Groundwater Salinity and a Perennial Pasture Decision
A 600-hectare grazing property in the South East has rising water tables under 180 hectares of annual pasture. Bore readings show groundwater at 1.8 m depth with an electrical conductivity of 9 dS/m, and the pasture on the lowest ground is thinning with salt-tolerant weeds appearing. The owner wants to sow a deep-rooted perennial pasture on the affected area to lower the water table. Establishment costs are $180/ha, and the perennial is expected to persist for six years. The property runs 1,400 ewes and depends on the affected paddocks for autumn feed. Neighbouring land has already been abandoned to dryland salinity, and the catchment group offers a cost-share of 40% on perennial establishment in priority areas.
1. Assess whether sowing the perennial pasture is a sound response to the salinity problem, and identify the conditions under which it would fail.
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The proposal targets the cause rather than the symptom: rising water tables are driven by annual pastures using less water than perennials, so a deep-rooted perennial that dries the soil profile can lower the water table and reduce capillary rise of salt. That is the correct mechanism, and the 1.8 m water table with 9 dS/m groundwater indicates a real and developing problem.
However, the response is conditional:
- Scale. Perennials must be sown across enough of the recharge area, not only the discharge (lowest) ground. Sowing only the salt-affected paddock treats the symptom's location, not the recharge zone, and the water table will keep rising. The 180 hectares affected may be smaller than the recharge area that feeds them.
- Time. Lowering a water table takes years, and the perennial is expected to persist only six years. If it thins before the water table responds, the investment is lost.
- Economics. Establishment is $180/ha; with 40% cost-share the grower pays $108/ha, or about $19,440 over 180 ha. The feed value of the perennial must offset the loss of autumn feed from annual pasture, since the flock depends on those paddocks.
- Persistence. Perennials fail on saline discharge sites where salt is already at the surface; those areas may need exclusion or salt-tolerant species instead.
It would fail if sown only on the discharge area, if rainfall is insufficient to support the species, if grazing pressure during establishment destroys the stand, or if the water table is driven by an off-farm source the grower cannot control. A sound answer recommends perennials on the recharge area plus monitoring of bore levels, with the cost-share improving the economics.
2. The owner wants to abandon the worst 40 hectares. Discuss the consequences and a better alternative.
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Abandonment removes grazing from 40 of 600 hectares, about 7% of the property, and reduces autumn feed at a time when the flock of 1,400 ewes is most dependent on it. It also leaves bare ground, which increases evaporation and can worsen surface salt accumulation, and it may allow wind erosion and weed spread onto neighbouring land.
Abandonment also has an off-farm dimension. The catchment group's cost-share signals that salinity is treated as a catchment problem; abandoning land may shift the water table problem downslope rather than solving it, and could affect the neighbour whose land is already affected.
A better alternative depends on the site:
- On the true discharge area, sow salt-tolerant species or manage as a sacrifice paddock with careful grazing rather than leaving it bare.
- Fence and revegetate the worst 40 hectares with salt-tolerant shrubs or trees to lower the water table locally and provide shelter, while keeping the area in some productive use.
- Concentrate the perennial establishment on the recharge area upslope, where the water-table benefit is greatest.
- Use the catchment cost-share to fund the revegetation rather than losing the land entirely.
The judgement the exam rewards is that abandoning land treats the symptom, transfers risk to neighbours, and forgoes feed, so it should be a last resort after recharge-zone treatment and salt-tolerant use are considered.