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 subse quent streamflows on the South Island of New Zealand; however, the rel ationship between streamflow and SOI may be nonlinear and heteroscedas tic. To deal with such difficulties, the conditional probability of st reamflow's being below a critical magnitude, given a prior observation of SOI, can be considered a random variable, with its probability den sity function (pdf) estimated by Bayesian analysis of existing observa tions of streamflow and SOI. The conditional pdf can yield a probabili stic 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 non exceedance of 360 m3/s of approximately 17%. However, during a moderat e La Nina, average austral spring SOI = 12, the conditional probabilit y 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 pro bability of nonexceedance of seasonal streamflow can be seen to vary b y a factor of more than 5 as a function of the SOI.