Abstract

Increased attention directed at permafrost hydrology has been prompted by climate change. In spite of an increasing number of field measurements and modeling studies, the impacts of permafrost on hydrological processes at the catchment scale are still unclear. Permafrost hydrology models at the catchment scale were mostly developed based on a “bottom-up” approach, hence by aggregating prior knowledge at the spot/field scales. In this study, we followed a “top-down” approach to learn from field measurement data to understand permafrost hydrology at the catchment scale. In particular, we used a stepwise model development approach to examine the impact of permafrost on streamflow response in the Hulu catchment in western China. We started from a simple lumped model (FLEX-L), and step-wisely included additional complexity by accounting for topography (i.e. FLEX-D) and landscape heterogeneity (i.e. FLEX-Topo). The final FLEX-Topo model, was then analyzed using a dynamic identifiability analysis (DYNIA) to investigate parameters’ temporal variation. By enabling temporal dynamics on several parameters, we diagnosed the physical relationships between parameter variation and permafrost impacts. We found that in the Hulu catchment: 1) the improvement associated to the model modifications suggest that topography and landscape heterogeneity are dominant controls on catchment response; 2) baseflow recession in permafrost regions is the result of a linear reservoir, and slower than non-permafrost regions; 3) parameters variation infers seasonally non-stationary precipitation-runoff relationships in permafrost catchment; 4) permafrost impacts on streamflow response mostly at the beginning of the melting season; 5) allowing the temporal variations of frozen soil related parameters, i.e. the unsaturated storage capacity and the splitter of fast and slow streamflow, improved model performance. Our findings provide new insights on the impact of permafrost on catchment hydrology in vast mountain regions of western China. More generally, they help to understand the effect of climate change on permafrost hydrology.

Highlights

  • IntroductionPermafrost covers 24% of the exposed land surface of the Northern Hemisphere (Zhang et al, 2005; Woo, 2012; Walwoord and Kurylyk, 2016)

  • Permafrost is the ground that is at or below 0°C for at least two consecutive years.Permafrost covers 24% of the exposed land surface of the Northern Hemisphere (Zhang et al, 2005; Woo, 2012; Walwoord and Kurylyk, 2016)

  • Suggest that topography and landscape heterogeneity are dominant controls on catchment response; 2) baseflow recession in permafrost regions is the result of a linear reservoir, and slower than non-permafrost regions; 3) parameters variation infers seasonally nonstationary precipitation-runoff relationships in permafrost catchment; 4) permafrost impacts on streamflow response mostly at the beginning of the melting season; 5) allowing the temporal variations of frozen soil related parameters, i.e. the unsaturated storage capacity and the splitter of fast and slow streamflow, improved model performance

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Summary

Introduction

Permafrost covers 24% of the exposed land surface of the Northern Hemisphere (Zhang et al, 2005; Woo, 2012; Walwoord and Kurylyk, 2016). The high Asia region is largely covered by permafrost and is characterized by a fragile cold and arid ecosystem (Immerzeel et al., 2010; Ding et al, 2020). As this region serves as the “water tower” for nearly 1.4 billion people, understanding the permafrost hydrology is important for regional and downstream water resources management and ecosystem conservation. Permafrost substantially controls surface runoff and its hydraulic connection with groundwater. The freeze–thaw cycle in the active layer significantly impacts soil water movement direction, velocity, storage capacity, and hydraulic conductivity (Bui et al, 2020; Gao et al, 2021)

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