Diminishing Streamflows on the East Coast of the South Island New Zealand and Linkage to Climate Variability and Change

A tendency towards lower rainfalls and flows for eight east coast catchments on the South Island of New Zealand is investigated. Exceptionally dry conditions are shown to have occurred for catchments located north of the city of Christchurch, especially over the period 2001-2007. A series of mean sea-level pressure ('weather') patterns typically associated with flood-producing storms in these catchments are identified, with certain weather patterns shown to be very strongly associated with flood conditions in certain rivers. The annual frequencies of occurrence of key flood-related patterns are shown to have decreased significantly over the 50-year period 1958-2007. The reasons for these apparent decreases are not currently understood.