Contact information

Jonathan Mackay email: joncka@bgs.ac.uk tel: +44 (0)115 9363236

• In the foreland areas of glaciated catchments unconsolidated,

unconfined, high-storage alluvial aquifers are commonly found.

• These aquifers are recharged by lateral flows from the mountain front,

diffuse rainfall and snow melt and focussed infiltration from meltwater

channels draining neighbouring mountains.

• Groundwater provides social, economic and ecological benefits in

proglacial regions by providing a reliable water source for abstraction and by

supplementing low flows in glacier-fed rivers outside of the melt season.

• Proglacial aquifer storage and groundwater contribution to glacier-fed rivers

will change in response to climate warming and glacier retreat.

• No studies have investigated climate change impacts on proglacial

groundwater dynamics and groundwater-surface water (GW-SW)

exchanges. This study seeks to do this.

Modelling impacts of glacier retreat on proglacial groundwater dynamicsMackay, J D, Barrand, N E, Hannah, D M, Krause, S, Jackson, C R, Everest, J, Aðalgeirsdóttir, G, MacDonald, A M, Ó Dochartaigh, B É.

1. Build MODFLOW-NWT model of unconfined sandur aquifer at Virkisjökull

Glacier Observatory, south-east Iceland (Fig. 1).

2. Determine twenty-first century changes (2080s) in:

• seasonal groundwater level dynamics.

• GW-SW exchanges (river leakage and surface drainage).

3. Determine the principal drivers of change: climate-driven diffuse recharge

or meltwater-driven river runoff.

• Regular Cartesian layered model grid: 50 m horizontal, 10 m layers (~1 km

wide, 4 km long and up to 100 m thick)

• SFR1 river routing package: Manning’s equation for uniform open channel

flow and Darcy’s law for river leakage.

• Melt and rainfall-runoff inflow at upstream end of river and diffuse recharge

simulated using glacio-hydrological model (Mackay et al. 2018) (Fig. 2).

• Monte Carlo calibration against observation borehole time-series (green

circles in Fig. 1) used to specify homogeneous parameterisation.

• Cluster analysis used to select five scenarios from ensemble of projections

for the 2080s (Mackay et al. 2019).

• Scenarios chosen to capture range of possible future glacio-hydrological

and climatic behaviours (Table 1).

• Scenarios incorporate uncertainty in future climate and glacio-hydrological

model structure and parameterisation.

• Winter (DJF) and spring (MAM) levels projected to fall (Fig. 3c).

• Groundwater contributes 3-8% less to winter (DJF) runoff (Fig. 4) - buffering of low

flow periods outside of the melt season will lessen in the future.

• Wettest scenarios project summer (JJA) and autumn (SON) levels to rise.

• Shifts in timing of min/max levels for G1-Q1-P1 and G3-Q1-P1 (Fig. 3b).

• Diffuse recharge drives changes in groundwater level dynamics and surface drainage

(Fig. 3d).

• Winter (DJF) river leakage increases. Mixed response in summer (JJA) (Fig. 5a)

• Projections closely follow leaking river length (Fig. 5b) (controlled by groundwater level)

• For G2-Q2-P3, fall in summer river leakage, drop in specific river leakage (Fig. 5c) and

reduced diurnal flow amplitude (Fig. 5d).

• Melt-driven diurnal river discharge cycle induces a downward head gradient at the

river-aquifer interface, driving river leakage to aquifer (Fig. 6).

• Combining a glacio-hydrological model and MODFLOW-NWT allowed us to evaluate impact

of climate change and glacier retreat on proglacial groundwater levels for the first time.

• Groundwater level dynamics are controlled by changes in diffuse recharge over the sandur.

• They are projected to fall due to projected drying of climate over sandur.

• River leakage and surface drainage are controlled by evolution of groundwater levels.

• But river leakage is partly controlled by melt in neighbouring mountains.

• Loss of diurnal melt signal in meltwater streams due to glacier and snowline retreat could reduce

focussed river recharge to aquifer.

• Future research should:

• Expand the presented modelling approach to other glaciated catchments.

• Utilise larger ensemble of climate and river discharge projections to gain greater

understanding of projection uncertainties.

Figure 1: The Virkisjökull Glacier Observatory includes the glaciated Virkisá River

basin (purple border) and proglacial sandur aquifer (black dash border). Inset

shows location of the observatory in Iceland. Photo shows sandur looking north-

east towards glacier.

Melt and rainfall runoff

River recharge

Diffuse recharge

Groundwater flow

Groundwater drainage

Bedrock

Sandur

Glacio-hydrological model

River flow

MODFLOW-NWT

Figure 2: Conceptual model of mountain to alluvial hydrological system at the Virkisjökull Glacier

Observatory (developed from Ó Dochartaigh et al. 2019). Black-dashed lassos show which model

is used to simulate principal processes.

Mackay, J D, et al: Glacio-hydrological melt and run-off

modelling: application of a limits of acceptability framework for

model comparison and selection, THE CRYOSPHERE, 12,

2175-2210, 2018.

Mackay, J D, et al: Future evolution and uncertainty of river flow

regime change in a deglaciating river basin, HYDROLOGY AND

EARTH SYSTEM SCIENCE, 23, 1833-1865, 2019.

Ó Dochartaigh, B É, et al: Groundwater / meltwater interaction in

proglacial aquifers, HYDROLOGY AND EARTH SYSTEM

SCIENCE DISCUSSIONS, in review, 2019.

Scenario Scenario code ΔT (oC) ΔG (km2) ΔQ (m3 s-1) ΔP (mm d-1)

1 G1-Q1-P1 3.1 -8.1 -1.1 -0.6

2 G2-Q2-P3 3.1 -6.4 -0.7 0.2

3 G2-Q2-P1 1.9 -5.6 -0.9 -0.7

4 G3-Q1-P1 1.0 -4.8 -1.2 -0.9

5 G3-Q3-P2 0.7 -3.7 -0.3 -0.2

Table 1: Scenarios listed in descending order according to their projected glacier retreat for the

2080s. Each includes a code with the format G*-Q*-P* where G, Q and P represent glacier

coverage, mean annual river discharge and mean annual sandur precipitation respectively and *

is either 1, 2 or 3 representing low, moderate or high respectively. Also shown are the changes in

near surface air temperature (ΔT), glacier coverage (ΔG), upstream river discharge (ΔQ) and total

precipitation over the sandur (ΔP).

Figure 3: 30-day moving average plots of simulated groundwater level for each

scenario over historic reference period (a) and future period (b). Also shown are the

projected changes in groundwater level (c) and diffuse recharge (d) between the two

periods.

Figure 4: 30-day

moving average of

projected change in

proportional

contribution of

groundwater to

surface runoff in

sandur.

Figure 6: Simulated river

stage (a), stage minus

head (b) and river

leakage (c) time-series

for a single river node 1

km downstream for three

days during the

reference period (July

2013).

Figure 5: 30-day moving average plots of projected changes in river leakage (a),

length of leaking river (b), specific river leakage (c) and diurnal flow amplitude (d).

1. Groundwater in glaciated catchments 3. MODFLOW-NWT configuration and calibration 5. Twenty-first century groundwater level dynamics and surface drainage

2. Study aims

4. Twenty-first century diffuse recharge and river flow

scenarios

6. Twenty-first century river leakage

7. Conclusions and future work

8. References