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WATER RESOURCES RESEARCH, VOL. 49, 2228–2240, doi:10.1002/wrcr.20213, 2013
Global assessment of vulnerability to sea-level rise in
topography-limited and recharge-limited coastal
groundwater systems
Holly A. Michael,1,2 Christopher J. Russoniello,1 and Lindsay A. Byron1
Received 15 June 2012; revised 16 March 2013; accepted 22 March 2013; published 26 April 2013.
[1] Impacts of rising sea level on the hydraulic balance between aquifers and the ocean
threaten fresh water resources and aquatic ecosystems along many world coastlines.
Understanding the vulnerability of groundwater systems to these changes and the primary
factors that determine the magnitude of system response is critical to developing effective
management and adaptation plans in coastal zones. We assessed the vulnerability of two
types of groundwater systems, recharge-limited and topography-limited, to changes caused
by sea-level rise over a range of hydrogeologic settings. Vulnerability in this context is
defined by the rate and magnitude of salinization of coastal aquifers and changes in
groundwater flow to the sea. Two-dimensional variable-density groundwater flow and salt
transport simulations indicate that the response of recharge-limited systems is largely
minimal, whereas topography-limited systems are vulnerable for various combinations of
permeability, vertical anisotropy in permeability, and recharge. World coastlines were
classified according to system type as a vulnerability indicator. Results indicate that
approximately 70% of world coastlines may be topography-limited, though variability in
hydrogeologic conditions strongly affects classification. Future recharge and sea-level rise
scenarios have much less influence on the proportion of vulnerable coastlines than
differences in permeability, distance to a hydraulic divide, and recharge, indicating that
hydrogeologic properties and setting are more important factors to consider in determining
system type than uncertainties in the magnitude of sea-level rise and hydrologic shifts
associated with future climate change.
Citation: Michael, H. A., C. J. Russoniello, and L. A. Byron (2013), Global assessment of vulnerability to sea-level rise in
topography-limited and recharge-limited coastal groundwater systems, Water Resour. Res., 49, 2228–2240, doi:10.1002/wrcr.20213.
1.
Introduction
[2] Nearly a quarter of the world’s population lives
within 100 km of a coastline and 100 m of sea level [Small
and Nicholls, 2003]. Fresh water is essential for sustaining
these dense populations and critical ecosystems, yet coastal
water resources are threatened by salinization due to overpumping and climate change. The combined effects of sealevel rise and hydrologic shifts predicted to occur due to climate change, including changes in rainfall and evapotranspiration [Earman and Dettinger, 2011; Intergovernmental
Panel on Climate Change, 2007], will alter hydraulic gradients between land and sea. Changes in this hydraulic balance affect aquifer-ocean water fluxes, with implications for
both water resources and ecosystems. When sea level rises
1
Department of Geological Sciences, University of Delaware, Newark,
Delaware, USA.
2
Department of Civil and Environmental Engineering, University of
Delaware, Newark, Delaware, USA.
Corresponding author: H. A. Michael, Department of Geological Sciences, University of Delaware, Newark, DE 19716, USA. (hmichael@
udel.edu)
©2013. American Geophysical Union. All Rights Reserved.
0043-1397/13/10.1002/wrcr.20213
relative to hydraulic heads on land, salinization of fresh
groundwater can occur. Salinization mechanisms include
lateral saltwater intrusion at depth and vertical infiltration at
the surface due to coastline transgression and storm surge
overtopping [Kooi et al., 2000]. The land-sea hydraulic
balance also affects groundwater flow to the sea: both fresh
groundwater discharge and circulation of saltwater through
the offshore subsurface. Alteration of this submarine
groundwater discharge (SGD) can have important implications for coastal aquatic ecosystems and chemical ocean
budgets. Fresh groundwater contributes nutrients to estuarine environments, resulting in increased ecosystem productivity and in many cases eutrophication [Hu et al., 2006;
Johannes, 1980; Kim et al., 2011; Valiela et al., 1990].
Both fresh and saline groundwater discharge contribute constituents that affect ocean chemistry [Bone et al., 2007;
Johannesson and Burdige, 2007; Windom et al., 2006]. Despite the importance of groundwater as a resource and a
transport vector, it has received little attention relative to
surface flooding in climate change vulnerability assessments
[Kundzewicz et al., 2007]. Potential effects of climate
change on groundwater resources are important to assess as
coastal adaptation and management strategies are developed
now and in the coming decades [Green et al., 2011; Milly
et al., 2008; Werner et al., 2013].
