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Retorts

Drought Doubt

Earl Bardsley in his article ["The 100-Year Drought?" August, 1992] suggests that a 1-in-100 year drought may be more common than thought. The uncertainty of estimating droughts is illustrated by a calculation that if just one event, a severe drought, is recorded in 100 years then there is a 5% chance of its return period being less than 21 years.

While there is uncertainty in calculating the true return period of droughts, it is not nearly as uncertain as illustrated by the above calculation, which is inappropriate. Dr David Rhodes of Industrial Research Ltd, Applied Mathematics, confirms this view.The very nature of droughts is that over a period of years a range of drought severities will be observed, rather than simplistic "drought" or "no drought" outcomes. This range of severities provides additional information on extreme events, and so narrows the uncertainty of the estimate.

ECNZ has referred to the recent drought as being 1-in-100 for the potential energy content of South Island hydroelectric inflows for the November-May period in terms of its prior expectations of probabilities. With a limited set of inflow data, only 60 years, obviously the 1-in-100 quoted was an estimate of an event of 0.01 probability. The occurrence of the 1991/92 drought, being the worst for the time of the year, in now 61 years of inflows, alters downward the expectations of what event has a 0.01 probability. Consequently ECNZ with the added information of 1991/92 inflows would modify its reservoir operating strategy accordingly.

Earl Bardsley raises the interesting possibility that local climatic factors might shift temporarily. In view of the Mt Pinatubo eruption, with temporary cooling effects possibly influencing inflows, it now seems prudent for ECNZ to be more pessimistic than normal about inflows in the next couple of years and so further modify its reservoir operating strategy.

Kieran Devine, Fuel Resource Manager, ECNZ