Review of sediment connectivity: Conceptual connotations, characterization indicators, and their relationships with soil erosion and sediment yield

Soil erosion is an important driver of land and ecological degradation, with hydraulic erosion in particular leading to widespread impacts and damage. As an important concept and indicator for characterizing the potential and pathways of sediment production and transportation within watersheds or on slopes, sediment connectivity has gained global attention and thus been analysed since its proposal in 2003. Sediment connectivity has become an effective metric for analysing the sources, processes, and potentials of soil erosion and sediment yield (SY) in watersheds, and it has been considered a popular research topic in the field of soil erosion over the past decade. Considering the lack of up-to-date systematic reviews of conceptual connotations, characterization indicators for sediment connectivity, and quantitative relationships between these indicators and erosion and SY, a bibliometric analysis of “sediment connectivity” was conducted via the CiteSpace tool, which is based on the Web of Science (WOS), Scopus (Elsevier) and China National Knowledge Infrastructure (CNKI) databases. In this research, the current state, popular topics, and trends in relevant studies were identified, and the conceptual connotations, influencing factors, and indicator algorithms of sediment connectivity and their quantitative relationships with soil erosion and SY were summarized. Furthermore, the contents and directions to be strengthened and improved in the future were determined. The results indicated that over the past 21 years, sediment connectivity has been analysed in 123 countries or regions over the past 21 years. Researches have focused primarily on related concepts, indicators, scales, and influencing factors. This concept has been widely applied in various practices such as soil and water resource regulation, land use optimization, and soil erosion control. In previous studies, several linear ( SY = a · IC + b ) and exponential ( SY = a · e b · IC ) increasing relationships between sediment connectivity indicators (such as the index of connectivity (IC)) and SY at the slope or watershed scale have been established, facilitating the development of research on prediction and attribution analysis for the identification of sediment sources and changes. There is a consensus on “what sediment connectivity is” to date, but a unified and complete system has not been yet formed for sediment connectivity and several of its derived concepts. The Index of Connectivity (IC), as the primary means for quantitatively characterizing the status and distribution of sediment connectivity, has led to the creation of more than 20 different algorithms, whereas the included parameters mainly reflect the internal factors influencing topography and land use/cover. The effects of climatic factors and human activities have not been fully considered in previous research, which has led to relatively backwards researches on functional connectivity indicators. Hence, the classification systems and theoretical frameworks for a series of concepts must be further refined on the basis of sediment connectivity, such as the objective openness, scale dependence, comprehensive impact, and distribution heterogeneity. Moreover, the amount of research on the influences of external drivers and the coupled effects of different factors on indicators of sediment connectivity should be increased. Nevertheless, it is still necessary to explore certain aspects, such as the parameter combinations and normalization methods of the upslope and downslope components of the IC algorithm, and to continuously improve the explanation of the dynamic changes in sediment while considering both hydrological connections along flow paths and off-site impacts on underlying surface variations. Moreover, there is a need to increase the spatiotemporal scale of research on sediment connectivity, explore its feedback mechanisms and close quantitative relationships with soil erosion and SY, focus on the integrated application of different indicators (methods), and validate and results via multisource information to promote relevant applications. The obtained results provide valuable reference data for the refinement of theories and methods for sediment connectivity and enhance its support of studies of soil erosion and SY in watersheds.