Effects of Stirring and Sampling Methods on the Accuracy of Sediment Concentration Measurements in a Runoff Collection Tanks

ABSTRACT Measurement of sediment concentrations in runoff plots has previously lacked refined quantitative operational standards specifically targeting the stirring and sampling procedures within the collection tank, often leading to substantial representativeness issues in unevenly stirred samples. Consequently, sediment concentration measurements have frequently underestimated true values, incurring significant errors. In this study, we investigated how sampling parameters (stirring time, sampling interval and sample volume) affect the accuracy of sediment concentration measurements. For the experiment, we utilised topsoil from a quaternary red soil with sediment concentrations set at 5.07, 50.72 and 101.45 g/L, at a water depth of 60 cm. Relative measurement errors were compared for different stirring times, sampling intervals and sample volumes. Additionally, three sampling methods—manual, mechanical and mechanical plus depth profile—were evaluated for sediment concentrations of 1.05, 5.07, 10.49, 50.72, 101.45 and 439.1 g/L under identical water depth conditions (60 cm). In each method, water samples were collected from the surface, middle and bottom layers of the collection tanks, then thoroughly mixed and weighed in aluminium boxes to measure the sediment concentration. The findings indicate that the impacts of the assessed sampling parameters on measurement errors stabilise at a stirring time of 180 s, a sampling interval of 0 s, and a sample volume of 800 mL. Under these conditions, the average relative errors obtained using the manual, mechanical and mechanical plus depth profile methods were 37%, 24% and 14%, respectively, with the mechanical plus depth profile method thus demonstrating the highest measurement accuracy. Notably, this study was conducted using a single soil type (quaternary red soil), which may limit the generalizability of the findings to soils with different textures (e.g., sandy or clayey soils) or physicochemical properties. On the basis of these findings, we recommend the adoption of the mechanical plus depth profile method, coupled with a precise control of stirring time, sampling interval and sample volume, to enhance the accuracy of sediment concentration measurements in collection tanks. This approach promises to provide significant improvements in the reliability and precision of sediment concentration assessments, while future research could expand to investigate multiple soil types to further validate the conclusions.