Understanding of critical zone structures and hydrological connectivity: a review

Upstream areas are the major flood contributing areas in humid regions. Upstream hillslopes,valleys and ephemeral channels provide quick pathways for flows during flood events. These pathways generate flows seasonally and only after a rainfall event,i.e.,the hydrological connectivity is ephemeral. Thus the dynamics of drainage networks such as expansion/contraction,and connection/disconnection,may offer important clues to understanding the patterns and processes of runoff generation. However,the mechanism of what controls the hydrological connectivity and how it connects the hillslope,valley and channels is yet to be understood. Extensive field studies in diverse catchments around the world continue to characterize and catalogue the enormous heterogeneity of hillslope structures and complexity of rainfall runoff processes in multiple watersheds,and at different scales. But,these field findings seem to be meaningless for the modeler,as they usually fail to incorporated the experimentalist's knowledge into their models. There is plenty of knowledge gap in the fundamentals with regard to how catchment are composed,organized and connected through hillslopes,valleys and channels,and how catchment storage affects rainfall-runoff responses. In this study,through comparisons of hillslope experiments,we find that process based connectivity is deeply affected by hillslope soil depth,bedrock terrain and drainage network structures. This review also showed that present research works are focusing on micro-scale mechanism (e.g.,soil pores and flow),and there is a gap between the hydrological connectivity experiments and the modelers. We suggest that experimentalists should find a macroscale pathway hidden in the critical zone that is the pivot of the runoff generation and shapes the flow hydrodynamics in the entire catchment. That is to say hydrological connectivity of hillslopes,valleys and ephemeral channels should focus on the cumulative effects of hillslope processes instead of individual soil pore processes. Finally,there should be a balance between conceptualizing of complex hillslope structures and moderate depiction of runoff generation.