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MICHAEL ET AL.: GLOBAL ASSESSMENT OF VULNERABILITY TO SEA-LEVEL RISE
[3] The geologic and hydrologic settings of coastal
groundwater systems are critical to assessing vulnerability
of groundwater resources to salinization. These factors
have been considered at particular sites [e.g., Hughes et al.,
2009; Loaiciga et al., 2012; Oude Essink et al., 2010;
Vandenbohede et al., 2008], providing insights into local
impacts. More general studies [e.g., Masterson and Garabedian, 2007; Webb and Howard, 2011; Werner and
Simmons, 2009; Werner et al., 2012] improve mechanistic
understanding, and large-scale analyses [Ferguson and
Gleeson, 2012; Ranjan et al., 2006, 2009] assess worldwide impacts. Due to the long timescale over which
changes in climate and sea level occur, the majority of
studies focused on groundwater impacts involve model predictions. Model analyses that have explored controls of
hydrogeologic characteristics on groundwater salinization
due to climate change simulated the difference between the
steady-state position of the freshwater-saltwater interface
before and after a rise in sea level [Sherif and Singh, 1999;
Werner and Simmons, 2009] as well as transient saltwater
intrusion using sharp [e.g., Ranjan et al., 2009] or dispersed
[e.g., Chang et al., 2011; Watson et al., 2010; Webb and
Howard, 2011] interface models with varying hydrogeologic properties. These modeling studies suggest that important hydrogeologic factors affecting salinization due to
sea-level rise are recharge, hydraulic gradient, and permeability; less important are specific yield, specific storage,
and dispersivity. Perhaps the most important control on the
extent and rate of seawater intrusion in simulations is the
nature of the freshwater boundary condition: specified flux
or specified head [Werner and Simmons, 2009]. Despite the
finding that in steady state, systems with specified head experience a greater magnitude of salinization [Werner and
Simmons, 2009], the majority of studies investigate system
response using specified flux boundary conditions.
[4] Physically, a specified flux boundary condition represents a system that has sufficient thickness of unsaturated
zone to accommodate any water-table rise. The elevation
of the water table is limited only by the flux of water to the
system, or its recharge : a flux-controlled system [Werner
and Simmons, 2009] or in this work, a recharge-limited system (Figure 1). The piezometric rise in recharge-limited
systems caused by an increase in sea level has been called
a ‘‘lifting’’ effect [Chang et al., 2011]. Recharge-limited
systems are less vulnerable to sea-level rise because the hydraulic gradient between land and sea can be maintained
(Figure 1). Conversely, in topography-limited systems the
water table is near to land surface such that an increase in
base level results in intersection of the water table with land
surface and increased runoff. Topography-limited systems
are more vulnerable to sea-level rise because the hydraulic
head on the freshwater, landward side cannot rise in response
to a rise on the seaward side (Figure 1). Conditions which
would produce these types of systems are similar to conditions for topography-controlled and recharge-controlled
water tables described by Haitjema and Mitchell-Bruker
Figure 1. Conceptual model of coastal groundwater systems for (a) recharge-limited and (b) topography-limited systems. (left) Representative initial salinity distributions, groundwater flow patterns, and
SGD. (right) Changes in sea level, water-table elevation, and resulting magnitude of salinization (red)
due to movement of the freshwater-saltwater interface in response to sea-level rise.
2229
MICHAEL ET AL.: GLOBAL ASSESSMENT OF VULNERABILITY TO SEA-LEVEL RISE
[2005] and mapped over the contiguous United States by
Gleeson et al. [2011a]. Recharge-limited systems tend to be
more arid, more mountainous, and/or more permeable,
whereas topography-limited systems are humid, low-lying,
and/or less permeable. However, the criterion developed by
Haitjema and Mitchell-Bruker [2005] indicates the tendency
of the water table to follow the shape of the topography,
which determines the nature of the flow system, whereas the
distinction in this work relates to the capacity of the system
to accommodate water-table rise due to an elevated sea level.
[5] Because the type of system impacts the land-sea hydraulic balance, it will also affect the response to sea-level
rise of groundwater flow to the sea. Topography-limited
systems will experience a reduction in fresh SGD relative
to recharge-limited systems due to a reduced hydraulic gradient [Werner et al., 2012]. The change in the saline component of SGD expected in each system with a rise in sea
level is not obvious, however. While a global typology for
the importance of SGD has been proposed [Bokuniewicz
et al., 2003], the relationship between system type and sealevel rise induced changes in fresh and saline SGD has not
been well studied.
[6] The objectives of this study are to explore the vulnerability of a range of coastal groundwater systems to effects of sealevel rise and to consider the distribution of system type,
recharge-limited and topography-limited, over world coastlines.
We isolate effects on the natural system by neglecting anthropogenic effects, such as pumping or mitigation measures. We
define and evaluate vulnerability by the magnitude of three primary effects: salinized aquifer volume, seawater intrusion rates,
and changes in fresh and saline groundwater flow to the sea.
