Shifts in flood and low-flow regimes in New Zealand due to interdecadal climate variations

Thirty-one of the longest available streamflow records for New Zealand were analysed to see whether shifts in flood and low-flow regime occurred in 1977/1978 corresponding to a shift in phase of the Interdecadal Pacific Oscillation. Annual peak flows, and annual minimum flows averaged over 28 days, for two periods, 1947-1977 and 1978-1999, were compared using box plots and statistical tests. The plots and the tests show that a decrease of flood size has occurred since 1978 in the Bay of Plenty region of the North Island, and that increases in flood size and low-flow magnitude have occurred in the South Island for most rivers with headwaters draining from the main divide of the Southern Alps and Southland. For much of the North Island apart from the Bay of Plenty, and the north of the South Island, no consistent pattern of shifts was identified.