Simulation of Climate Change Effects on Hydropower Lake Inflows in a Mountain Basin, South Island, New Zealand

Abstract:

Hydroelectric power is a valuable renewable energy resource and the primary source of electricity in New Zealand, but climate change is predicted to have varying effects on mountain rivers and hydropower in the future due to strong topographic, precipitation, and temperature gradients over space. This study models climate change impacts on river inflows to three headwaters hydropower lakes (Pukaki, Tekapo, and Ohau) in the Upper Waitaki Basin of the central South Island using the TopNet hydrological model based on the 1990s (baseline), 2040s, and 2090s periods. The IPCC 4th assessment report A1B emissions scenario and average temperature and precipitation results of 12 Global Circulation Models with statistical downscaling were used as input to TopNet. Indicators of hydrologic alteration were used to estimate and compare 63 flow metrics across the full flow regime based on TopNet output. Kruskall-Wallis tests were used to evaluate significant differences between metrics among the three periods. Implications for hydropower operations, other water resources management needs, and braided river flows and ecology were also evaluated. Results show that annual and cumulative inflows increase for future climate periods for all three lakes. However, seasonal effects cause flows to generally increase in winter and early spring and decrease slightly in summer due to warmer temperatures with more winter rain and less snow. The number of inflow metrics with statistically significant changes is greatest for Lake Pukaki, with the most glacierized catchment, and least for Lake Ohau. High flows and floods generally increase, and low flows/extreme low flows decrease. Overall these changes may be beneficial for hydropower and some braided river floodplain ecological features, but the seasonal shifts will result in greater flood risk in winter and spring, more drought risk in summer and autumn, and additional challenges managing water volumes in these mountain hydropower lakes.