Evaluating the influence of atmospheric moisture transport on runoff variability across New Zealand

Atmospheric moisture transport is a key driver of flooding, yet its role in shaping runoff across New Zealand has not been assessed nationally. Using 40 years of daily runoff data from 166 near-natural flow sites and catchment-averaged integrated vapour transport (IVTC), strong regional contrasts were identified. Correlations between IVTC and runoff were strongest along the western Southern Alps. Atmospheric Rivers (ARs) and high IVTC events accounted for majority of annual high runoff across most regions. Persistent events amplified runoff markedly: ARs of multiday duration increased cool-season runoff by up to 225% above baseline flow in the western South Island, compared to 50–100% in most North Island regions. High IVTC also raised the likelihood of extreme runoff (>90th percentile) by more than 70% in western South Island catchments. Overall, the study reveals how topography and moisture-transport dynamics drive regional contrasts in New Zealand runoff response and flooding hazard.