Catchment hydrologic function is widely time-variant

Catchment hydrologic function describes how meteorological conditions are transformed into streamflow. Whether this transformation is stable or varies from event to event, and what controls that variability, remains unknown at the global scale, limiting confidence in flood-risk and water-security assessments. Here, we analyze about 1.7 million runoff-producing events across 4838 catchments on six continents. We classify each event by its water-input type (rain, snowmelt, or rain-on-snow) and by its antecedent soil moisture (dry, moderate, or wet). These two descriptors together define the event’s hydro-meteorological conditions. We summarize each catchment’s dominant hydro-meteorological event type in the dormant and growing seasons separately, and find a wide diversity of dominant types across the global catchment population. We then quantify, for each catchment, the extent of event-to-event time-variance in the precipitation–streamflow transfer function that represents catchment hydrologic function. Time-variant catchment hydrologic function is pervasive: 95.6% of catchments in the dormant season and 99.0% in the growing season show moderate to strong event-to-event variation, and even catchments experiencing highly similar hydro-meteorological conditions across events remain functionally time-variant. Within-catchment spatial heterogeneity in hydro-meteorological conditions modulates this variation. Greater time-variance in hydrologic function is associated with greater event-to-event variation in a catchment’s runoff response.