Dairy farms are likely to feel the wet end of sea level rises in coming years, with even small increases and relatively minor storm events likely to double the number of farms at risk of inundation.
For the first time, in-depth modelling on a specific land use impact of sea level rises has been published. The work was done by New Zealand researchers in a collaborative Deep South National Science Challenge study, published in the International Journal of Disaster Risk Reduction.
Researchers modelled differing sea level increase scenarios, combining them with varying intensities of weather events ranging from one-in-10-year events, through to one-in-500.
Based on a 1m sea level rise – the commonly accepted level NZ could experience – the number of dairy farms at risk of inundation even from a relatively mild one-in-10-year event doubles from 470 to 1075. This would have 10% or more of their land covered in water.
But even with only a 0.5m lift in average sea levels, the number of farms affected still rises to 800 after a one-in-10-year weather event.
Overall, the researchers found it was the relative increase in sea levels rather than the extremity of the events that contributed most to greater inundation risk.
This was to the extent that a one-in-10-year event comes to have almost the same impact as a one-in-500 event with no sea level rise.
Study lead author Dr Helen Craig of University of Canterbury said this signals that we can expect severe impacts to large numbers of farms previously only exposed to large, infrequent events such as one-in-200-year or one-in-500-year events.
The impacts will now occur much more frequently, even with relatively small ESL (extreme sea level) events.
The regions with the greatest proportion of dairy farms affected are Otago, Northland and Bay of Plenty, at 10-20% of their dairy farms.
But Waikato has the highest physical number of total farms impacted, with 367 affected by a 0.5m sea level rise and a one-in-10-year event. With a 1m lift in sea levels, that number increases to 420 dairy farms.
The modelling worked to determine the level of intervention required post-event. It found that all affected farms would require supplementary feed and possible milking plant and shed repairs, and experience a temporary decline in milk yield.
Typically, these farms would have 10-25% of their pastures inundated with water.
The impact on dairy farm infrastructure, including farm dairies, was also calculated by the researchers. They estimated that with a 2m sea level rise there was about a quarter of a billion dollars in dairy sheds exposed to a one-in-10-year event.
The number of dairy cows at risk from the rises are currently 175,000 for a one-in-10-year event at current sea levels, but this rises to over half a million at 2m and a one-in-500-year event.
With over a quarter of cows in Bay of Plenty exposed to that risk, the report notes movement out of the region after such an event may prove challenging.
Earlier flood research has also found mastitis and lameness increases 30-50% after floods, while leptospirosis surges over 170%.
NZ has about 12,000 dairy farms covering 1.7 million hectares and generating $15 billion-$18 billion in export earnings a year.
The authors maintain that understanding the impacts of sea level rises on dairy is critical in order to improve response and recovery plans in targeted areas most likely to experience the impact of events and higher sea levels.
The full report can be read here