L Whitelock-Bell, S Innes, D Leong, S Reed (Tonkin & Taylor Limited)
A Sood (NIWA)
A Lester (Watercare Services Limited)
Watercare is responsible for supplying high quality drinking water to more than 1.7 million people with more than 400 million litres supplied every day. Following Auckland’s 2019/2020 drought, the security of the city’s water supply both today and into the future has been brought into sharp focus.
Over the past twenty years Watercare has used an Integrated Source Management Model (ISMM) to assist operational and planning decisions. For example, it is usedto help identify when new water sources may be needed. One of the functions of the model is to determine the current system yield.
Climate change is expected to impact the pattern and reliability of precipitation as well as to increase potential evapotranspiration (PET). ISMM has recently been updated to consider multiple potential climate futures and assess how the system yield might be expected to change in the future.
In alignment with Watercare’s 2019 Climate Change Strategy, the analysis presented here considered two time horizons (2040 and 2090) and two Representative Concentration Pathways (RCP 4.5 and 8.5). Six Global Climate Models (GCMs) (selected for dynamical downscaling based on past performance and model diversity) have been used to simulate each combination of time horizon and RCP, resulting in a total of 24 modelled future scenarios.
The project team applied the changes predicted by the GCMs to an existing rainfall record to create synthetic future records. This approach was selected to preserve natural variability captured in the long-term historical rainfall record, as was necessary for modelling the future performance of an operational water supply system. Rainfall perturbations were applied to the historical data set using a method based on gamma distributions, and a stochastic weather generator was then used to extend the datasets for each scenario.
The impacts of climate change have been modelled and quantified by estimating the yield of the system under the historical baseline climate and then each future climate scenario. Yield, as it is discussed here, represents the annual average daily demand that can reliably be provided by a water supply system, while meeting the required security of supply standard. Preliminary conjunctive-use yields have been estimated using the ISMM model to consider the performance of the integrated system. Stand-alone yields for each of the ten reservoir sources have also been modelled to better understand the possible impacts on the stored-water sources.
The outcomes of this work will be used to inform Watercare’s long term strategic planning to ensure security of supply to Auckland under a changing and uncertain climate. This paper details the work done and provides a valuable example of climate change impact modelling for the New Zealand water industry.