Impact of Climate Cycles and Trends on Selwyn District Water Assets

Aqualinc carried out an assessment of the impact of climate cycles and trends on Selwyn District Council’s potable water, wastewater, stormwater, land drainage, and water race assets. The assessment looked out to 2048 to align with their 2018 to 2048 Infrastructure Strategy. The project considered projected changes in climate in the light of historically observed climate cycles and trends and assessed what the impact of future changes could be on water assets.

A robust analysis of long term historical climate (100 years+) was carried out, which allowed climate projections to be placed in context. The results suggested that, for certain climate variables, natural variability is often much greater than climate change effects, particularly over a 30 year horizon.

The study was a high level risk assessment, to identify the assets that were most likely to be affected by climate change. Priority areas were guided by a risk matrix that Aqualinc developed in consultation with the council at the outset of the study. The variables that were identified as having the greatest potential impact for infrastructure were groundwater levels (high shallow groundwater levels and reduced deep groundwater levels), extreme rainfall, high river flows, and sea level rise.

Across the district no evidence of an increasing trend in extreme rainfall events was found. If a trend exists, it is masked by the high variability. Rainfall is projected to increase in the mountain regions of the district so the alpine rivers (i.e. the Waimakariri and the Rakaia) could show an increase in flow, with implications for associated water supply, stormwater and wastewater infrastructure in the alpine settlements.

Over the next 30 years, sea level rise will increase the likelihood of issues with wastewater, stormwater and land drainage in the areas close to Te Waihora/Lake Ellesmere and the mouth of the Rakaia.

Changes in groundwater levels associated with projected climate change are small compared to irrigation effects and the natural inter-annual variability.