We use transient advective-dispersive flow and transport
models to characterize the extent to which hydrogeologic
factors (permeability, vertical anisotropy in permeability,
and recharge) control the magnitude of these effects for
recharge-limited and topography-limited systems. We show
that the transient response to sea-level rise is greater for topography-limited systems compared to recharge-limited
systems over the range of parameter values explored for
both salinization and changes in SGD. We then use an analytical model to classify world coastlines by system type as
a first indicator of geographic vulnerability to sea-level rise.
2.
Methods
[7] Two sets of analyses were performed. The first was a
generic assessment of aquifer response to sea-level rise. We
used 2-D variable-density numerical models to assess effects
of aquifer properties and recharge rate on the magnitude and
rate of salinization as well as changes in groundwater discharge to the sea for the two types of hydrogeologic systems:
recharge-limited and topography-limited. We then classified
world coastlines by system type for a range of hydrogeologic
characteristics to assess global vulnerability.
2.1. Vulnerability Assessment
[8] Numerical modeling of the transient response of flow
and salinity distributions in response to a sea-level change
was carried out using the U.S. Geological Survey SUTRA
(Saturated-Unsaturated TRAnsport) code [Voss and Provost, 2002]. SUTRA is a finite-element model capable of
simulating variable-density groundwater flow and advec-
tive-dispersive solute transport. We chose this numerical
modeling approach rather than steady-state or sharp-interface modeling approaches in order to incorporate rates of
interface movement and the inland extent of low-salinity
groundwater. The rate of interface movement, which is not
estimated in steady-state approximations, may be slow
(occurring over millennia), so salinization over management timescales may be small. Thus, the rate of movement
is an important management consideration. Sharp-interface
models can simulate transient movement, but the full width
of the interface is not simulated, potentially leading to
overestimation of the freshwater resource since only water
containing <2% seawater is generally considered potable.
[9] Simulations were run using a 2-D, cross-section
model domain (Figure 2a). After a steady-state flow simulation to obtain an initial pressure distribution, simulations
were run to steady state (1000 years) with current sea level
to obtain pressure and concentration distributions prior to
sea-level rise. A 1 m instantaneous rise in global sea level
was considered without coastline transgression, assuming
fortification that would maintain the position of the coastline. This assumption allows isolation of the lateral component of seawater intrusion because vertical infiltration of
seawater above inundated coastline [e.g., Kooi et al., 2000]
is not simulated. Only salinization due to lateral movement
of the freshwater-saltwater interface was considered, infiltration due to saltwater overtopping on land was not. A 1 m
rise is within the range of 0.5–1.4 m predicted by the Intergovernmental Panel on Climate Change (IPCC) for the period 1990–2100 [Rahmstorf, 2007]. Simulations were run
for a period of 200 years post sea-level rise, a length of
time considered relevant for management. To evaluate aquifer vulnerability, the area salinized (volume salinized per
meter length of coastline), salinization rate (the rate of
movement of the 2% seawater contour at the model base,
or toe), and changes in fresh and saline SGD over the 200
year period were analyzed.
[10] The model domain was 500 m deep and extended
50 km landward and 25 km seaward of the coastline. Mesh
size was 100 m in the x direction (horizontal) and 20 m in
the y direction (vertical), except for the region 10 km landward and 5 km seaward of the coastline in which horizontal
mesh size was 50 m. Vertical side and bottom boundaries
were zero flux. The offshore top boundary was specified
pressure, initially hydrostatic mean sea level at the elevation of the coastline node. We note that this configuration
does not allow for an increase in aquifer thickness with
higher sea level or water tables, an effect that would tend to
cause landward movement of the interface toe [see Werner
et al., 2012]. However, in this case, the ratio of the change
in thickness to the total thickness is very low (1:500), so
the effect is probably minimal.
[11] A specified flux representing average annual recharge
was applied to the landward top boundary for recharge-limited simulations, which produced a pressure distribution
increasing landward that could change with changes in sea
level. Topography-limited systems were represented by specifying the pressure distribution developed along the landward
boundary from the steady-state recharge-limited simulation
with the same set of hydrogeologic parameters; this distribution represents an unmoving water table limited in elevation
by the topographic surface. These boundary conditions
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MICHAEL ET AL.: GLOBAL ASSESSMENT OF VULNERABILITY TO SEA-LEVEL RISE
Figure 2. Model setup, initial salinity distributions, and salinized areas. (a) Model domain and boundary
conditions. (b–i) (left) Steady-state salinity distributions before sea-level rise for both recharge-limited
and topography-limited systems with the same permeability and recharge, and (right) salinized area (yellow) for topography-limited systems due to movement of the 2% seawater contour after a 1 m sea-level
rise (red and blue are >2% seawater and <2% seawater, respectively, both before and after sea-level
rise). (b and c) Base-case parameters, (d and e) high anisotropy (kz ¼ 1013 m2), (f and g) low recharge
(R ¼ 118 mm yr1), and (h and i) high recharge (R ¼ 772 mm yr1). All other parameters are base case
(kx ¼ kz ¼ 1010 m2, R ¼ 300 mm yr1).
