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Title: Using Cesium-137 to quantify sediment source contribution and uncertainty in a small watershed

Author
item Zhang, Xunchang
item ZHANG, GUANGHUI - Beijing Normal University
item LIU, BENLI - Chinese Society Of Agricultural Engineering
item LIU, BING - China Institute Of Water Resources

Submitted to: Catena
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/21/2016
Publication Date: 3/1/2016
Citation: Zhang, X.J., Zhang, G., Liu, B., Liu, B. 2016. Using Cesium-137 to quantify sediment source contribution and uncertainty in a small watershed. Catena. 140:116-124.

Interpretive Summary: Knowledge of sediment provenance is needed for precision conservation and calibration of soil erosion models. The objectives are to evaluate the ability of using the fingerprint Cs-137 to apportion sediment source contributions and to quantify the reliability of the estimates. We collected 50 surface samples from overland of a watershed, 28 subsoil samples from gully bank, and 44 sediment samples in channels. The Cs-137 concentration was measured by gamma spectrometry. Proportional contributions from each source were calculated by solving a linear mixing equation. There are significant differences in Cs-137 concentrations between sources in the whole watershed. The mean proportions predicted for the watershed were 0.42 from the overland areas with a 35% relative error, and 0.58 from the gully area with a 25% relative error. Overland source contributed most uncertainty to the estimated proportions, followed by sediment mixtures, with minimum from gully. In general, 30 to 50 samples are needed for each source and sediment to generate reliable estimates of proportional contributions. In addition, the results showed that the fingerprint Cs-137 was capable of discriminating the three sources of croplands, rangelands, and gully. These findings will be useful to soil scientists, soil conservationists, and engineers who are interested in determining sediment sources for better conservation planning or model calibration.

Technical Abstract: Knowledge of sediment provenance is critical for precision conservation and calibration of soil erosion models. The objectives are to evaluate the ability of Cs-137 to apportion sediment source contributions, quantify uncertainty of the estimates, and estimate desirable sample size. We collected 50 surface samples from overland of a watershed, 28 subsoil samples from gully bank, and 44 sediment samples in channels. The Cs-137 activity was measured by ' spectrometry. Proportion means were calculated by solving a linear mixing model. Uncertainty was estimated by first-order approximation and Monte Carlo (MC) simulation. The Cs-137 tracer was conservative for the fraction of <63 µm. The Cs-137 activities differed between sources in the whole watershed at P=0.05. The mean proportions (relative error) predicted for the watershed using the mixing model were 0.42 (35%) for the overland source and 0.58 (25%) for the gully source, with a 95% confidence interval of ±0.145 for both. Overland source contributed most uncertainty to the estimated proportions, followed by sediment, with minimum from gully. The MC simulation predicted the same mean proportions, but with relative errors being <3% for both. Compared with the first-order approximation, MC underestimated uncertainty of the means due to the use of large sample size. In general, 30 to 50 samples are needed for each source and sediment to generate reliable estimates. In addition, Cs-137 was capable of discriminating the three sources of croplands, rangelands, and gully. No adjustment or weighting should be made directly to the mixing model unless the tracer’s conservativeness is violated.