Spatiotemporal Trends in Near-Natural New Zealand River Flow

Anthropogenic climate change is affecting rivers worldwide, threatening water availability and altering the risk of natural hazards. Understanding the pattern of regional streamflow trends can help to inform region-speciflc policies to mitigate and adapt to any negative impacts on society and the environment. We present a benchmark dataset of long, near-natural streamflow records across Aotearoa New Zealand (NZ) and the flrst nationwide analysis of observed spatio-temporal streamflow trends. Individual records rarely have signiflcant trends, but when aggregated within homogenous hy-drologic regions (determined through cluster analyses), signiflcant regional trends emerge. A multitemporal approach that uses all available data for each region and considers trend signiflcance over time reveals the influence of decadal variability in some seasons and regions, and consistent trends in others. Over the last 501 years, winter streamflow has signiflcantly in-creased in the west South Island and has signiflcantly decreased in the north North Island; summer streamflow has signifl-cantly decreased for most of the North Island; autumn streamflow has generally dried nationwide; and spring streamflow has increased along the west coast and decreased along the east coast. Correlations between streamflow and dynamic and thermodynamic climate indices reveal the dominant drivers of hydrologic behavior across NZ. Consistencies between the observed near-natural streamflow trends and observed changes in circulation and thermodynamic processes suggest possi-ble climate change impacts on NZ hydrology.

Automated summary

Anthropogenic climate change is impacting rivers globally, jeopardizing water availability and changing natural hazard risks. Analyzing regional streamflow trends can provide valuable information for developing policies to mitigate and adapt to the associated negative impacts on society and the environment. This study presents a comprehensive dataset of long-term, near-natural streamflow records in New Zealand (NZ) and the first nationwide analysis of observed spatio-temporal streamflow trends. By aggregating data within homogeneous hydrologic regions, significant regional trends are revealed. The findings demonstrate the influence of decadal variability in specific seasons and regions, with both increasing and decreasing streamflow trends observed across NZ. Correlations between streamflow and climate indices highlight the primary drivers of hydrologic behavior and suggest possible impacts of climate change on NZ hydrology.