ensured that each set of recharge-limited and topographylimited simulations was hydrologically comparable. To simulate response to sea-level rise, the specified pressure on the
offshore top boundary was increased by the equivalent of 1m
of seawater, with the position of the coastline assumed stationary. Solute transport boundary conditions were zero flux
along the vertical sides and bottom, concentration C ¼ 0
along the top landward boundary and C ¼ Cseawater ¼ 0.0357
kg salt kg seawater1 along the top offshore boundary for
inflow, and zero concentration gradient for outflow.
[12] Aquifer permeability (k) and vertical anisotropy (ratio of horizontal to vertical permeability, kx :kz) were considered to be the primary aquifer characteristics affecting
system response to sea-level rise ; these were varied in the
analysis. The range of simulated k values was chosen based
on the permeability of common unconsolidated aquifer
materials from well-sorted gravel (1010 m2) to silt (1014
m2) [e.g., Fetter, 2001]. Vertical anisotropy was achieved
by using the same value of kx, 1010 m2, while decreasing
kz by factors of 10, 100, and 1000.
[13] Freshwater recharge (R) was considered the primary
hydrologic factor affecting system response. Recharge categories were chosen based on a global analysis of recharge
rates [Döll, 2009]. The global recharge distribution was separated into seven quantiles, each represented in simulations
by the median value. The base-case model represents an isotropic, well-sorted sand aquifer receiving a moderate
amount of recharge (R ¼ 300 mm yr1, kx ¼ kz ¼ 1010 m2).
[14] Model sensitivity to changes in dispersivity was
found to be minimal, and so the dispersivity was chosen to
minimize numerical instability (longitudinal ¼ 50 m, transverse ¼ 5 m). Specific storage was 104 m1 ; this parameter has been found to have little effect on salinization
processes in this study and other studies [e.g., Ranjan et al.,
2009; Webb and Howard, 2011], likely because pressure
equilibrates much faster than the salinity distribution.
2.2. Global Classification
[15] ARCMAP 10.0 [Environmental Systems Research
Institute, 2011] geographic information system (GIS) software was used to classify coastlines as either recharge-limited or topography-limited for different values of k, R, and
shore-perpendicular distance to a hydraulic divide (L). This
classification was achieved by comparing topographic
slope, as calculated in the GIS from a digital elevation
model (DEM) [U.S. Geological Survey (USGS), 1996], to
the average slope of the phreatic surface with vector analyses at points equally spaced every 1 km along world coastlines. The average slope of the phreatic surface was
calculated using global data sets of current and future
recharge, permeability, and an analytical solution for the
elevation of freshwater head [Custodio, 1987]:
2231
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2qo x Rx2
h¼
;
K ð1 þ Þ
MICHAEL ET AL.: GLOBAL ASSESSMENT OF VULNERABILITY TO SEA-LEVEL RISE
where h is the freshwater head, qo is the discharge per unit
length of coastline, x is the distance inland from the coast,
R is the uniform net recharge, K is the hydraulic conductivity, and is the density ratio, f/(s f), where f and s
are the freshwater and saltwater end-member densities,
respectively. The discharge qo was conceptualized as the
recharge rate multiplied by the shore-perpendicular distance to a hydraulic divide. The Custodio [1987] solution
was chosen because it accounts for the freshwater-saltwater
interface of coastal systems, provides a similar pressure
profile to that obtained by numerical modeling for the same
input parameters, and has been similarly applied in recent
coastal hydrogeology literature [e.g., Werner and Simmons,
2009].
[16] In order to describe a curved surface in terms of a
simple linear slope (the slope of the phreatic surface), the
average slope over 1 km was considered. This definition of
average slope is arbitrary and was chosen to match the 1
km resolution of the elevation raster data. To preserve distance values in geographical calculations, the global shoreline line file was clipped and reprojected into files
corresponding to each of the 60 unique UTM (Universal
Transverse Mercator) zones. In each file, a line was buffered 1 km shoreward of the coastline along which points
were picked at 1 km intervals. By working in UTM projections, we maintained precision and reduced distance and
angular distortion that would have resulted from performing these buffer calculations in the original geographic projection. Following point delineation, the 60 point files were
merged back into a global data set. Elevation [USGS, 1996]
and current and future recharge values [Döll, 2009] were
extracted at each point location from the nearest raster cell,
which allowed calculation of the phreatic surface for each
point with the Custodio solution. Because each point was
1000 m from the shoreline (assumed head ¼ 0 m), the average slope of the phreatic surface could be calculated and
compared to the average topographic slope at each point.
