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An assessment of the impacts of climate change in the Waikato region: Applying CMIP5 data

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Report: TR 2015/26

Author: Drs Meng Wang, Yinpeng Li and Chonghua Yin

About this report

Climate change is projected to alter long-term average climatic conditions and climatic variability in New Zealand. The best estimates are for a temperature increase of 0.2–2.0°C by 2040 and 0.7–5.1°C by 2090, with marked seasonal changes in rainfall and extreme events (Ministry for the Environment, 2008 (based on CMIP3 data). This report provides a broad technical assessment of the physical effects of climate change in the Waikato region. Projections cover short (2030), medium (2070) and long-term (2100) timeframes. Ensembles (or parallel scenarios) of up to 40 CMIP5 Global Climate Models (GCMs) were used to generate the future scenarios. Eight climatic indices were used to represent the major climate change-induced effects, including mean temperature, mean precipitation, extreme precipitation change, peak streamflow change, Potential Evapotranspiration Deficit (PED), Temperature-Humidity Index (THI), Growing Degree Days (GDD), and extreme wind. 

Baseline and future scenarios for the climate change-related indices exhibit both spatial and temporal variability across the region. The projections indicate that mean annual temperature could increase by 1.17 (0.95-1.33)°C by 2070 and by 1.25 (1.00-1.45)°C by 2100. Summer annual mean temperature could increase with the highest seasonal change: 1.23 (0.95-1.43)°C by 2070 and up to 1.27 (1.00-1.50)°C by 2100.

Mean annual precipitation change is highly variable both annually and seasonally, ranging between a decrease of 4.56% and an increase of 9.22% by 2070; and between a decrease of 4.84% and an increase of 9.90% by 2100. The increase in precipitation is the highest (up to 4.6%) in autumn, by 2070, in the northern part of Thames-Coromandel, Waitomo, Otorohanga and Taupo districts. The largest decrease in precipitation is projected to occur in the Thames-Coromandel districts with up to 2.7% decrease in spring. The highest mean precipitation changes are projected for the Waitomo, Otorohanga, South Waikato and Taupo districts. 

Extreme daily precipitation changes indicate an increase that is consistently located in north-east parts of the Waikato region – i.e., in the Hauraki and Thames-Coromandel districts and the north part of the Waikato district.

Peak streamflow changes are expected to increase in rivers such as the Kauaeranga and Waihou rivers which were selected for further analysis on the basis that extreme precipitation increases were projected to be highest in the areas through which these rivers flow. The Average Recurrence Intervals (ARIs) for peak streamflows are also expected to decrease (i.e. peak streamflows of specific values occur more frequently).

Projections for changes in Potential Evapotranspiration Deficit, PED, used as an indicator for drought, also highlight the spatial variability of impacts with the highest PED values (where PED>400mm) located in the Hauraki and Matamata-Piako districts by 2100. However, PED>200mm projections are more widespread across the Waikato region by 2070 and 2100, particularly in the central, northern and eastern parts. Drought-related stress on agricultural production systems and ecosystems in the Waikato region is therefore an issue that regional resource management will need to address into the future, in order to identify and assess feasible adaptation options that will ameliorate projected impacts.

The Temperature-Humidity Index, THI, indicates that mildly stress-inducing conditions (THI>72) for animals such as dairy cows are widespread across the Waikato region by 2070 and 2100. Moderately stress-inducing conditions (THI>78) are more restricted to the central parts of the Waikato region by 2070 and 2100, with two ‘hotspots’ evident in the Hauraki and Matamata-Piako districts and further south in the Otorohanga district, particularly by 2100.

Growing Degree Days, GDD, projections indicate that a lengthening of the growing season is expected across the entire Waikato region, although the increase is spatially variable and highest in northern districts where temperatures are warmer and lowest in the Taupo district. This projected lengthening of the growing season is likely to have beneficial effects on pasture and crop productivity, and may create opportunities for new commercial crops to be cultivated. The increase in GDD also may have an impact on native species or species of importance to biosecurity, where GDD changes are likely to induce more favourable growing conditions. 

