Potential climate change impacts on water resources in the Auckland Region (New Zealand)

Potential climate change impacts on water resources in the Auckland region (New Zealand) are assessed using scenarios of future climate change (2020, 2050, 2100) and a daily water balance model to transform the scenarios into seasonal impacts on the soil water regime and catchment water yield. The climate change scenarios are derived in the form of best guesses and envelopes, the latter incorporating known quantifiable sources of uncertainty at the global and regional scales. The water balance model is driven by historical time series of precipitation and potential evaporation from 2 sites selected to represent regional differences in climate regime. Simulations for a range of site characteristics (vegetation, soil water storage capacity, soil drainage characteristics) are undertaken by making appropriate adjustments to model parameters. Impact assessments are nested within more general sensitivity analyses (response surfaces) of seasonal summary variables, derived from multiple runs of the water balance model with systematic changes to the input variables. Potential impacts are assessed by superimposing the climate scenarios onto the seasonal response surfaces and from frequency analysis across the range of plausible impacts. The scenario overlays give potential impacts on mean conditions, whereas the frequency analyses focus on changes at the 5 yr return period level, important for water resource planning. A key result from the soil water regime analysis is that the direction of impacts is uncertain, for all scenarios, seasons, climate regimes, and site characteristics. The best guess is for negligible change. Intra-regional climate regime differences are important and vegetation cover significantly affects sensitivity. In contrast, water yield results indicate an unambiguous change to increased yield, especially over winter, although with very large uncertainties concerning the magnitude of the change. Differences in climate regime and site characteristics are again important. From a water resource perspective the study results show no cause for alarm. Within the context and Limitations of the regional climate change scenarios used, detrimental impacts appear rather less likely than potential benefits, the latter possibly being substantial.