Modelling a Perennial Firn Aquifer usingMODFLOW 6: A case study on the Lomonosovfonna ice cap

Abstract

Perennial Firn Aquifers (PFA's) are bodies of fresh water stored in pore spaces in firn (granular snow) for multiple consecutive years on glaciers and ice caps. PFA's can can delay glacier discharge into the ocean, affect sliding processes of glaciers and contain microbiological life. There are indications that PFA's are forming in places formerly thought to have unsuitable formation conditions, the abundance of PFA's is expected to increase in a future warming climate. The dynamic characteristics of PFA's, such as flow rates, water table depth variations and the reaction to a changing climate, are poorly understood. To date, a model of lateral flow of water in firn in general is not available. In this study, a 3D flow model of a PFA is created using the USGS modular hydrological model (MODFLOW 6) and FloPy. The area that is modelled is a grid of about 10 x 7 km on top of the Lomonosovfonna ice cap on Svalbard. The hydraulic conductivity is calibrated against field data of water table depth extracted from ground-penetrating radar data collected in 2017. It is found in this study that for a third order polynomial hydraulic conductivity as function of the density, the RMSE between model output and observations is lowest. The model was then run from 1957 to 2019, and for two RCP scenarios (RCP 4.5 and RCP 8.5) from 2019 - 2060. Our results suggest that the aquifer has been present since at least 1957 and increased in volume up to present day. The model predicts a rise in the water table in both future scenarios, more pronounced in the RCP 8.5 scenario. The modelled water table reaches the surface around 2044 (RCP 8.5) and 2048 (RCP 4.5), more cells will eventually have a water table at the surface in the RCP 8.5 scenario compared to the RCP 4.5 scenario. Future research can use this model as a starting point to model more elaborate firn-PFA interactions, such as freezing of the water table. Also, more research can be done to develop an above-surface routine for the water, such as runoff or lake formation. The density-dependent hydraulic conductivity can be used on other glaciers or ice caps to perform similar model experiments.