Identifying the pattern and drivers of spatio-temporal trends in New Zealand streamflow

Climate change is affecting rivers worldwide, threatening water availability and altering the risk of natural hazards. Identifying the pattern and causes of regional streamflow trends is imperative for informing region-specific policies to mitigate and adapt to the negative impacts on society and the environment. Changes in the strength and position of the southern hemisphere storm track resulting from both greenhouse gases and ozone depletion are known to project strongly onto New Zealands long mountainous spine. For example, increasing westerly winds in the winter have increased rainfall on the west coast of the South Island, and this should be detectable in river flow trends. Using a network of long, near-natural streamflow records representative of New Zealands hydroclimate diversity, we identify spatio-temporal trends in winter streamflow. Individual records rarely have significant trends, but when aggregated within regions, which we determine through cluster analyses, significant trends emerge. In winter, streamflow on the West Coast of the South Island has significantly increased while streamflow in Northland has significantly decreased. Correlating the observations with dynamic and thermodynamic indices, such as surface pressure differentials, temperature and precipitation (separating rain- and snow-fall) over New Zealand, we assess the main drivers of the observed changes in winter streamflow.