Seasonal low flow regimes reveal compound drought processes and catchment heterogeneity across New Zealand

Study region: New Zealand shows strong climatic and hydrological gradients, ranging from humid, snow-influenced alpine catchments to temperate lowland and groundwater-dominated systems. This diversity produces distinct seasonal low flow regimes, making New Zealand an ideal setting for investigating the climatic controls and seasonality of low flow extremes. Study focus: This study investigates seasonal low flow behaviour across 227 catchments in New Zealand by comparing summer and winter low flow frequency characteristics. Seasonal low flow frequency analysis was performed using a mixed distribution approach, and the dominant season at each catchment was identified. To investigate climatic controls on seasonal low flows, drought drivers were classified using the Standardized Precipitation Index (SPI), Precipitation Evapotranspiration Index (SPEI), and Temperature Index (STI). New hydrological insights for the region: Seasonal low flow behaviour varies substantially across New Zealand, demonstrating that annual analyses can mask important differences in low flow generation processes. Summer was identified as the dominant low flow season in most catchments. The dominant climatic drivers differed between seasons. Compound drought events were the most common driver category in summer. In contrast, winter low flow events showed no dominant relationship with the selected indices, suggesting a stronger influence of hydrological storage and lagged catchment processes. These findings demonstrate the value of seasonal frequency analysis for identifying distinct low flow-generating mechanisms and improving climate-informed water resource management in New Zealand.