Risks to drinking water from future drought

The effect of drought on water availability is receiving an increased amount of attention as we head into a future where the impacts of climate change are already being felt. The level of risk to New Zealand from restricted drinking water availability under “as yet unexperienced droughts” emerged as one of the priority questions and knowledge gaps from the Climate
Change and Drought Dialogues held at Motu in May and June 2018.
The level of risk we are likely to face in relation to drinking water availability in times of drought is addressed in this report.

What level of risk are we likely to face in relation to drinking water availability in times of drought?

This study uses projected climate change information derived from simulations by six global climate models, driven by four IPCC Fifth Assessment emission scenarios (known as RCPs). The global models are dynamically downscaled to a 5-km grid covering the whole country, with the outputs used to generate stream flows using NIWA’s TopNet model. These inflows together with other physical characteristics of the water supply schemes were used as inputs to a spreadsheet-based model to calculate water yields at 99.5%, 99%, 98% and 95% assurance of
supply.

The projected risk of shortfalls in surface water supply are assessed for 10 case study water supplies – seven in the North Island and three in the South Island. Surface water inflows for each case study are used to calculate water yield (availability) for a baseline period from 1986 to 2005 and is then compared to water yield for 2040, 2060, and 2080 time horizons. In each of the 10 locations, we calculated the water availability at each of the future time horizons using outputs from the six Global Climate Models (GCMs) using four Representative Concentration Pathways (RCPs).

Some key findings of this report are:

• This method represents a first substantial step towards the development of a practical, transparent procedure for assessing the potential impact of climate change on reservoir and catchment yield for water supply.
• In conjunction with the new drought index (Climate-NZDI) developed by NIWA (in response to Question 1 above), this approach provides a tool for rapid assessment of water availability trends under various climate change conditions, where the outputs from these assessments can be used to prioritize water resources planning efforts.
• The projected impact of climate change on long-term reservoir yield and stream flows varies across the RCPs and levels of assurance of water supply that are modelled. The relative changes in yield pose challenges to water managers in making decisions about water supply resilience, and potentially calls for an adaptive approach to water supply management.
• Our approach however can be used as a long-term planning tool. Decisions about when new water resources or supplementary schemes are needed can then be based on the best available information and with sufficient lead time to allow for implementation. Water availability from alternative surface water resources under climate change conditions can now also be considered as part of an adaptive integrated system planning approach to the provision of water supply.
• Our approach would also benefit from a more detailed understanding of water demand (for example to recognize increased demand brought about by population growth, or to consider changing demand patterns), and including changes to demand that may be required in response to regulatory changes to water resource management. This approach would be significantly strengthened if it were to incorporate a Te Ao Māori perspective on water.