4.3 Distal hazards
Pyroclastic Fall Deposits - (Ash)
4.3.1 Impact On Livestock
Ashfall is unlikely to immediately kill livestock, except when deposition rates are exceptionally high and thickness is great. From the experience at Mt St Helens, the majority of stock survived ashfalls up to 300 mm (MAF Volcanic Alert, 1995). However, survival of stock in the short term would be dependent on the availability of feed and good quality water, with ashfalls of 10 mm affecting pasture growth.
The actual effects on livestock depend on:
- consistency of the ash (tephra)
- amount of ash deposited
- poisonous aerosols attached to the ash
- amount of rainfall immediately following any ash deposit
- metabolic and nutritional demand of the livestock at the time
- age of livestock
- pasture length
- stocking rate
Where the ashfall significantly destroyed pastures, stock survival in the short term would be dependent on the ability to feed up to 100% of their diet in supplement, until they could be evacuated out of the area, or slaughtered, or pasture species re-established. Restoring quality water supplies would also be essential for stock to stay on the land. Even with the evacuation to good quality feed and water, stock's long term productivity may still be effected by ash inhalation and fluorine poisoning. Some stock may not recover in the long term, with humane slaughtering being the best option.
Where ashfalls affect a large area, evacuation of stock would be extremely difficult due to the logistics of moving large numbers of stock and sourcing feed in areas unaffected by the ash. This may result in large losses of livestock through dehydration and starvation.
Where there is a significant ashfall, clean water would be in short supply, with natural water supplies and dams contaminated, and pump functions severely reduced by the abrasive nature of the ash. Following ashfalls from Ruapehu in 1995 and 1996, farmers noted that animals were readily put off their feed by 2-5 mm ash deposits. Hence, even with very light deposits of ash, supplementary feed would be required.
Young stock are more at risk than mature animals. Close grazing animals such as sheep and deer are more likely to be affected by light ash showers.
An eruption in early spring would have the greatest impact on both sheep and beef and dairy farms. On dairy farms, milk yields would be severely depressed in early lactation - an effect that would carry over for the whole lactation. On sheep and beef farms, lamb and calf survival/thrift would be poor as ewes and beef cows reduce/stop lactating. Livestock losses from the eruption of Ruapehu in 1995 were greatest in lactating ewes, grazing short pasture. Wool quality is likely to be severely effected where sheep are close to shearing.
Ashfalls may be poisonous to stock resulting in clinical diseases which include hypocalcaemia, fluorosis, forestomach and intestinal damage and secondary metabolic disorders. Nutritional and stress related diseases may also occur. The high sulphur concentration in the ash may also induce copper and cobalt deficiencies in the long term (DF Shanks, 1997).
Fluorine aerosols attached to fine tephra pose a significant threat to livestock (Gregory & Neil 1996). Poisoning occurs where the fluorine content of dry grass exceeds 250 ppm. Before death, the poisoning causes lesions in the nose and mouth and hair to fall out around the mouth. Fluorine poisoning of livestock has occurred a number of times in Iceland (Thorarinsson 1979). The eruption of Lakuggar in 1783 killed 50% of the island's horses and cattle and 79% of the sheep. In the 1947 eruption of Hekla, only sheep were effected with other animals (cattle, horses, cats, dogs and poultry) unaffected. In 1970 fluorine poisoning occurred in areas of Iceland which received only 1 mm of ash.
As a result of _ 5 mm ash fall on the Rangitaiki Plain (Taupo) during the 1995 Ruapehu eruption, approximately 2000 ewes and lambs (2.5% of the area's sheep population) were killed as a result of eating ash-affected pastures. Autopsies of the dead animals suggest fluorine poisoning or pregnancy toxaemia was the cause of death (Gregory & Neall 1996). Three Ayrshire dairy cows died at Atiamuri in June 1996 (pers. comm. MAF). It was reported that they stopped eating, showed signs of lethargy before dying after swallowing quantities of ash. Toxic levels of fluorine were found in the dead animals blood. The Department of Conservation also reported the death of a number of wild deer in Kaimanawa Ranges, downwind from Ruapehu, following the two largest October 1995 eruptions (possibly up to 5% of the sika deer population).
4.3.2 Impact on Pastures
Damage to small vegetation and the soils on which they depend will vary with tephra thickness and composition of the ash. The effects in Table 3.4 are based on observations from past eruptions described by Folsom (1986) and Blong (1984).
At light rates of ash, i.e. up to 50 mm, plant survival and regrowth will be dependent on :
- Chemical nature of the ash.
- Compaction of the ash after the eruption.
- Degree of continuing disturbance.
- Amount and reliability of rainfall.
- Length of feed at time of ashfall.
