Identification of Hydrologically Sensitive Areas Considering Watershed Process Dynamics

Hydrologically sensitive areas (HSAs) largely control watershed response to rainfall, along with water and pollutant transport processes. Thus, their identification is critical in watershed management planning. Although watershed processes have been studied enough to provide a good understanding of HSA dynamics, only a few concepts and methods are available for HSA delineation, and they rely on spatial indices that do not consider temporal variation in hydrologic processes. This study introduces alternative concepts and methods to delineate HSAs. Three unique maps showing watershed dynamics were created using the outputs of a long-term hydrologic simulation implemented with a grid-based distributed model. The spatial distributions of HSAs identified using the newly proposed methods were compared with those of topographic indices. Results demonstrated that the new methods highlight transport processes, such as routing (or connection) and travel time, and the roles of soil and land covers, which have not been the focus of other concepts and approaches for HSA identification. In contrast to topographic index-based approaches, the proposed methods provided HSA boundaries with clear physical meanings to improve the interpretability and applicability of HSA maps. The methods are expected to enhance our ability to tackle water issues for improved water resource management by providing unique concepts and alternative ways to explicitly delineate HSAs. Keywords: Grid-based distributed model, Hydrologic connectivity, Hydrologically sensitive area, HYSTAR, Time-area method, Topographic wetness index.