Information on sediment sources in river catchments is required for effective sediment control strategies, to understand sediment, nutrient and pollutant transport, and for developing soil erosion management plans. Sediment fingerprinting procedures are employed to quantify sediment source contributions and have become a widely used tool. As fingerprinting procedures are naturally variable and locally dependant, there are different applications of the procedure. Here, the auto-evaluation of different fingerprinting procedures using virtual sample mixtures is proposed to support the selection of the fingerprinting procedure with the best capacity for source discrimination and apportionment. Surface samples from four land uses from a Central Spanish Pyrenean catchment were used i) as sources to generate the virtual sample mixtures and ii) to characterise the sources for the fingerprinting procedures. The auto-evaluation approach involved comparing fingerprinting procedures based on four optimum composite fingerprints selected by three statistical tests, three source characterisations (mean, median and corrected mean) and two types of objective functions for the mixing model. A total of 24 fingerprinting procedures were assessed by this new approach which were solved by Monte Carlo simulations and compared using the root mean squared error (RMSE) between known and assessed source ascriptions for the virtual sample mixtures. It was found that the source ascriptions with the highest accuracy were achieved using the corrected mean source characterisations for the composite fingerprints selected by the Kruskal Wallis H-test and principal components analysis. Based on the RMSE results, high goodness of fit (GOF) values were not always indicative of accurate source apportionment results, and care should be taken when using GOF to assess mixing model performance. The proposed approach to test different fingerprinting procedures using virtual sample mixtures provides an enhanced basis for selecting procedures that can deliver optimum source discrimination and apportionment.
Increasing risk of soil loss as a result of climate change, has generated a need for reliable information on erosion rates at different temporal scales. Use of the fallout radionuclides 137 Cs, 210 Pb ex and 7 Be as tracers of sediment mobilisation and redistribution makes it possible to obtain estimates of soil redistribution rates within both undisturbed and cultivated landscapes over a range of timescales. Mediterranean landscapes are characterized by a great diversity of physiography and land use, and as a consequence erosion and deposition patterns are highly variable spatially. To document such spatial variability, a slope transect located in the subhumid Pre-pyrenean mountains (NE Spain) was selected to use 137 Cs and 210 Pb ex to assess medium-and longer-term soil redistribution rates. A total of 23 sectioned soil cores spaced 50 m apart were collected along the slope transect, where 7 Be had been previously used to document soil redistribution resulting from an individual storm event. The inventories of both radionuclides varied markedly, between 409 and 6080 Bq m-2 for 137 Cs, and between 0 and 6734 Bq m-2 for 210 Pb ex. Estimates of soil redistribution, derived from the 137 Cs depth profiles, using appropriate conversion models, show that erosion rates along 2 the transect vary between 2.6 and 31.9 Mg ha-1 year-1 , and that sedimentation rates vary between 0.2 and 24.5 Mg ha-1 year-1. The highest soil losses occur in cultivated fields, within the midslope zone of the transect, while the highest deposition rates are found in tilled fields within the lower part of the transect. Erosion rates from 210 Pb ex varied widely between 0.1 and 83.7 Mg ha-1 year-1 on the lower slope, whereas sedimentation rates ranged between 0.8 and 110 Mg ha-1 year-1 also at the bottom slope. The spatial distribution of the radionuclides along the transect reflects the effects of different land use and slope gradient on water erosion. The results obtained confirm the potential for using 137 Cs and 210 Pb ex measurements for assessing soil redistribution on slopes in the Mediterranean environment over different temporal scales.
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