Variability of suspended sediment transport dynamics between seasons and flow pathways in a small agricultural catchment

Abstract Suspended sediment dynamics in small agricultural catchments are shaped by the interaction of overland and subsurface flow processes, yet no event-scale, flow pathway-specific comparisons of seasonal discharge (Q)–suspended sediment concentration (SSC) relationships within a single catchment exist. This study addresses this gap by analysing Q–SSC relationships during 18 overland flow events (2014–2016) using high temporal resolution (1-min) discharge and turbidity data combined with event-based SSC monitoring at three different flow pathways within the Hydrological Open Air Laboratory (HOAL), Lower Austria: a partly tile-drained piped headwater inlet, an overland flow station, and the aggregated catchment outlet. The catchment is under annual crop rotation of mainly winter wheat, maize, barley, and oilseed rape. Seasonal Q–SSC relationships were fitted for each station, and event-based coefficients were compared. Results revealed clear scale effects between flow pathways and strong seasonal contrasts. SSC was highest in overland flow, lowest at the partly tile-drained headwater inlet, and in-between at the catchment outlet. All flow pathways showed increased SSC magnitude and variability in the growing season. Event-scale Q–SSC coefficients showed no systematic seasonal variation for the partly tile-drained piped headwater inlet, while both the overland flow and catchment outlet showed higher intercepts and flatter slopes in the growing season, reflecting higher baseline SSC during the growing season and sediment transport concentrated at higher discharges during the dormant season.