Points where the average slope of the phreatic surface
exceeded the topographic slope were considered to be topography-limited, whereas points where the average slope
was less than the topographic slope were considered to be
recharge-limited. The effects of sea-level rise on system
type classification were evaluated by subtracting 1 m from
all terrestrial elevation values; this is equivalent to adding
1 m to the elevation of mean sea level.
[17] A global map of permeability developed by Gleeson
et al. [2011b] was used as the base-case spatial distribution.
Current and future recharge rates were based on the analysis of Döll [2009]. The climate change scenario based on
the IPCC B2 greenhouse gas emission scenario and
ECHAM4 global climate model predicted the greatest
change to global recharge values. This scenario was used to
calculate future values of expected recharge from the current recharge estimate and ECHAM4 B2 future recharge
multiplier given by Döll [2009].
[18] Sensitivity of system type characterization to permeability, recharge, and shore-perpendicular distance to a hydraulic divide were evaluated. Permeability was varied across
a range of reasonable values from 1014 to 1010 m2, and
recharge rates were varied by a factor of two above and below
the base-case distribution. The base-case divide distance was
considered 10 km and was varied between 1 and 50 km.
3.
Results
3.1. Vulnerability Assessment
[19] Three vulnerability criteria were considered for
recharge-limited and topography-limited systems : salinization rate, salinized area, and changes in SGD. These were
assessed for an instantaneous rise in sea level over a 200
year management time period. Recharge-limited systems
exhibited little to no salinization, whereas the response of
topography-limited systems was substantial and dependent
on hydrogeologic factors.
[20] The initial salinity distributions, which were identical for both system types, and salinized portion of the aquifer for selected topography-limited simulations are shown
in Figure 2. In recharge-limited systems, the elevation of
the water table rose with a rising sea level, nearly maintaining the position of the freshwater-saltwater interface and
largely preventing salinization. In topography-limited systems, the water table remained stationary, allowing the
interface to move inland at a rate dependent on model parameters. In both systems, low recharge conditions (Figure
2f) and high values of vertical anisotropy in permeability
(Figure 2d) resulted in highly dispersed interfaces with
great landward extent ; these tended to move more slowly
than the sharp interfaces exhibited by systems with greater
throughflow of fresh groundwater. Vertical anisotropy in
permeability also affected the interface position. Lower
values of kz relative to a constant kx resulted in wider offshore freshwater discharge zones and salinity transition
zones that were farther offshore (Figure 2d).
3.1.1. Salinization Rate
[21] The rate of movement of the 2% seawater contour,
the approximate potable water limit, on the interface toe
(the intersection of the interface and the model bottom
boundary) for each topography-limited simulation is shown
in Figure 3. Although the salinized area in recharge-limited
systems was negligible compared to topography-limited
systems (Table 1), the interface moved in response to sealevel rise, consistent with previous studies [Chang et al.,
2011; Werner et al., 2012]. The 2% seawater toe initially
oscillated, eventually stabilizing in its initial location, usually within the 200 year simulation period. Salinization
rates for topography-limited systems (Figure 3 and Table
1) were generally highest after the initial sea-level perturbation and tended to decrease with time. In some cases,
there was a time lag between the perturbation and the peak
rate of movement of the toe. This effect was most pronounced in simulations with low recharge, with the peak
salinization rate for a recharge rate of 90 mm yr1 occurring 50 years after sea-level rise, and the 2% toe only beginning to noticeably move after about 100 years for the 35
mm yr1 recharge rate. The time lag in peak rate only
occurred (within the 200 year simulation period) for the
isotropic case and the highest value of permeability simulated, 1010 m2.
[22] The peak salinization rate was highest in simulations
with high permeability, decreasing with lower horizontal and
vertical permeability values (higher-anisotropy ratios; Figures
3b and 3c). The effect of recharge rate on peak salinization
rate was nonlinear: maximum peaks occurred for intermediate recharge rates of 146 and 300 mm yr1 (Figure 3a and
Table 1). Equilibration occurred quickly at higher recharge
2232
MICHAEL ET AL.: GLOBAL ASSESSMENT OF VULNERABILITY TO SEA-LEVEL RISE
Figure 3. Rate of movement of the 2% seawater interface
toe after an instantaneous 1 m sea-level rise for topography-limited systems with different (a) recharge rates, (b)
horizontal permeability, and (c) ratio of horizontal to vertical permeability. Corresponding salinized area (see also
Table 1) is shown in legends for reference.
rates, but the extent of intrusion was lower due to high hydraulic gradients and freshwater throughflow. At low recharge
rates there was a lag in response and lower rates of intrusion
over a longer timeframe, resulting in greater overall salinization than occurred at higher recharge rates.
