When farmland is flooded by seawater
Storm surges and king tides
Published on Tuesday, January 9, 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.
- 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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