The ash would also affect insect populations, which would be severely checked with ashfalls greater than 25 mm. This may have a beneficial effect where pastoral and crop pest populations are reduced due to the ash. Highly mobile insects, many of which have a dense covering of body hairs which trap the dust such as honey and pollination bees are more susceptible to ash and their activity may be reduced (R J Cook et al 1981).
As outlined in Table 4.1, ashfalls greater than 150 mm result in complete burial of the pastures and soil. Where soil burial is complete, the soil is sterilised, as it is deprived of oxygen, resulting in death of all the existing pasture species and crops.
Survival of pastoral plants will be influenced by the timing of rainfall. With rain, the ash will compact to approximately one-third it's original depth. Where there is rain soon after an eruption (within 2-3 days) plant survival will be improved, with compaction of the ash.
On hill country, rain will wash ash into the gullies and low lying catchments, causing more damage. Wind erosion may also cause the ash to sand dune, where the ash is not consolidated or incorporated into the soil profile.
Where the ash results in complete burial of pastoral plants for 5-7 days, it is likely all plants will die, as occurs with heavy silting and flooding, e.g. Waimana floods July 1998.
Even where the ash could be removed within 5 days, plants may still die from burning if the ash is acidic.
The acidity and nature of the ash varies between volcanoes and eruptions. The chemicals in the ash from Mt St Helen's eruption did not have a significant detrimental effect on plants. In the Ruapehu eruption in 1996, ashfalls of 2mm on pastoral land elevated soil sulphur levels and were expected to lower soil pH by 0.2 - 0.3 units prior to pre-eruption values (Cronin et al 1996). Therefore, an ash layer 50 mm or greater, high in sulphur, could significantly reduce soil pH. In turn, this would reduce the availability of phosphate and other essential minerals and alter the soil's characteristics to such an extent, arable and pasture plants would not survive.
Where there is acid rain following an eruption, pastures will be scorched and die (N G Gregory et al 1996).
4.3.3 Impact on Arable Crops
As with pastures the type and depth of ash will influence crop survival and yield. Ash falls accompanied with acid rain would destroy crops.
The timing of the ash fall would affect the chances of survival of grain and cereal crops. Maize yields are not likely to be affected by light ash falls in the first two months of growth when the plant is in the vegetative period. Heavy ash falls burying much of the plant and changing the soil characteristics will result in crop failure.
The most critical period for maize yields are three weeks before tasselling to two weeks after pollination. Even light ash falls during this period could result in barren stalk and crop failure.
Although there is little information on the effects of ash, they could be similar to hail and frost damage. Table 4.2 below gives general guidelines on yield loss due to leaf defoliation. Yields may also be reduced by damaged stalks being more susceptible to disease.
Table 4.2
% Leaf Defoliation in Maize |
||||
20% |
50% |
80% |
100% |
|
Growth Stage |
% Yield Loss |
|||
7 Leaf |
0 |
2 |
6 |
9 |
9 Leaf |
0 |
4 |
9 |
13 |
11 Leaf |
1 |
7 |
14 |
22 |
13 Leaf |
1 |
10 |
22 |
34 |
15 Leaf |
2 |
15 |
34 |
51 |
17 Leaf |
4 |
21 |
48 |
72 |
19 Leaf |
6 |
27 |
64 |
96 |
Tassel |
7 |
31 |
68 |
100 |
Maize requires many heat units for a crop to reach maturity. An eruption could delay crop maturity if sunshine hours were reduced during the eruptive period.
With most cereal crops being grown in the Manawatu, the likelihood of a 50mm ash shower or greater on these crops is low. An ash shower of this degree would effect these crops most over the pollination period.
Ash showers near crop maturity will make harvesting difficult and reduce the quality of grain. The weight of ash on crop leaves is likely to cause lodging, increasing harvesting costs and reducing yield. Ash collected within and among the spikes will cause some contamination of the harvested grain. Although from the Mt St Helens' experience 80% or more of ash was removed by cleaning procedures already used in flour mills, with the greatest loss to the farmer in reduced price (RJ Cook et al, 1981). However, this was at ash showers less than 50 mm.
Where ash depth was greater than 100mm, replanting of crops would only be possible where the ash could be incorporated to 20cm (deep ploughing).
Where it is possible to replant crops, yields would be depressed. The degree dependent on the change in the soil characteristics with the incorporation of the ash and amount of fertiliser applied.
The weight of the ash on leaves affects plant survival. Lucerne and pea crops regardless of stage of growth would either fail or have poor yields from ash falls of 10mm or greater as the plants have abundant delicate leaves and stems which are easily damaged, reducing the rate of photosynthesis and making the crop susceptible to lodging.
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