Marginal snowpacks are a critical part of the hydrology and ecology of temperate mountain regions. However, their sensitivity to temperature increase is uncertain due to their phenology which sits between seasonal and ephemeral snowpacks. In this paper, we investigate the sensitivity and the potential response of marginal snowpacks to a warmer climate by leveraging simulated snow series under various combinations of mean winter temperature (T) and precipitation (P), and varied meteorological conditions over 21 years, at 24 mountain sites globally. Marginal snowpacks are found to be more sensitive to temperature increases compared to both seasonal, and ephemeral snowpacks. According to all considered T and P combinations, 64% of marginal snowpacks would transition to ephemeral or disappear, compared to only 21% and 24% for seasonal and ephemeral snowpacks, respectively. The study reveals new insight on the impacts of a warming climate on marginal snowpacks, including: the advance of melt-out dates (-21 days) and a reduction in snowmelt contribution to runoff (-11%). Further, we estimate the global distribution of marginal snowpack based on T and P from ERA5-Land reanalysis. The total area covered by marginal snowpack would decrease by 2.7% under +1º C, with a more significant decreases under +2 ºC (5.7%) and +3 ºC (10.5%) warming. This study places marginal snowpacks at the forefront of where climate change will pose the greatest threats to mountain and montane water resources and environmental systems globally, with profound shifts even under optimistic future climate scenarios (+1ºC).

The interception of rainfall by plant canopies alters the depth and spatial distribution of water arriving at the soil surface, and thus the location, volume, and depth of infiltration. Mechanisms like stemflow are well known to concentrate rainfall and route it deep into the soil, yet other mechanisms of flow concentration are poorly understood. This study characterises pour points, formed by the detachment of water flowing on the lower surface of a branch, using a combination of field observations in Western Australian banksia woodlands and rainfall simulation experiments on Banksia menziesii branches. We aim to establish the hydrological significance of pour points in a water-limited woodland ecosystem, along with the features of the canopy structure and rainfall that influence pour point formation and fluxes. Pour points were common in the woodland and could be identified by visually inspecting trees. Water fluxes at pour points were upto 15 times rainfall and were usually comparable to or greater than stemflow. Soil water content beneath pour points was greater than in adjacent control profiles, with 20-30% of seasonal rainfall volume infiltrated into the top 1m of soil beneath pour points, compared to 5% in controls. Rainfall simulations showed that pour points amplified the spatial heterogeneity of throughfall, violating water balance closure assumptions. The simulation experiments demonstrated that pour point fluxes depend on the interaction of branch angle and foliation for a given branch architecture. Pour points can play a significant part in the water balance, depending on their density and rainfall concentration ability.

The Australian seasonal snowpack can be classified as a marginal maritime snowpack with a temperature near 0 °C throughout the snow season. Subtle changes in atmosphere – snow energetics therefore result in rapid change in snowpack properties, which occur against a background of a warming climate. This has been attributed to a 40% decline in spring snow depths in the past 40 yrs. and geologic records suggest the seasonal snowpack is now near a 2000 yr. minimum. Modelled future snow cover predicts further decline by 57 % to 78 % of current maximum snow depth by the 2040s. Such research primarily attributes this decline in snow cover to global warming. However, the past decline in Australian snow cover can also be attributed to change in synoptic wintertime precipitation patterns that have resulted in a dramatic increase in proportional winter and spring precipitation of tropical origin since the 1950s. Tropical moisture is predominantly transported into southeast Australia during negative phases of the Indian Ocean Dipole (IOD) by northwest cloud bands – visible expressions of atmospheric rivers coupling tropical moisture sources northwest of Australia to the Australian Alps. Here we present a case study of one such event that occurred from the 21 to 23 July 2016 when 118 mm of rain-on-snow over a 12 hr period led to near complete ablation of the snowpack. While predictions of future variability of the IOD due to global warming remain uncertain, we suggest that warming atmospheric temperatures increase the risk of such extreme rain–on-snow events during negative IOD events. Combined with reduced snow cover in response to warmer ambient wintertime temperatures, such rain-on-snow events may further accelerate the reduction in seasonal snow cover in the Australian Alps, possibly on occasions after which the snowpack does not recover before spring. These conditions would present significant challenges to the Australian snow sports industry which is worth $2 billion annually and lead to change in snow dependent ecosystems and alpine hydrology.