Design and Performance Evaluation of an Adjustable-Slope Erosion Plot under Simulated Rainfall

Fixed-slope erosion plots limit controlled examination of the effects of slope. This study introduced an adjustable-slope erosion plot, evaluated its application with a DeltaLab rainfall simulator, and compared hydrosedimentary responses among three land uses in the Toshan watershed, Golestan Province, Iran. Rainfall at an intensity of 80 mm h⁻¹ was applied for 30 min to burned cropland, broadleaf forest, and coniferous forest plots (five replicates per land use); 30% was the only experimentally measured slope, and results for other slopes represent only baseline-dependent engineering responses. Runoff initiation time, interval runoff and sediment, event totals, and time to reach 50% of the cumulative response were calculated. Robust estimates, 95% bootstrap confidence intervals, randomization tests, Cliff’s delta, and leave-one-plot-out analyses were used. Median runoff initiation times were 1.15, 3.00, and 4.37 min in cropland, coniferous forest, and broadleaf forest, respectively. Median total sediment was 114, 48.5, and 62 g, and event sediment concentrations were 8.99, 3.67, and 5.33 g L⁻¹, respectively; these patterns remained stable in the sensitivity analyses. There was insufficient evidence of differences in total runoff. In an engineering sensitivity analysis anchored at a 30% slope, increasing the slope to 40% was associated with a 23.6% increase in sediment, a 4.3% increase in runoff, and a 6.8% reduction in runoff initiation time. The proposed system is a practical tool for training, screening critical slopes, and designing erosion experiments.