Abstract. Perennial snow, or firn, covers 80 % of the Greenland ice sheet and has the capacity to retain surface meltwater, influencing the ice sheet mass balance and contribution to sea-level rise. Multilayer firn models are traditionally used to simulate firn processes and estimate meltwater retention. We present, intercompare and evaluate outputs from nine firn models at four sites that represent the ice sheet's dry snow, percolation, ice slab and firn aquifer areas. The models are forced by mass and energy fluxes derived from automatic weather stations and compared to firn density, temperature and meltwater percolation depth observations. Models agree relatively well at the dry-snow site while elsewhere their meltwater infiltration schemes lead to marked differences in simulated firn characteristics. Models accounting for deep meltwater percolation overestimate percolation depth and firn temperature at the percolation and ice slab sites but accurately simulate recharge of the firn aquifer. Models using Darcy's law and bucket schemes compare favorably to observed firn temperature and meltwater percolation depth at the percolation site, but only the Darcy models accurately simulate firn temperature and percolation at the ice slab site. Despite good performance at certain locations, no single model currently simulates meltwater infiltration adequately at all sites. The model spread in estimated meltwater retention and runoff increases with increasing meltwater input. The highest runoff was calculated at the KAN_U site in 2012, when average total runoff across models (±2σ) was 353±610 mm w.e. (water equivalent), about 27±48 % of the surface meltwater input. We identify potential causes for the model spread and the mismatch with observations and provide recommendations for future model development and firn investigation.

In spring of 2011, a perennial storage of water was observed in the firn of the southeastern Greenland Ice Sheet (GrIS), a region of both high snow accumulation and high melt. This aquifer is created through percolation of surface meltwater downward through the firn, saturating the pore space above the ice-firn transition. The aquifer may play a significant role in sea level rise through storage or draining freshwater into the ocean. We carried out a series of active source seismic experiments using continuously refracted P-waves and inverted the first P-arrivals using a transdimensional Bayesian approach where the depth, velocity, and number of layers are allowed to vary to identify the seismic velocities associated with the base of the aquifer. When our seismic approach is combined with a radar sounding of the water table situated at the top of the firn aquifer, we are able to quantify the volume of water present. In our study region, the base of the aquifer lies on average 27.7 ± 2.9 m beneath the surface, with an average thickness of 11.5 ± 5.5 m. Using a Wyllie average for porosity, we found the aquifer has an average water content of 16 ± 8%, with considerable variation in water storage capacity along the studied east-west flow line, 40 km upstream of the Helheim glacier terminus. Between 2015 and 2016, we observed a 1-2 km uphill expansion of the aquifer system, with a site dry in summer 2015 exhibiting a water content of 530 kg m −2 in summer 2016. We estimate the volume of water stored in the aquifer across the entire region upstream of Helheim glacier to be 4.7 ± 3.1 Gt, ∼3% of the total water stored in firn aquifers across the GrIS. Elucidating the volume of water stored within these recently discovered aquifers is vital for determining the hydrological structure and stability of the southeastern GrIS.

Within the lower percolation zone of the southeastern Greenland ice sheet, meltwater has accumulated within the firn pore space, forming extensive firn aquifers. Previously, it was unclear if these aquifers stored or facilitated meltwater runoff. Following mixing of a saline solution into boreholes within the aquifer, we observe that specific conductance measurements decreased over time as flowing freshwater diluted the saline mixture in the borehole. These tests indicate that water flows through the aquifer with an average specific discharge of 4.3 × 10−6 m/s (σ = 2.5 × 10−6 m/s). The specific discharge decreases dramatically to 0 m/s, defining the bottom of the aquifer between 30 to 50 m depth. The observed flow indicates that the firn pore space is a short‐term (<30 years) storage mechanism in this region. Meltwater flows out of the aquifer, likely into nearby crevasses, and possibly down to the base of the ice sheet and into the ocean.

Firn aquifers have been discovered across regions of the Greenland ice sheet with high snow accumulation and melt rates, but the processes and rates that sustain these aquifers have not been fully quantified or supported by field data. A quantitative description of the hydrology of a firn aquifer upslope from Helheim Glacier that integrates field measurements is presented to constrain melt and recharge rates and timing, temporal variations in temperature and water levels, and liquid‐water residence time. Field measurements include weather data, firn temperatures, water levels, geochemical tracers, and airborne radar data. Field measurements show that once the firn column is temperate (0°C), meltwater from the surface infiltrates to the water table in less than 2 days and raises the water table. Average recharge is 22 cm/year (lower 95% confidence interval is 13 cm/year and upper 95% confidence interval is 33 cm/year). Meltwater within the recently formed aquifer, which flows laterally downslope and likely discharges into crevasses, has a mean residence time of ~6.5 years. Airborne radar data suggest that the aquifer in the study area continues to expand inland, presumably from Arctic warming. These comprehensive field measurements and integrated description of aquifer hydrology provide a comprehensive, quantitative framework for modeling fluid flow through firn, and understanding existing and yet undiscovered firn aquifers, and may help researchers evaluate the role of firn aquifers in climate change impacts.

Some regions of the Greenland ice sheet, where snow accumulation and melt rates are high, currently retain substantial volumes of liquid water within the firn pore space throughout the year. These firn aquifers, found between ∼10 and 30 m below the snow surface, may significantly affect sea level rise by storing or draining surface meltwater. The hydraulic gradient and the hydraulic conductivity control flow of meltwater through the firn. Here we describe the hydraulic conductivity of the firn aquifer estimated from slug tests and aquifer tests at six sites located upstream of Helheim Glacier in southeastern Greenland. We conducted slug tests using a novel instrument, a piezometer with a heated tip that melts itself into the ice sheet. Hydraulic conductivity ranges between 2.5 × 10 −5 and 1.1 × 10 −3 m/s. The geometric mean of hydraulic conductivity of the aquifer is 2.7 × 10 −4 m/s with a geometric standard deviation of 1.4 from both depth specific slug tests (analyzed using the Hvorslev method) and aquifer tests during the recovery period. Hydraulic conductivity is relatively consistent between boreholes and only decreases slightly with depth. The hydraulic conductivity of the firn aquifer is crucial for determining flow rates and patterns within the aquifer, which inform hydrologic models of the aquifer, its relation to the broader glacial hydrologic system, and its effect on sea level rise.