[23] Equilibration of the system, indicated by a salinization rate of zero, occurred within decades for some systems
and much longer than 200 years for others. Equilibration
time was greatest for low recharge rates and higher anisotropy ratios, which were also scenarios with more dispersed
interfaces (Figure 2). Rates of movement for the 90% seawater toe were similar to those of the 2% toe, though generally of slightly lesser magnitude, indicating a widening
interface after sea-level rise. Under low recharge conditions
(<146 mm yr1), the 90% toe initially moved offshore in
response to increased sea level, which may have been an
artifact of the instantaneous sea-level rise. Within the 200
year management timeframe, the interface toe moved
inland between 0 and 2 km depending on the hydrogeologic
scenario, 1 km in the base case (Table 1).
3.1.2. Salinized Area
[24] The area salinized after 200 years of equilibration
with a 1 m sea-level rise for topography-limited and
recharge-limited systems is shown in Figures 4a–4c. Salinization was not observable in recharge-limited systems
except under conditions of very low recharge (35 mm yr1),
for which effects were minimal. In topography-limited systems, salinized area tended to decrease with increasing
recharge (Figure 3a). This was not apparent in the lowest
recharge case (35 mm yr1) for salinized area after 200
years (Figure 4a) because the time lag between sea-level
rise and interface movement was large (Figure 3a), and a
new equilibrium was not reached within that timeframe.
[25] In topography-limited systems, aquifer permeability
was a major control on interface movement. The area salinized increased with permeability : values typical of fine to
coarse sandy aquifers exhibited orders of magnitude more
salinization than lower-permeability systems (Figure 4b).
System vertical anisotropy greatly affected the position of
the interface (the initial salinity distribution; Figure 2) but
had only a minor effect on salinized area (Figure 4c). An
exception is highly anisotropic systems (anisotropy ratio of
1:1000 in our simulations), in which the interface position
near the surface was far offshore and seawater intruded
through another mechanism: seawater infiltrated into the
previously fresh outflow face seaward of the initial coastline, creating a lobe of dense saltwater (Figure 2e).
3.1.3. Changes in SGD
[26] Changes in the hydraulic balance between land and
sea can affect groundwater fluxes to the sea as well as seawater fluxes to aquifers (salinization). We analyzed the
change in fresh and saline SGD as a result of sea-level rise
for each of the simulations. For recharge-limited systems in
this analysis, the model setup prescribed the fresh flux
through the system, so fresh SGD could not change. Saline
recirculation could be affected, but the minimal changes in
the nature of the interface in recharge-limited systems
resulted in negligible changes in saline SGD driven by the
density gradient along the interface. No other mechanisms
for saltwater exchange were simulated; thus, changes were
not evaluated. In topography-limited systems, both the
fresh throughflow and the position and thickness of the
interface changed with changes in sea level: in nearly all
cases a rise in sea level resulted in a reduction of both fresh
and saline SGD.
[27] The reduction in SGD (defined as discharge seaward
of the armored coastline) rate varied among topographylimited systems with different recharge rates. In systems
with low recharge rates (35–146 mm yr1), a 1 m rise in
2233
MICHAEL ET AL.: GLOBAL ASSESSMENT OF VULNERABILITY TO SEA-LEVEL RISE
Table 1. Model Simulation Results for Topography-Limited and Recharge-Limited Boundary Conditionsa
Toe of 2% Seawater Contour
Varied
Parameter
Salinized
Area (m2)
Base case
R ¼ 35
R ¼ 90
R ¼ 118
R ¼ 146
R ¼ 453
R ¼ 772
kx ¼ 1011
kx ¼ 1012
kx :kz ¼ 10:1
kx :kz ¼ 100:1
kx :kz ¼ 1000:1
440,000/0
496,000/40,000
1,326,000/6,000
1,111,000/0
895,000/0
282,000/0
159,000/0
34,000/0
2,000/0
420,000/0
367,000/0
853,000/0
Peak Salinization
Rate (m yr1)
Equilibration
Time (yr)
Inland Toe
Movement (m)
Change in SGD
Saline
(m3 d1)
Topography-limited simulations/recharge-limited simulations
48/0.0
82/0
1000
232.5
5.3/0.0
>200
610
14.3
30/0.0
>200
3300
22.2
38/1.1
>200/>200
2600
24.9
46/3.9
150/>200
2100
27.0
42/0.0
34/0
650
24.2
25/0.0
27/0
350
11.9
12/0.0
27/0
91
0.05
1.1/0.0
48/0
14
0.01
42/0.0
62/0
900
8.08
26/0.0
170/0
790
0.54
8.1/0.0
>200/0
630
1.50
% Change in SGD
Fresh
(m3 d1)
Saline
Fresh
241.0
4.8
12.3
16.1
19.9
56.5
78.3
13.0
1.16
35.2
23.6
12.1
295
100
100
100
100
64
29
0.98
1.2
56
9.1
83
2100
100
100
100
100
91
74
31
2.8
86
58
30
a
Parameter values are base case (bold; R ¼ 300 mm yr1, kx ¼ 1010 m2, kx :kz ¼ 1:1) unless otherwise specified; recharge (R) in mm yr1 and permeability (k) in m2. Salinized area and toe movement (maximum salinization rate, equilibration time, and distance toe moved inland) over the 200 year sealevel rise response period are given for topography-limited/recharge-limited systems. Change in SGD is negligible for recharge-limited systems and is not
shown. Salinization and change in SGD are negligible for low-permeability (kx ¼ 1013 to 1014 m2) simulations and are not shown.
