Quantifying within-catchment spatial variability of hydrological droughts in cold, humid regions

Abstract. Although catchments serve as the primary unit for water resources management, the spatial distribution of hydrological droughts within catchments remains poorly documented. Many drought assessments rely on sparse gauge networks and presume spatial coherence across the hydrometric network, an assumption that is rarely verified. This study provides one of the first large-scale assessment of within-catchment spatial variability in hydrological droughts, focusing on cold, humid regions. Using 52-year streamflow timeseries of thousands of stream reaches spread across 109 catchments, we examined the duration, severity, and spatial extent of droughts identified with the Standardized Streamflow Index (SSI). Hydrological droughts showed greater within-catchment spatial variability than previously documented: 37 % of events were widespread (>90 % of the catchment), while 14 % were highly localized (<10 % of the network). As a result, a single downstream stream gauge would have missed about 30 % of drought events within a given catchment, whereas increasing monitoring density to one gauge per 100 km² raised detection rates to nearly 100 % in most catchments. The spatial extent of droughts varied significantly with their severity: events spanning over 90 % of the network were, on average, twice as severe as those affecting less than 10 %. Our findings show that hydrological droughts can be highly variable across hydrometric networks in cold, humid regions, highlighting the importance of integrating spatial variability into drought management and investigating its controlling factors.