The long-term impacts of glacier retreat on runoff in the Waitaki Catchment, New Zealand

Runoff from the Waitaki catchment is relied upon for hydroelectricity generation and long-term water storage. The Waitaki catchment holds some of the country’s largest glaciers. Ice melt contributes to runoff, along with snow melt and rainfall. Understanding the implications of climate change and glacier retreat on downstream hydrology is important for long-term water resource planning. Using an enhanced temperature index model coupled with an ice flow model, glacier mass balance within the Waitaki was projected through the 21st century for four future climate scenarios based on CMIP5.

Glacier volume in the Waitaki catchment is projected to decrease over the 21st century for all future climate scenarios by up to 86%. Average ablation is projected to begin earlier in spring and continue later into autumn by the end of the century. Conversely, average accumulation is projected to start later in autumn and end earlier in spring. These shifts become more pronounced with increasing severity of climate change scenarios. The largest increases in average rainfall and ablation are projected to occur in the winter and spring months when runoff is typically at its lowest, as a result of warmer winter temperatures.

Maximum ice melt contributions to runoff, known as peak water, are projected to occur before 2040 in the Waitaki catchment for all future climate scenarios. Before peak water is reached, increases in total ablation may mitigate the frequency and magnitude of low flow events during dry periods, but may also increase flood risks in and around the summer season when rainfall is high. Once peak water passes and ice melt contributions to runoff decrease, extreme low flow events may be more of a concern during dry periods with reduced rainfall.