sea level caused the salinity interface to move entirely
onshore. This means that the fresh and saline water that discharged offshore prior to sea-level rise discharged landward of the armored coastline after the rise (as into
streams), resulting in 100% reduction and a linear reduction
in the magnitude of discharge with increasing recharge rate
(Figure 4d, open symbols). In simulations with greater
recharge, the interface remained at least partially offshore,
allowing some fresh offshore discharge to occur: fresh
SGD reduction increased with recharge rate, though the relative reduction decreased (Figure 4d, closed symbols, and
Table 1). In the higher-recharge cases, saline SGD was
reduced; this was also due to partial movement of the interface landward across the shoreline. Simulations with the
Figure 4. (a–c) Salinized area for both system types and (d–f) change in fresh and saline components
of SGD for topography-limited systems 200 years after an instantaneous sea-level rise of 1 m with different (a and d) recharge rates, (b and e) horizontal permeability, and (c and f) ratio of horizontal to vertical
permeability. Open symbols in (d)–(f) represent 100% reduction in SGD.
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MICHAEL ET AL.: GLOBAL ASSESSMENT OF VULNERABILITY TO SEA-LEVEL RISE
highest recharge rates have less reduction in saline discharge than intermediate recharge simulations.
[28] Low-permeability systems exhibited minimal changes
in SGD and very low initial SGD, but in systems with permeability values typical of sandy aquifers, reduction was pronounced and initial values were greater. The salinity
interface was offshore for low k values (1012 to 1014 m2)
and the hydraulic gradient was high enough (determined by
the recharge rate) that there was little change in fresh SGD;
the same was true for saline SGD. For a permeability of
1011 m2 the interface was partially onshore: the shoreline
intersected the 2% salinity contour 200 years after the sealevel rise. In this case, fresh discharge was reduced by about
one third, while saline SGD remained approximately constant
because nearly all of the saline part of the interface remained
offshore throughout the simulation. The highest permeability
base case exhibited the greatest change in offshore discharge.
Initially, the shoreline bisected the interface. After 200 years
the interface migrated almost completely onshore, resulting
in nearly 100% reduction in offshore discharge.
[29] The effect of vertical permeability, or anisotropy ratio,
was similar to that of horizontal permeability: higher-anisotropy
systems exhibited a lower reduction in discharge. The primary
reasons for this are the position of the interface relative to the
shoreline and its rate of movement. Higher-anisotropy systems
display offshore interfaces that move more slowly than those
with a lower-permeability contrast. In the isotropic system, the
shoreline initially approximately intersected the 90% seawater
salinity contour, meaning that most fresh and saline groundwater discharged onshore initially, and nearly all of it discharged
onshore after the rise in sea level when the interface moved
landward of the shoreline. The simulated interface for anisotropy ratios of 10 and 100 also straddled the shoreline but did
not migrate completely onshore after 200 years; thus, the reduction in SGD is slightly less than the isotropic case for both fresh
and saline SGD. The interface in the highly anisotropic case
remained offshore throughout the simulation. The reduction in
fresh SGD occurred due to the reduced hydraulic gradient, while
the increase in saline SGD was a result of the development of a
saline circulation cell near the shoreline (Figure 2e).
3.2. Global Classification
[30] The results of section 3.1 illustrate the dependence
of aquifer vulnerability to sea-level rise on system type:
recharge-limited or topography-limited. This means that
type classification of coastlines may serve as a first indicator of potential vulnerability. The relationship between the
topographic slope and water-table elevations determines
whether a system is recharge-limited (low vulnerability) or
topography-limited (high vulnerability).
