Assessing changes in runoff-sediment response in a restored Loess Plateau watershed using long-term monitoring

Long-term environmental monitoring provides a basis for detecting changes in runoff-sediment relationships in restored watersheds. Using 35 years of outlet runoff-sediment observations from the Luoyugou catchment, together with daily sediment records, flood-event data, rainfall indices and MODIS normalized difference vegetation index (NDVI), we assessed nonstationarity through annual indicators, stage-varying sediment rating curves, discharge-stratified comparisons, sediment-response breakpoints, effective discharge, quantile regression, event-scale evidence and robustness tests. Annual runoff depth showed no monotonic decline, whereas sediment yield and sediment efficiency decreased markedly after 2010. Median sediment efficiency declined from 193.7 kg m⁻³ in 1990-1999 to 50.5 kg m⁻³ in 2011-2024. Daily rating curves shifted downward, with the exponent decreasing from 2.026 to 1.521 and predicted sediment discharge at Q = 0.5 m³ s⁻¹ declining by 49.3%. Within shared discharge support, year-clustered regressions estimated recent-period reductions of 55.4% and 57.4% in the high and very-high flow classes, respectively. Event-scale models also showed lower sediment loads after controlling for event runoff, rainfall amount and rainfall intensity. Together, these patterns are consistent with increasing sediment-supply limitation at the catchment outlet. The study demonstrates how long-term monitoring can diagnose changes in hydro-sediment response and guide targeted process investigations in restored dryland watersheds.