Dear Colleagues,

The hillslope is a fundamental spatial unit of headwater catchments. It is recognized that hillslopes are of key importance for a reliable description of both catchment runoff generation and biogeochemical processes. However, understanding hillslope processes has been limited by problems posed by the presence of heterogeneity. Quantifying the threshold hydrological responses of hillslopes to rainfall as controlled by soil heterogeneity, preferential flow, and the spatiotemporal connectivity of soils’ saturated patches requires distinct and multi-scale monitoring procedures which are rare in most environments. The inability to measure or map heterogeneity has restricted the development of robust physically based models, critically required for getting the right answer for the right reason in hydrologic predictions as well as for understanding the interplay between hydrology, biogeochemistry, and ecohydrological feedbacks. To advance the current models, heterogeneity should be replaced by the ecosystem function that it performs, instead of characterizing and specifying its exact details. This would allow to move from a micro-scale Newtonian response to a hillslope-scale, functionally based response. The recent literature indicates that the residence times of water and the landscape transit time distribution are the crucial macro-scale descriptors of how catchment functions, providing a fundamental basis for studying hydrological and biogeochemical transformations in the subsurface. Flux partitioning of water and dissolved substances occurring along various flow pathways (e.g., above and on the soil surface, above the soil–bedrock interface, evaporation, and transpiration) with different residence times plays a major role in the overall hillslope water and material balance. The connectivity of hillslope soil water with the riparian zone and the underlying geological structures remain also poorly understood. To address the above-mentioned challenges, novel monitoring and observation techniques, as well as new modeling approaches, are needed to foster a better quantification of hillslope hydrologically functioning descriptors, including water transit time, residence time, and flow pathways.

Dr. Jaromir Dusek
Dr. Ali Ameli
Dr. Matthias Sprenger
Prof. Shiqin Wang
Guest Editors

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• hillslope hydrology
• stormflow
• rainfall–runoff relationship
• modeling
• flux partitioning
• threshold behavior
• travel time of water
• bedrock
• preferential flow
• soil heterogeneity
• water residence time