Quantification of the spatio-temporal variations in hydrologic connectivity of small-scale topographic surfaces under various rainfall conditions

Land surfaces are generally not smooth and show certain irregularity. Overland flow on such surfaces is essentially discontinuous and exhibits strong variability and complexity. Modeling of the spatio-temporal variability and heterogeneity of overland flow and the hydrotopographic effects has been proven to be a challenge. The objective of this study is to quantitatively describe the intrinsic spatio-temporal variations in hydrologic connectivity associated with overland flow generation. Firstly, a puddle-to-puddle (P2P) hydrologic connectivity concept was proposed to characterize runoff generation processes and the related spatio-temporal dynamics. Secondly, a laboratory overland flow experiment was conducted to characterize the dynamic puddle filling and spilling processes, hydrologic connectivity, and outlet discharge. Thirdly, a conceptual P2P model was applied to simulate the P2P overland flow dynamics, calculate flow discharge, and track the evolution of hydrologically connected areas. Particularly, two modified hydrologic connectivity indices, time-varying connectivity function and connectivity length, were proposed to characterize the properties and dynamic changes in hydrologic connectivity. Furthermore, the influences of surface topography, rainfall, and surface slope on hydrologic connectivity were evaluated. The proposed hydrologic connectivity indices effectively revealed the variability and the threshold behavior of overland flow generation. It was demonstrated that the dynamic P2P processes governed hydrologic connectivity, controlled surface runoff generation, and altered the flow drainage patterns. Temporal variations of rainfall intensity changed the occurrence timing of the P2P dynamics and evolution of connected areas, while spatial variations of rainfall intensity directly influenced the overall development of hydrologic connectivity of a hydrologic system. Surface slope showed considerable influences on hydrologic connectivity. The results suggested that critical slope(s) could exist, at which a sharp change in flow drainage area and hydrologic connectivity occurred.