Assessment of the Rangitata River basin’s Food-Energy-Water Nexus under a changing climate using WEAP : A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Water Resource Management at Lincoln University

With well documented and nationally discussed ongoing deterioration and imminent threat to New Zealand’s waterways from global change, triggering a wholesale of freshwater reforms, this study sought to develop a technique that would quantify in high resolution the effects of anthropocene-induced climate change on the hydrology of the Rangitata River basin and the interdependent economic sectors. The conceptual hydrologic and water management model, Water Evaluation and Planning Tool (WEAP) was chosen for the study. The model was calibrated and validated across an equally split 1980-2020 timespan. A Nash-Sutcliffe Efficiency of 0.70 was obtained for calibration, and 0.60 for validation. The percent bias was 3% and 2.5%, respectivey, and the root mean squared error to observed standard deviation was 0.55 and 0.61. Dynamically downscaled General Climate Model (GCM) climate change data of daily precipitation and mean temperature were used as forcings in the hindcasting and forecasting of water budget dynamics in the catchment, and assessed in interdecadal time series spanning 1972 to 2100. Future climate change simulations were based on the 5th Climate Model Intercomparison Project (CMIP5) warmest emissions scenario of anticipated radiative forcings of 8.5 Watts/m2 (RCP8.5), also known as the ‘business-as-usual’ pathway. Climate change datasets from an ensemble of six out of over forty GCMs from the CMIP5 experiments were used to simulate climate change signals across the Food-Energy-Water nexus in the Rangitata basin. Intercomparison among the 6 models used revealed notable level of uncertainty with the projected signals leading to a communication of that uncertainty in the form of ranges of lowest to highest, and average among the 6 variables. Relative to the baseline historical period (1972-2005), and forecasting for the last 21st century decade of 2090s, the simulations reveal that the Rangitata River Upper Reach basin will experience an increase in temperature by 155%±51% during winters that are 15%±13% wetter. Summers will be moderately warmer with an increase of 31%±9%, but projected to be dryier by -3.5%±18.5%. Rangitata River discharge at abstraction point will be 1.5% higher in general, while winter and summer seasons discharge will increaseby 27%±12% and 7.5%±8.5%, respectively. This is due to warmer winters leading to faster snowmelt, coupled with projected more precipitation. Notwithstanding the forecast increase in precipitation in winter in the plains, the hydrologic model predicts an overall 6.5% increase in irrigation demand by the farmers. In winter the demand increases by 7%±4%, while summer needs will be higher by 10%±3%. Actual irrigation would on the whole rise by 7%±4% with winter and summer having 7%±4% and 8%±2% more water being used, respectively. The corresponding supply reliability remains unchanged for the two seasons. For these scenarios it is assumed that the current Rangitata River allocation framework and local production model remains in place for the rest of the 21st century. Under the same conditions, inflows to the Highbank hydropower station located at the end of the supply or delivery canal, which for the greater part of the year outside winter only gets unwanted or residual water, are projected to drop by 7% causing a proportionate reduction in power production. However, winters will have 10%±17% increase in inflows, while summer inflows may decline by -14%±22%. The hydroenergy sector in the RDR scheme would hence be able to produce less under climate change, assuming the current river water allocation framework stands into the future. In summary, the study findings confirm and unpack the likely impacts of climate change in the catchment and demonstrate how the water resource assessment and management tool WEAP can be useful to multisectoral decision makers in an alpine basin of New Zealand.