Comparison of measurement methods of sediment concentration in collecting tanks of slope runoff plots

The accurate measurement of sediment concentration in collecting tanks of slope runoff plots is a key to quantitatively monitor slope soil erosion. The researches on measuring accuracy, influencing factors and applicability of field runoff plot observation methods are needed. In this study, four methods of sediment concentration were compared. The test was taken in quaternary red earth. The soil samples were taken from surface of farmland (0-20 cm). Six sediment concentrations (1.05, 5.07, 10.49, 50.72, 101.45 and 439.10 kg/m3) were designed based on the measured sediment concentration range from 2012 to 2016 in slope farmland. Three water depths were set up including 30, 60 and 90 cm. Four methods included Artificial Method (AM), Mechanical Method (MM), Dept Profile Method (PM) and MM+PM. The measurement accuracy was evaluated by relative error. Each treatment was replicated three times. The results show that compared with the true value, the measurement accuracy of different method for determining sediment concentration was low. The averages of absolute relative error for the AM, MM, PM and MM+PM were 37%, 24%, 38% and14%, respectively. The measurement accuracy of different methods was 14% or above. Among them, the accuracy of MM+PM was the highest. In general, the relative error of each method was large under the condition of high sediment concentration. The depth of water had different influence on different sampling methods. Comparatively, the measurement error of AM was most significantly affected by water depth. Under low sediment concentration, when the water depth increased to 60 cm, the measurement error significantly increased, but when the water depth continued to increase, the measurement error did not increase. When the sediment concentration was 101.45 kg/m3, the relative error of MM only increased significantly after the water depth increased to 90 cm. When the sediment concentration was 59.72 kg/m3, no obvious relationship was found between the measurement accuracy of the PM method and the water depth. When the sediment concentration was more than 5.07 kg/m3, the measurement accuracy of the MM+PM method was almost not affected by water depth. The linear equation was used to modify the sediment concentration measurement results of the four methods. All R2 were greater than 0.99 (P<0.01), and the root-mean-square error was 4.53-11.37 kg/m3, which indicated the reliability of the linear regression equation and the equation could be used to calibrate the measured values of each method. The relative error of the measured values of each method was significantly reduced and the measurement accuracy was significantly improved. The practical range of these equations was 1.05-439.10 kg/m3 and the water depth was 60 cm. The results of this study could support the selection of sampling methods under different conditions. The results of this study could be used to revise the sediment concentration in collecting tanks of slope runoff plots in red soil area.