09, 2018

What happens to farmland which has been flooded by seawater? 

  • The impact will depend on how long the crop or pasture is covered by seawater. If it’s only for a short time – up to 48 hours – and gets good rainfall, grasses should bounce back. 
  • Rain is needed to wash the salt away. It’s likely the saltwater was diluted by the rains during the storm which measured up to 70mm.
  • The last similar event in the areas was 15 July 1995. This was during winter, there was plenty of rain to wash out, and pasture recovered well over a three month period.

Pasture and soil

  • Salt should wash away fairly quickly with rainfall. The more porous the soils the quicker it washes out.
  • Pasture dies after three days without oxygen. Overseas studies on inundation suggest:
    • 24-48 hours most pasture will recover, some discolouration
    • 48-96 hours most of the grass will die but some stems survive, clover survives
    • After 96 hours all pasture dies and requires renewal.
  • Most of the area impacted now falls in the 24-48 hour category; some pooling remains and those areas have surpassed 96 hours.
  • Your soil/texture profile affects how much seawater can impact it. Clay soils have low penetration (1-2cm); sandy soils have higher penetration (up to 15cm). This is good news for many farmers whose soil is marine clay. Near Kaiaua there is more sand-based soil.
  • Soils are not expected to become sodic.

Animal health:

  • Precautionary approach of deferring grazing until after the next heavy rainfall. 
  • Ensure stock have access to fresh clean water.
  • Talk to your vet if there are any concerns.

Actions farmers can take:

  • If it’s possible to pump seawater off, do so.
  • Make a feed budget. Consider there may already be feed shortages in the area and take this into account.
  • You may need to offload stock – make a plan and make decisions early.
  • Talk to your accountant about possible tax flexibility or other standard income assistance for you/workers.
  • Talk to your farm consultant if your pasture has been covered for several days or longer - will there be a need to regrass?
  • Talk to your farm consultant about soil testing for sodium or conductivity. Some consultants may be able to offer this as a rapid field test.
  • Talk to your local Rural Support Trust who are working with regional councils, CDEM, MPI and agri-business.

Read a more detailed analysis by Matthew Taylor, Soil Scientist and Diffuse Contamination Specialist at the Waikato Regional Council

 

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