4.4 Ash Depths Scenarios
4.4.1 Greater than 500 mm
An ash deposit 500mm or greater would result in building collapse, destruction of services, limited to no access until there was consolidation of the ash with rain. Both livestock and humans would be killed, if not evacuated before the eruption. The land would be unsuitable for agriculture for a significant period as the ash sterilises the soil, with no organic matter or natural fertility to support plant growth. (Folsom 1986)
The environment after such an eruption would be very harsh and difficult for human survival until basic facilities were restored, such as water supply, power, access and shelter.
Reworking and erosion of the material would cause damage tens of kilometres away from the vent.
4.4.2 100 mm
Most plants would die immediately, including lucerne. Brassica and swede crops would fail if the ash fell before crops were well established. At 100mm a large proportion of the plant cover would be eliminated for more than one year (Folson, 1986 & Blong, 1984).
With feed supplies being decimated, productivity for the season would be severely checked. The long term impact is dependent on the ability to rehabilitate the land.
4.4.3 25-50 mm
The greatest impacts from a 25-50mm ash shower initially would be the loss of feed, due to burial and burning of pastures. Pastures and stock are more likely to be affected under a sheep and deer grazing regime as the feed is generally shorter in length.
On hill country, rain would result in ash being washed off hillsides, resulting in greater damage to gullies and land flooded with sediment laden water.
High amounts of supplementary feed would be required until pastures recovered or were re-established. The effect on production would be devastating for at least 3-6 months, until pastures were producing 70-80% of pre-eruption levels.
Where there is widespread reduction in stocking rate through slaughtering of breeding animals, increasing productivity to previous levels may be delayed due to the loss of high genetic merit stock.
Yields of maize and cereal crops would be reduced - the degree dependant on the time of the ash fall. The impact being the least in vegetative and late maturing stage.
4.4.4 5 mm - 25 mm
Although the initial impacts of 5-25 mm of ash would appear devastating, pastoral land should recover quickly,
- providing rain followed within 5-10 days of the ash fall,
- the ash was not acidic, and
- the ash did not contain poisonous gases.
The initial problems would be feeding livestock put off their feed by ash contaminated pastures. Once rain fell and the ash was reduced to 2-8 mm in depth pastures should start to recover. The effects being similar to a severe drought, e.g. summer 1998. A proportion of farms will require undersowing.
A 25mm ash shower would alter soil characteristics, both pH and permeability. This may reduce the productivity of the land for up to 5 years.
Animals may suffer ill-thrift, effects of fluorine poisoning and increased teeth wear, particularly deer and sheep.
Farm operations would be affected by the abrasive nature of the ash. Water pumps, cowshed plant and vehicles are all likely to need increased maintenance until the ash is consolidated and stabilised.
4.4.5 < 5 mm
The impacts of 5mm or less of ash will be minimal, unless the ash is high in fluorine. Even where the ash contained poisonous aerosols, the effects would be localised.
The biggest impact will be on farmer perceptions of the damage the ash may cause and the inconvenience of ash covered pastures, vehicles and buildings. This was borne out in the Ruapehu eruptions in 1995 and 1996 when lack of knowledge and fear of the impacts of ash created great uncertainty.
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