Variations in transport of suspended sediment and associated elements induced by rainfall and agricultural cycle in a Mediterranean agroforestry catchment

Abstract

Soil erosion and fine particle exports are two of the major concerns of soil nutrient loss and water quality decrease nowadays. In Mediterranean mountainous environments, agricultural practices during different cropland stages likely increase sediment supplies and the export of fertilisers and pesticides out into the drainage system. In this study, we attempt to evaluate the soil response to different agricultural practices implemented during the agricultural cycle by monitoring the bare soil cropland area through the use of remote sensing and applying the sediment fingerprinting technique together with the newly consensus-based tracer selection method. To this purpose, 128 source samples were distributed over the four main land use/land covers and geomorphic elements existing in the study area. To analyse the spatio-temporal variability of source contributions, three sampling stations were established along the catchment and collected during two hydrological years. The consensus method was used to show the individual messages of each tracer, revealing non-conservative and dissenting tracers, followed by a discriminant function analysis (DFA) to select the best set of tracers for each mixture. Overall, the unmixing model outputs displayed channel bank and agriculture as the main contributing sources for all the seasonal campaigns. Nevertheless, the agricultural contribution was higher during the periods when the soil surface in croplands had no plant cover protection. Certain elements such as As, Co, Li, Mn, Zn and ²³⁸U were above source ranges in the sediment mixtures. The enriched elements showed higher content in the sediment mixtures during sowing and after harvest periods. Besides, an enrichment of phosphorous during both agricultural practices periods points out to agricultural activities as the main cause of sediment and elements export to streams. Thus, in the subcatchment with less bare soil cropland area, the agriculture source contributed with the lowest percentages. Our results support the protection of croplands, especially in periods of vegetation cover absence to prevent the loss of fertile soil and the export of potential pollutants to downstream water bodies.

Keywords: Agricultural practices; Consensus-based method; Element export; FingerPro unmixing model; Temporal scale fingerprinting.