5.0 Impacts of Ash Fall on New Zealand Horticulture
5.1 Introduction
The impact on horticulture of a volcanic event could be severe. The most destructive events are likely to occur close to the volcanic vent, but cover a relatively small geographic area. The most significant effects in terms of affecting a wider range of crops over a larger area are from ash fall. Remobilisation of ash as it is eroded and distributed in water systems could also have a significant effect, particularly on plains areas. These areas are largely formed from alluvial material, often including ash from earlier eruptions. However, significant quantities of ash could render drainage systems ineffective and reduce usefulness of the land for long periods. This is discussed further under the effects on pastoral agriculture (section 4.2 on Lahars).
The severity of the impacts of ash fall would depend on the depth of ash, the particle size, the chemical composition and the area over which it was distributed. Distribution is very dependant on windfall on the day of the event. Even depths of ash as large as 1 metre (unconsolidated depth) would fall in a relatively short period of about 12 hours following each eruptive event. Because of the climate and weather variables, although there may be a significant warning of an eruption, prediction of where the ash fall is most likely would not be able to be assessed until close to the time the event was occurring. Where a series of eruptions occurred, as Mount Ruapehu in 1995, the ash fall distribution may be quite different from each eruption depending on the low and high level wind directions at the time. Thus growers over a wide geographic area would need to be aware of possible impacts, any steps to mitigate the effects of ash fall, and rehabilitation strategies. Major horticultural areas such as the Bay of Plenty, Auckland and Hawke's Bay are in volcanic risk areas. A wide range of annual and perennial crops are grown in these areas and are significant on a national scale.
There has been limited experience with ash fall on horticultural crops. Relevant examples are the effects of the Ruapehu ash in 1995, the Mount St Helens ash in the USA in 1980 (Cook et al, 1981; Folsom, 1986), Paricutin in Mexico (Luhr and Simkin), and studies on vegetation recovery from the Tarawera eruptions. (Keam, 1988). Some of the main impacts from these eruptions are summarised in the Table 4.1 and Table 3.5.
One of the significant effects of the Mount Pinatubo eruption was atmospheric dust reducing temperatures over a wide area. This reduced the summer temperatures in New Zealand by 1-2 degrees, with consequent retarded plant growth and reduced productivity. Lingering ash or repeated falls could have a similar effect over a more localised area. For example, after the Mount St Helens eruption, temperatures were reduced over 600 miles away in Canada. (C Van Eaton, personal communication).
During the 1995 Ruapehu eruptions one of the most significant effects was crop losses on cauliflower in Gisborne, 250km away, which received a trace level of ash. Broccoli crops were not similarly affected. The ash from this eruption was chemically fairly inert, so the main losses were due to the dark coloured ash on the white cauliflower curd detracting from curd appearance. This is a cosmetic effect but sufficient to downgrade the cauliflower crop causing economic losses. Other crops shed the ash before significant damage occurred.
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