With sea levels expected to rise in New Zealand, the Future Coasts Aotearoa programme is researching how rising seas could raise groundwater levels and affect lowland farming. To support this, AgResearch Group of the Bioeconomy Science Institute was tasked with evaluating how higher water tables may harm pasture production by causing waterlogging and pugging.
What did they find?
AgResearch have found minimal effect on annual pasture production if the water table is deeper than around one metre from the soil surface. However, if the water table rises closer to the surface, pasture production starts to drop and then declines rapidly.
To quantify pasture impacts of progressive groundwater rise, AgResearch first simulated many combinations of weather, soil type and water table depth with a dynamic process-based model (APSIM). From the outputs of that model they have generated a simulated production vs water table depth function:
Production = Productionmax x (1 − exp[ 𝑤𝑡0 – 𝑊𝑇 x 𝛼 ]), where:
Production is the annual harvestable dry matter (DM), (kg DM /ha /year),
Productionmax is the site-soil annual pasture harvestable (kg DM /ha /year) with no water table restriction,
α is a curvature parameter (mm),
wt0 is the water table depth above which there is no harvestable pasture (mm), and
WT is the mean annual water table depth below the soil surface(mm).
Values for Productionmax, α and wt0 have been estimated for many different soil types and locations, so it is now possible to estimate the impact of progressive waterlogging on pasture at different sites around the country.
How did they do it?
To do the work, the AgResearch team:
- Conducted a literature survey to identify the key effects of waterlogging and associated livestock treading damage on soil properties and pasture production.
- Used the APSIMmodel (see www.apsim.info) to estimate the direct effects of progressive waterlogging and treading on pasture growth and harvestable pasture mass. The treading effect model was newly developed as part of this work. This newly developed model estimates the damage to pasture and soil properties from stock treading on overly wet soils.
- Ran pasture production simulations at six regional locations across New Zealand, using Future Coasts Aotearoa-supplied water table depths and daily weather with soil types derived from online information. The six regional locations chosen for the simulations were: Northland, Waikato, Bay of Plenty, Manawatū, Canterbury and Southland.
- Summarised the impacts of water table depth and potential treading damage, as affected by water table rise, suitable for incorporation into a hazard/risk framework.
Report
Quantifying the impacts of higher water tables on pasture production [PDF 1.2 MB]
This work was undertaken under the Future Coasts Aotearoa programme Research Aim 2.