Charles P. Pearson

THE SOUTHERN OSCILLATION INDEX AS A PREDICTOR OF THE PROBABILITY OF LOW STREAMFLOWS IN NEW ZEALAND

The Southern Oscillation index (SOI) can yield information about subsequent streamflows on the South Island of New Zealand; however, the relationship between streamflow and SOI may be nonlinear and heteroscedastic. To deal with such difficulties, the conditional probability of streamflows being below a critical magnitude, given a prior observation of SOI, can be considered a random variable, with its probability density function (pdf) estimated by Bayesian analysis of existing observations of streamflow and SOI. The conditional pdf can yield a probabilistic forecast of critically low streamflows given a precursor value of SOI. In the Clutha River basin of New Zealand, average austral summer inflow to the headwater lakes has an unconditional probability of nonexceedance of 360 m3/s of approximately 17%. However, during a moderate La Nina, average austral spring SOI = 12, the conditional probability of nonexceedance has an expected value of 55%. For an El Nino with a spring SOI of −12, the expected probability of nonexceedance is 18%; for a neutral SOI of 0, the expected probability is 10%. Thus, the probability of nonexceedance of seasonal streamflow can be seen to vary by a factor of more than 5 as a function of the SOI.

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 km2) 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 Nina conditions) are shown to be significantly larger than in springs when SOI is neutral or negative (El Nino 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 Nina conditions, there is a relative absence of snow accumulation, and hence less summer snowmelt.