Climate change and water resources in the Auckland region

Abstract

This thesis assesses the sensitivity of selected water resources in the Auckland region to climate variation and the potential water resource impacts of anticipated global warming. Based on a review of regional water resource planning imperatives, the soil-water regime and catchment water yield are the two water resources selected for detailed investigation. Preliminary sections of the thesis review the prospects for climate change and derive a generalised conceptual model for climate change impact assessment. The latter is a significant component of the research, arising out of perceived weaknesses in the conceptual basis of much of the impacts-related research being undertaken in the mid-1980s. One outcome is the identification of regional climate change scenarios, derived from multiple sources, as an appropriate basis for impact assessment. Others include the development of methodologies for linking these synthesis scenarios with "transfer functions" used to assess biophysical impacts, and methodologies for coping with the myriad uncertainties associated with projections of future climate. Consideration of essential criteria for biophysical transfer functions used for impact assessment, combined with a review of hydrological modelling, leads to the identification of process-based hydrological models as appropriate tools for assessing potential climate change impacts on water resources. A daily water balance model is adopted as most suited to the specific research aims of this study. The model specifics are derived from a review of theory, observational evidence, and existing models and modelling experience. A substantial body the research presented here involves detailed calibration and verification of the daily water balance model. By virtue of its intended application, the model is calibrated and verified in both field and catchment-scale modes against observational data for three sites. Results indicate that the model is an appropriate transfer function for soil-water regime and catchment water yield applications, but that it is not appropriate for low-flow applications. The sensitivity of the soil-water regime and catchment water yield to climate variation is examined by running the daily water balance model for a range of site characteristics and climate regimes. Results are presented in the form of "response surfaces", representing the effect of changes in precipitation and potential evaporation on selected soil-water regime and catchment water yield variables. Potential climate change impacts are assessed by superimposing regional climate change scenarios onto these response surfaces. The climate change scenarios represent a range of plausible changes in precipitation and potential evaporation, together with "best-guess" estimates. Derived from multiple sources for the years 2020, 2050, and 2100, the scenarios indicate substantial uncertainty about projected regional climate change. This uncertainty is translated into a range of potential impacts by deriving envelopes of potential water resource impacts which encompass the full range of future climates indicated by the climate change scenarios. The results are presented in terms of inter-annual variability, permitting potential changes in the frequency of more extreme conditions to be assessed, as well as changes in the mean. In addition to the soil-water regime and catchment water yield, potential impacts on the demand for water for irrigation are assessed by incorporating an irrigation scheduling scheme into the daily water balance model. Potential impacts on irrigable potential are also assessed. Impact assessment results are dominated by uncertainty about regional climate change and are significantly influenced by site characteristics and the baseline climate regime. This results in a wide range of potential impacts for all impact variables analysed. An increase in catchment water yield is indicated, but even the direction of change is uncertain in the case of the soil-water regime and irrigation. In general, the results indicate potentially significant climate change impacts, but no cause for alarm. Potential benefits appear more likely than detrimental impacts. Planning response options to minimise adverse impacts and exploit beneficial impacts are outlined. A series of water resource planning recommendations is made in this context.