POTENTIAL CLIMATE CHANGE IMPACT ON FLOOD RISK ACROSS NEW
ZEALAND
Christian Zammit1 , Scott Graham ,1 Bruce Dudley1 , Abha Sood1
1
NIWA
Aims
Climate change is projected to have a potentially significant but spatially variable impact on New
Zealand’s water cycle and flood generation mechanism. Following the IPCC5 report 9IPCC
2015), the Ministry for the Environment (MfE) has engaged NIWA to update the current 2010
flood guidance manual (MfE 2008). This guidance manual aims to provide guidance not only on
potential climate change, but on potential change in different flow characteristics at daily time step
for specific locations across New Zealand. However daily time step is usually not fine enough to
correctly represent flood generation mechanism. This study aims to look at potential climate
change impact on sub-daily flood characteristics and flood generation mechanism (ie
precipitation) under IPCC5 assessment.
Method
Climate change fields (precipitation and temperatures) were generated using Regional Climate
Models (RCM) for different Representative Concentration Pathway (RCP) at sub-daily time step
over the period 1971-2120. Those climate change scenarios were generated as part of the MBIE
contestable Climate Change Impact and Implication project. The RCM climate change field were
used to drive hydrological simulations across New Zealand using the National hydrological model
(TopNet). Analysis of the change in precipitation characteristics (ie duration, frequency and
intensity) as well as flood characteristics (peak, duration, frequency) were carried out at National
and regional scales.
Results
Analysis of the change of subdaily precipitation characteristics over the period 1971-2120 was
carried out per decade and for precipitation event threshold of 2mm/hr. Figure 1 presents the
potential change in precipitation intensity (for precipitation event over 2mm/hr) for RCM
HadGEM2 and RCP4.5 across the West coast of New Zealand. Analysis indicates that sub-daily
precipitation intensity is expected to change across decades.
Further analysis indicates that large variation of the precipitation characteristics are expected
across the range of RCMs and RCPs resulting in potential large change in term of flood
characteristics and associated flood risk.
Figure 1: Hourly precipitation change for rainfall events above 2mm/hr forHADGEM2 model RCP 4.5.
Results are presented in term of change of intensity to the 1990 decade reference period. )
References
IPCC 5- Climate change 2013: Contribution of Working Group I to the Fifth Assessment Report of the
Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J.
Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge,
United Kingdom and New York, NY, USA, 1535 pp.
Ministry for the Environment. (2008) Climate Change Effects and Impacts Assessment: A Guidance Manual
for Local Government in New Zealand. 2nd Edition. Mullan B; Wratt D; Dean S; Hollis M; Allan S; Williams T,
Kenny G and MfE. Ministry for the Environment, Wellington. xviii + 149 p.