Quantifying risk to agriculture from volcanic ashfall: a case study from the Bay of Plenty, New Zealand

Quantitatively assessing long-term volcanic risk can be challenging due to the
many variables associated with volcanic hazard and vulnerability. This study presents a
structured first-order approach for considering variables in hazard and vulnerability anal-
yses, such as eruption style and cyclic fragility, in order to quantitatively estimate risk.
Probabilistic volcanic hazard data derived from advection–diffusion–sedimentation tephra
fall model TEPHRA2 and probabilistic volcanic hazard analysis tool BET_VH (Bayesian
Event Tree for Volcanic Hazards) are combined with fragility functions and seasonal
vulnerability coefficients for agricultural production to calculate volcanic risk indices
which represent the likelihood of damage or loss to farm production over a given time
frame. The resulting dataset allows for approximations of quantitative risk over a con-
tinuous range of ash thickness thresholds, at multiple levels of uncertainty, and in the
context of fluctuating hazard and vulnerability environments (e.g., seasonal wind patterns
and crop phases). We illustrate this approach through a case study which evaluates the risk
of incurring 90% damage to agricultural production at dairy and fruit farms in the Bay of
Plenty region of New Zealand (BoP) due to ashfall from a Plinian eruption phase at the
large local caldera volcano, the Okataina Volcanic Centre (OVC). Consideration of sea-
sonal wind profiles, seasonal fluctuations in fruit and dairy farm vulnerability, multiple
possible OVC eruption styles, different possible OVC vent locations, and a continuous
distribution of ash thickness and damage thresholds enables a multi-dimensional analysis
that aims to reflect the natural complexity and interdependencies associated with volcanic
risk. A risk uncertainty matrix is introduced as a conceptual scheme to help guide eval-
uation and communication of the results of such quantitative risk analyses by showing how different types of uncertainty can yield ‘‘maximum’’, ‘‘average’’, or ‘‘minimum’’ estimates
of risk. Results of this case study indicate that BoP fruit farms are at higher risk of
experiencing damage and production loss from OVC ashfall than dairy farms, and farms to
the east of the OVC are typically at higher risk than farms to the north of the OVC.
Forecasts based on the annual maximum estimate of risk for fruit farms show a regional
average of 2.3% probability (greater than 1 in 50 likelihood) of experiencing 90% damage
from a basaltic or rhyolitic Plinian eruption from anywhere within the OVC over a period
of 100 years. Seasonal-level analyses revealed that the risk of experiencing losses due to
OVC ashfall at fruit farms is cyclic and fluctuates with time of year and harvest season,
with the highest risk experienced during peak harvest season (15 October–14 April) when
crop vulnerability is high and westerly winds dominate in the BoP.