Understanding storage-discharge dynamics in permafrost-underlain catchments using a storage partitioning approach

Seasonal thawing and freezing cycles of soils fundamentally control storage dynamics in permafrost-underlain catchments. In spring, most of the snow-stored water melts and large volumes of water reach the stream without infiltrating into the frozen subsurface. During the soil thawing period (summer and autumn), storage capacity in the subsurface active layer increases and previously frozen water becomes available and may contribute to discharge of the receiving stream. Although research in temperate catchments indicates that not all stored water contributes to discharge dynamics, the proportion of storage controlling discharge (hydraulically connected storage) in permafrost regions and how it changes during freezing-thawing cycles remains unclear. Here, we tested whether thawing of subsurface ice over summer and autumn increases the hydraulically connected storage that controls discharge dynamics. To test this hypothesis, we applied a storage partitioning approach for a headwater catchment underlain by continuous permafrost located in Tombstone Territorial Park in Yukon, Canada. We applied the water balance to calculate the total storage and a recession curve analysis to derive the hydraulically connected storage. From the difference between these two storage compartments, we calculated the hydraulically disconnected storage, consisting of both saturated and unsaturated storage. Our preliminary results show that hydraulically connected storage remains stable during subsurface thawing, while disconnected storage increases. This finding suggests that a large proportion of the total storage becomes unsaturated during summer and autumn, reducing the relative proportion of hydraulically connected storage. The insights from the storage partitioning approach presented here deepen our understanding of how permafrost-underlain catchments store or release water throughout the open water season. Such knowledge is especially important given climate change impacts on freezing-thawing dynamics in permafrost regions.