## 2 Materials and Methods
#### 2.1.3 The FP Technique to Evaluate Soil and Water Resources' Degradation  
In addition to evaluating soil redistribution by FRN at the study site, the discrimination of sediment contributions from different sources is relevant for understanding sediment transport and distribution processes at mixing points [71; 72; 73]. The sediment FP technique has been successfully applied to quantify the contribution of mobilized sediment in catchments [45; 46; 74; 75]. To evaluate the source of suspended sediments using unmixing models, different tracers have been assessed for the FP technique: color, magnetism, geochemistry, stable isotopes such as CSSI [31; 46; 74; 76; 77], and the use of a variety of other tracers, such as biomarkers [75]. The central assumption underlying the FP technique is the direct comparison between tracer properties in sediment mixtures and the source's sediment [78]. Several unmixing models have been developed: ISOSOURCE, SIAR, CSSIAR, MIXSIAR, and FingerPro [78; 79; 80; 81; 82], using different approaches (frequentist, Bayesian, etc.). The objective of these models is to quantify the proportion of sediment from other sources in sediment mixtures. The technique has been applied to explore variations in source contribution and sediment origin during floods and extreme precipitation events, which are essential factors in landscape change [75; 83]. It has also been combined with remote sensing to evaluate the transport of suspended sediments and associated elements induced by rainfall and the agricultural cycle [83]. Several tracers, such as geochemical elements and other soil properties (radionuclides, soil sizes, etc. ), have been included in the FP application to improve the discrimination capacity [84; 82]. Recently, a new approach has been developed to incorporate CSSI (isotope ratio) data together with geochemical elements (scalar) as tracers in the FP technique [61; 85]. Associated with the relevance of the correct selection of tracers in FP techniques, a new consistency analysis method was applied in several landscapes to identify erroneous tracers [86; 87; 79; 88]. CSSIs included as tracers in the FP technique have demonstrated a high level of discrimination for assessing the proportions of sediments from different land use types as sources [45; 46; 89; 90]. Since this technique considers the fatty acids (FAs) produced for the plant as biomarkers, it can discern between plant types (C3, C4) and the relevance of vegetation for different land uses in the final contribution of the sediments. The resultsof [PERSON] in 2018 [45] identified the isotopic contribution of maize (C4 plants) soils containing less depleted \(\delta^{13}\)C values and other land uses in sediment cores, supporting the identification of past crop practices in the study, complemented by FRN techniques. Also, [PERSON] in 2018 [46] identified four sources at the study site and estimated the final proportions of the sediment mixture, confirmed by runs with four mixing models (IsoSource, SIAR, MIXSIAR, SIMMR) with similar results. A global study carried out in six catchments in South America, Africa, Europe, and Asia [76] shows the capacity of the CSSI technique to discriminate sediment contributions; for better results, the relevance was identified of the catchment size (>1 ha) and the selection of all relevant sources including non-vegetated areas associated with land uses and other hotspots. The CSSI discrimination capacity could be enhanced by integrating complementary tracing techniques (e.g., geochemical). The CSSI tracer, based on measuring \(\delta^{13}\)C signatures of organic biomarker compounds such as fatty acids (FAs), has been used since the late 2000s to strengthen our understanding of sediment production and its balance in various ecosystems [90]. CSSI emerged as a valuable technique to track the origin and fate of eroded soils in the landscape and to distinguish between land use types, including forested watersheds [29; 46; 74; 76; 90; 91].