Climate Change adaptation options for Greater Wellington Regional Council’s wholesale water supply
Greater Wellington Regional Council currently supplies water to 380 000 consumers. This number is expected to rise to about 480 000 by the year 2050. Water is currently drawn from three rivers in the region and the Lower Hutt aquifer. Only a small amount of off-river storage is available, so the system is heavily reliant upon frequent and adequate rainfall. Under projected climate change scenarios, rainfall is likely to become less frequent and falls will be of higher intensity. In addition, projected increases in temperature will cause the per capita demand to rise, and a rise in sea level will increase the potential for saline intrusion into the aquifer. For long term planning purposes the water supply system is modelled using the Sustainable Yield Model (SYM) based on the Wathnet network linear programming software. Inputs to the system consist of daily river flows, and rainfalls on the aquifer recharge zone. Outputs from the system are the daily demands of the four cities of the Greater Wellington region and evaporative demands on the aquifer recharge zone. Operation of the system is constrained primarily by environmental considerations: the aquifer must not be over-pumped to avoid salt water intrusion, and all the river sources have both minimum flow and sharing rules applied to abstractions. Examination of the likely effects of climate change began with downscaling to the Wellington Region the results of the IPCC third climate change scenario data. Both rainfall and temperature changes were considered for low, mid and high scenarios. The modified rainfall and temperature data were used as input to rainfall-to-runoff models for each of the three river systems. The resultant flows were then used as input to the planning model. Temperature and rainfall data were also used in a regression style model of per capita demand to estimate the likely impacts on demand. The revised river flows and demands were fed into the SYM for each climate change scenario and the likely population that could be supplied under a number of different scenarios has been investigated. By examining different demand management scenarios, options for what might be sustainable under different supply restrictions can be investigated. Although demand management is unlikely to be a politically palatable option, it may be a more economically acceptable one.