Prediction of summer inflows to lakes in the Southern Alps, New Zealand, using the spring Southern Oscillation Index

The relationship between the austral spring Southern Oscillation Index (SOI) and the austral summer runoff is investigated for a broad, contiguous region (20 000 km 2) of the Southern Alps, New Zealand, representing 70% of the length of the Alps. Using a Bayesian method developed in an earlier study (Moss et al., 1994, Water Resour. Res., 30(10): 2717–2723), probabilities of summer inflows to alpine lakes when the spring Sol is positive (La Niña conditions) are shown to be significantly larger than in springs when SOI is neutral or negative (El Niño conditions). This has major economic significance because the lakes feed hydroelectric power plants that produce typically 43% of the electrical energy used in New Zealand. The lake inflows peak in spring and summer, and use of the available controlled storage must be scheduled carefully to meet power demands that peak in winter. We hypothesise that this occurs because in springs with La Niña conditions, there is a relative absence of snow accumulation, and hence less summer snowmelt.