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, cont iguous region (20 000 km(2)) of the Southern Alps, New Zealand, repres enting 70% of the length of the Alps. Using a Bayesian method develope d in an earlier study (Moss et al., 1994, Water Resour. Res., 30(10): 2717-2723), probabilities of summer inflows to alpine lakes when the s pring SOI is positive (La Nina conditions) are shown to be significant ly larger than in springs when SOI is neutral or negative (El Nino con ditions). This has major economic significance because the lakes feed hydroelectric power plants that produce typically 43% of the electrica l energy used in New Zealand. The lake inflows peak in spring and summ er, and use of the available controlled storage must be scheduled care fully to meet power demands that peak in winter. We hypothesise that t his occurs because in springs with La Nina conditions, there is a rela tive absence of snow accumulation, and hence less summer snowmelt.