The adjacent agricultural watershed is a vital component of the Three Gorges Reservoir Region (TGRR); however, it is affected by serious soil erosion. Assessing soil erosion dynamics in such watersheds is useful for identifying its causes and tendencies to develop, in turn providing scientific information for soil and water conservation at the regional scale. In the present study, the spatial and temporal patterns of soil erosion of a small agricultural watershed in central TGRR were investigated from 2002 to 2014 using the Revised Universal Soil Loss Equation (RUSLE) model, combined with Geographic Information Systems (GIS). The trends and processes of the overall soil erosion intensity were analyzed using spatial overlay analysis and the Markov transition matrix model, respectively. The spatial distribution of soil erosion rates within this watershed was relatively consistent during the study period. Erosion intensity was moderate, with a mean soil loss of 35.1 t·ha −1 ·year −1 . Precipitation was a dominant factor influencing the intensity of soil erosion. Moreover, most erosion intensities shifted closely to middle grades from 2002 to 2008, and declined from 2008 to 2014, indicating that soil erosion in the Wangjiagou watershed has recently decreased. These results suggest that recently implemented integrated soil management practices were responsible for the recently observed erosion patterns.
Although river mixing occurs widely in nature, the corresponding evolution of dissolved organic matter (DOM) composition remains poorly understood. Here, surface water samples were collected at multiple transects in the lower Athabasca River (LAR) under base-flow conditions. Asymmetric flow field-flow fractionation (AF4) coupled to online excitation-emission measurements (EEMs) and parallel factor analysis (PARAFAC) were utilized to investigate the size distribution of fluorescent DOM components during river mixing and the corresponding variation in size-resolved fluorescence. The majority of fluorescent components occurred at 0.810 and 1.170 kDa, reflecting the small size of the DOM molecules with maximum fluorescence. The loadings of fluorescence normalized to absorbance at 254 nm (A254) were highest for most terrestrial humic-like components, followed by the microbial humic-like component, and the protein-like components. Differences in size-resolved fluorescence were observed between DOM in humic-rich tributaries and in the mainstem of the LAR upstream of tributary inputs. The trend of variations in the A254-normalized PARAFAC loadings of terrestrial humic-like components also illustrates conservative mixing of aromatic-rich terrestrial DOM across size fractions in the LAR. From a molecular point of view, the mixing of fluorescent DOM occurred linearly and simultaneously across sizes without any evidence of aggregation, sedimentation, or changes in the fluorescence or concentration of any size fraction over the >60 km required for complete mixing of the river and its tributaries. Overall, this study provides insights into the size characteristics of fluorescent components of DOM and their conservative mixing behavior in large boreal rivers.
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