Abstract
Abstract. Groundwater plays a significant role in glacial hydrology and can buffer changes to the timing and magnitude of flows in meltwater rivers. However, proglacial aquifer characteristics or groundwater dynamics in glacial catchments are rarely studied directly. We provide direct evidence of proglacial groundwater storage, and quantify multi-year groundwater–meltwater dynamics, through detailed aquifer characterisation and intensive high-resolution monitoring of the proglacial system of a rapidly retreating glacier, Virkisjökull, in south-eastern Iceland. Proglacial unconsolidated glaciofluvial sediments comprise a highly permeable aquifer (25–40 m d−1) in which groundwater flow in the shallowest 20–40 m of the aquifer is equivalent to 4.5 % (2.6 %–5.8 %) of mean river flow, and 9.7 % (5.8 %–12.3 %) of winter flow. Estimated annual groundwater flow through the entire aquifer thickness is 10 % (4 %–22 %) the magnitude of annual river flow. Groundwater in the aquifer is actively recharged by glacier meltwater and local precipitation, both rainfall and snowmelt, and strongly influenced by individual precipitation events. Local precipitation represents the highest proportion of recharge across the aquifer. However, significant glacial meltwater influence on groundwater within the aquifer occurs in a 50–500 m river zone within which there are complex groundwater–river exchanges. Stable isotopes, groundwater dynamics and temperature data demonstrate active recharge from river losses, especially in the summer melt season, with more than 25 % and often >50 % of groundwater in the near-river aquifer zone sourced from glacier meltwater. Proglacial aquifers such as these are common globally, and future changes in glacier coverage and precipitation are likely to increase the significance of groundwater storage within them. The scale of proglacial groundwater flow and storage has important implications for measuring meltwater flux, for predicting future river flows, and for providing strategic water supplies in de-glaciating catchments.
Highlights
A major challenge in modern hydrology is predicting changes in freshwater flows and storage resulting from glacier retreat in response to climate change (Jiménez Cisneros et al, 2014)
Most glaciers worldwide have been in retreat since the mid-19th century, with the loss of global glacier ice accelerating during the 21st century (Zemp et al, 2015). This change has the potential to affect over 1 billion people who live in catchments where glacier melt contributes to river flow (Kundzewicz et al, 2008)
Three methods were used to establish the physical aquifer properties of the sandur: (1) infiltration tests to 0.15 m depth at 20 locations, using a Guelph permeameter, and saturated hydraulic conductivity calculated by the Laplace method (Reynolds et al, 1983) (Table S2); (2) particle size analysis on 42 sandur sediment samples to 0.5 m depth, at 22 locations, and hydraulic conductivity estimated using a modified Hazen formula suitable for heterogeneous glacial deposits (MacDonald et al, 2012; Williams et al, 2019) (Table S3); and (3) constant rate pumping tests of between 3.5 and 6 h in each sandur piezometer, at rates of 0.5–1.8 L s−1, and transmissivity estimated by the Jacob time-drawdown and Theis recovery methods corrected for unconfined conditions (Kruseman and de Ridder, 1994)
Summary
A major challenge in modern hydrology is predicting changes in freshwater flows and storage resulting from glacier retreat in response to climate change (Jiménez Cisneros et al, 2014). Most glaciers worldwide have been in retreat since the mid-19th century, with the loss of global glacier ice accelerating during the 21st century (Zemp et al, 2015) This change has the potential to affect over 1 billion people who live in catchments where glacier melt contributes to river flow (Kundzewicz et al, 2008). As glacier ice loss continues, meltwater flows will decrease (Jiménez Cisneros et al, 2014) This lessening of the role of glaciers in regulating flows will change the nature of glacier-fed rivers and the importance of other water sources in glacier catchments: rainfall, snowmelt and groundwater. Predicted impacts include changes to the frequency and magnitude of flooding (Jiménez Cisneros et al, 2014); hydroelectric power production (Laghari, 2013); drinking water and irrigation (Kundzewicz et al, 2008); Published by Copernicus Publications on behalf of the European Geosciences Union
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