The hot spots for extreme wind events are in the Thames-Coromandel, Waitomo, Otorohanga, Rotorua and Taupo districts. The extreme wind speed is projected to change but in small magnitudes in this century. 

The uncertainties associated with the future projections of the nine climatic indices used in this assessment increase progressively between 2030 and 2100 scenarios, including uncertainties associated with future GHG concentration profiles, the earth’s climate sensitivity to the GHG emissions, and the extent of the effects of feedback mechanisms that may influence the rate and magnitude of climate change. The uncertainty has been represented in this assessment by illustrating the spread of individual GCM projections within the various ensemble results.The new projections with the new AR5 data are similar to the previous results in AR4, except PED and sea level rise. The reasons may be: 1) the radiative forcing of RCPs scenarios are different from SRES scenarios, 2) more GCMs employed in AR5 ensemble. The warming trend in Waikato is smaller than AR4 and the decrease in precipitation projection is also milder, so the PED change scarcely in the median scenario. The improvement in modelling land-ice contributions to sea level rise produced much higher projections of global sea level rise and this is reflected in sea level rise scenarios for the Waikato’s coastal areas. 

Based on the findings of this Assessment, we provide recommendations for more in-depth impact assessments that will inform adaptation strategies and processes for the Waikato region. Following from this, it should be possible to identify a range of indicators and adaptation options needed to respond to the projected future effects of climate change in the Waikato region.

[Note: Detailed analyses of bio-physical impacts, risks and specific adaptation strategies for human, managed or natural systems in the region are not a part of this report. These would include impacts that affect the Regional Council’s core functions and responsibilities of natural hazard management (floods, droughts and storm damage, in particular), biodiversity and biosecurity management, and coastal area management, water and land management and regional planning among others. In addition, climate change has implications for regional agricultural productivity, and infrastructure that require more detailed assessments].

Read or download the report

An assessment of the impacts of climate change in the Waikato region: Applying CMIP5 data (PDF 3MB)


   Executive Summary 1
   Table of Contents 3
   List of Figures 5
  List of Tables 7
   Glossary 8
1 Introduction 10
2 Baseline and future climate in the Waikato region 13
2.1 Mean temperature and precipitation change 15
2.1.1 Introduction 15
2.1.2 Methodology and data 15
2.1.3 Results 15
2.1.4 Discussion and conclusion 22
2.2 Extreme precipitation change 23
2.2.1 Introduction 23
2.2.2 Methodology and data 23
2.2.3 Results 23
2.2.4 Discussion and conclusion 27
2.3 Peak streamflow change 28
2.3.1 Introduction 28
2.3.2 Methodology and data 28
2.3.3 Results 29
2.3.4 Discussion and conclusion 31
2.4 Potential Evapotranspiration Deficit (PED) 31
2.4.1 Introduction 31
2.4.2 Methodology and data 32
2.4.3 Results 32
2.4.4 Discussion and conclusion 36
2.5 Temperature-Humidity Index (THI) 36
2.5.1 Introduction 36
2.5.2 Methodology and data 37
2.5.3 Results 37
2.5.4 Discussion and conclusion 40
2.6 Growing Degree Days (GDD) 40
2.6.1 Introduction 40
2.6.2 Methodology and data 41
2.6.3 Results 41
2.6.4 Discussion and conclusion 43
2.7 Extreme wind 44
2.7.1 Introduction 44
2.7.2 Results 44
3 Conclusions 45
4 Recommendations and future research direction 47
5 References 50
6 Appendices 54
6.1 Appendix 1: Pattern Scaling Methodology 54
6.2 Appendix 2: RCP Scenarios (IPCC, 2013) 58
6.3 Appendix 3: Methodology for extreme precipitation event analysis based on daily GCM data 58
6.4 Appendix 4: Computation of the Potential Evapotranspiration Deficit, PED 62
6.5 Appendix 5: Calculation of the Temperature Humidity Index (THI) 63
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