Bryan Jenkins, University of Canterbury: bryan.jenkins@canterbury.ac.nz; Private Bag 4800, Christchurch 8140 NZ; +6433642330
INTRODUCTION
On the dry east coast of New Zealand's South Island the Canterbury region is at the sustainability limits of water availability.
METHODS
Past trends and projections of changes to temperature, rainfall and snow were compiled for the Canterbury region. New Zealand's average surface temperature has increased 1°C over the last 100 years. Surface temperatures are projected to increase a further 2.1°C over the next 100 years.
Climate models project little change in Canterbury's summer rainfall but significant changes in Canterbury's winter rainfall with decreases of 7.5-10% on the Canterbury plains but increases of 5-12.5% in the Southern Alps and on the west coast.
With higher surface temperatures, less snowfall is projected. The warming over the last 100 years has led to glacier retreat with increased elevations of 25-125m.
The implications of these projected changes were considered in relation to the three types of rivers in Canterbury: alpine braided rivers that have their headwaters in the Southern Alps and provide 88% of the annual flow in the region with much of the flow from snow melt leading to peak flows in late spring and early summer; foothill rivers with their headwaters in the foothills that are predominantly rain-fed with winter peaks; and coastal lowland streams that are groundwater-fed with flows related to groundwater levels.
RESULTS AND DISCUSSION
Increased temperatures will increase evaporation rates and therefore irrigation demand. The crucial parameter is the potential evaporation deficit. For the Canterbury plains this is about 320mm. Modelled projections indicate an increase of 120-180mm by 2080, which is about a 50% increase in irrigation demand.
Rainfall recharge occurs primarily during winter so a rainfall reduction in winter will lead to a reduction in groundwater recharge. Therefore groundwater levels will reduce and the volume that can be abstracted will also reduce.
With reduced groundwater levels lowland streams will decline in flow. With reduced winter rainfall on the plains foothill river flows will also decline. The aquatic ecology of lowland streams and foothill rivers is already under stress at times of low flow.
With their headwaters in the Southern Alps the flow in the alpine rivers is expected to increase. However, the distribution of flow throughout the year will change. Winter flows will increase but spring/summer flows may decrease.
Detailed analysis of the flow patterns has been undertaken for the Waimakariri and Rangitata Rivers. Mid-range projections indicate a 7% increase in mean annual flow for the Waimakariri between 1990 and 2040, and an 8% increase for the Rangitata. There are large flow increases from May to September but little change or slight decreases between September and April (the irrigation season). Analysis of the reliability of supply for run-of-river irrigation indicates increased levels of restriction.
These projections are significant for a region whose environmental and economic health is dependent on water. However, there are approaches that can increase the resilience of the ecological and economic systems that are dependent on water.
For example, the increased winter flows of alpine rivers could be harvested and stored for summer use. This is occurring on the Rangitata at Arundel with off-river storage ponds. However a more cost-effective solution for the Canterbury plains that would have both environmental and economic benefits is to use the increased winter flows for aquifer recharge.
Managed aquifer recharge could maintain groundwater levels for abstraction and lowland stream flow as well as dilute groundwater contamination from land use intensification. It also avoids the evaporative losses and loss of land associated with surface storage. Analyses for the Canterbury Water Management Strategy demonstrated that managed aquifer recharge was only two thirds of the cost of equivalent surface water storage.
There can also be significant improvements in water use efficiency to reduce irrigation demand. This is beginning to occur with shifts to more efficient forms of irrigation and the use of piped rather than canal distribution of water in irrigation schemes.
However more can be done through integrated surface and groundwater management. Integrated approaches would involve a predominant use of surface water when river flows allow, and a predominant use of groundwater when river flows are restricted. Also the greater use of surface water for irrigation in the upper reaches of groundwater zones would enhance recharge and enable greater use of groundwater for irrigation in the lower reaches.
Increased water efficiency together with reduced groundwater leakage and reduced runoff can also be achieved by greater use of soil moisture demand irrigation.
Conversions to dairying are the major land use change putting pressure on water resources and are a major source of nitrous oxide and methane. The introduction of nitrogen inhibitors can reduce nitrous oxide emissions.
CONCLUSION
There are adaptations to the projected effects of climate change but they involve changes in the way water is managed. There are also opportunities for offsets and mitigation of greenhouse gas emissions 1. Canterbury Water (2009) Canterbury Water Management Strategy Strategic Framework. Environment Canterbury, Christchurch
2. Chinn T (1996)) New Zealand glacier responses to climate change of the past century. New Zealand Journal of Geology and Geophysics 39 (3):415-428
3. Di H, Cameron K, Sherlock K (2007) Comparison of the effectiveness of a nitrification inhibitor, dicyandiamide, in reducing nitrous oxide emissions in four different soils under different climate and management conditions. Soil Use and Management 23 (1):1-9
4. Jenkins BR (2007) Water allocation in Canterbury. Paper presented at the NZ Planning Institute Conference 2007, Palmerston North, 27-30 March 2007
5. Jenkins BR (2012) Water Issues in Canterbury. Paper presented at the Agronomy Society of New Zealand Annual Conference 2012, Lincoln University, 20 November 2012
6. Mason E, Ledgard S (2013) Making your farm greenhouse gas neutral. www.forestry.ac.nz/euan/carbonStockUnits.htm. Accessed 29 Dec 2013 2013
7. Morgan M, Bidwell V, Bright J, McIndoe I, Robb C (2002) Canterbury Strategic Water Study. Lincoln Environmental, Lincoln
8. Ministry for Agriculture and Forestry (2011) Projected Effects of Climate Change on Water Supply Reliability in Mid-Canterbury. Ministry for Agriculture and Forestry, Wellington
9. Ministry for the Environment (2013) New Zealand's Greenhouse Gas Inventory 1990-2011. Ministry for the Environment, Wellington
10. Ministry for the Environment (2008) Climate change effects and impacts assessment: A Guidance Manual for Local Goverment in New Zealand (2nd Edition). Ministry for the Environment, Wellington
11. Mullen B, Porteus A, Wratt D, Hollis M (2005) Changes in drought risk with climate change. Prepared for Ministry for the Environment and Ministry of Agriculture and Forestry). NIWA, Wellingtion
12. Srinivasan M, Schmidt J, Poyck S, Hreinsan E (2011) Irrigation Reliability Under Climate Change Scenarios: a Modelling Investigation in a River-Based Irrigation Scheme in New Zealand. Journal of the American Water Resources Association 47 (6):1261-1274