[31] The calculated global distribution of topographylimited coastlines for the base-case parameter values (permeability distribution from Gleeson et al. [2011b], recharge
rate from Döll [2009], and shore-perpendicular distance to
a hydraulic divide of 10 km) is shown in Figure 5. In this
best-estimate scenario, 67.8% of world coastlines are topography-limited (Table 2). Low-sloping coastal areas and
those that receive plentiful rainfall, such as the US Gulf
Coast and coastal Bangladesh, are more likely to be topography-limited, and thus more vulnerable sea-level rise, than
coasts with high slopes or in drier climates, such as the US
West Coast and northern Africa.
[32] System type is dependent on the hydraulic gradient,
which develops from the balance of recharge, permeability,
and sea level. Sensitivity to permeability, recharge rate, and
divide distance is shown in Table 2 and Figure 6. The percentage of topography-limited coastlines varies from 15.9%
to 77.9% over the range of values considered for current conditions. Over all current and future scenarios, 15.5% of
coastlines are always recharge-limited, and 14.1% are always
topography-limited.
[33] The sensitivity of system classification to permeability, which is highly variable both globally and locally
(subpixel scale), was assessed (Figure 6a and Table 2).
Between 15.9% and 75.4% of coastlines are topographylimited for the range of permeability values considered
(other parameters held at base-case values) ; in other words,
15.9% of world coastlines are always topography-limited,
and 24.6% of world coastlines are always recharge-limited.
Systems with higher permeability require a lower hydraulic
gradient to transmit a given fresh discharge per unit
Figure 5. Map of the distribution of recharge-limited (R-limited) and topography-limited (T-limited)
coastlines for base-case parameter values. (permeability distribution from Gleeson et al. [2011b],
recharge from Döll [2009], and a 10 km distrance to hydraulic divide).
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MICHAEL ET AL.: GLOBAL ASSESSMENT OF VULNERABILITY TO SEA-LEVEL RISE
Table 2. Percentage of Topography-Limited World Coastlines
Calculated for Different Values of Permeability and Current and
Future Recharge Scenariosa
Current Sea Level
Varied
Parameter (m2)
1 m Sea-Level Rise
Current
Recharge
Future
Recharge
Current
Recharge
Future
Recharge
67.8%
15.9%
40.2%
75.4%
46.8%
77.9%
63.1%
72.2%
68.4%
15.7%
40.6%
75.9%
47.2%
78.4%
63.8%
72.8%
69.0%
24.2%
42.2%
75.8%
49.6%
78.7%
64.6%
73.2%
69.5%
24.2%
42.5%
76.4%
50.0%
79.2%
65.1%
73.7%
Base case
k ¼ 1010 m2
k ¼ 1012 m2
k ¼ 1014 m2
L ¼ 1 km
L ¼ 50 km
Half R
Double R
a
Current base-case results are in bold. Parameter values are base case
unless otherwise specified. Permeability values in ‘‘distributed’’ scenario
are from Gleeson et al. [2011b].
coastline; thus, higher-permeability systems are less likely
to be topography-limited. The shore-perpendicular distance
to a hydraulic divide determines in part the fresh discharge
for a given recharge rate, so this is also an important parameter. The percentage of topography-limited shorelines
ranges from 46.8% to 77.9% if divide distance is changed
from 1 to 50 km (Figure 6b and Table 2; other parameters
held at base-case values). Recharge has a lesser effect on
classification because the assumed uncertainty associated
with recharge estimates is less than the uncertainty in
coastal aquifer permeability and geometry globally. The
percentage of topography-limited coastlines varies from
63.1% to 72.2% if recharge is halved and doubled, while
other parameters are held at base-case values (Figure 6c and
Table 2).
[34] Increases in recharge due to climate change have the
potential to convert previously recharge-limited systems to
topography-limited systems. A future scenario (scenario
ECHAM4 B2) [Döll, 2009] in which 55.1% of world shoreline locations are predicted to experience enhanced recharge
causes a 0.14% decrease to 0.64% increase in topographylimited world coastlines, while a 1 m sea-level rise will
increase the percentage of topography-limited coastlines by
between 0.0% and 8.3% (across tested k, L, and R values;
Table 2). This indicates that sea-level rise has a potentially
greater impact than changes in recharge, but effects will be
more strongly controlled by the hydrogeologic properties (k,
L, and R; Table 2). If both sea-level rise and recharge
changes occur, the percentage of global coastlines that are topography-limited will increase by 0.0%–8.3%. These results
suggest that values of and uncertainties in hydrogeologic
characteristics (such as permeability, recharge, and watershed
size) are more critical to quantify than uncertainties in predicted changes in recharge and sea-level rise for improving
estimates of the distribution of world coastlines highly vulnerable to effects of sea-level rise.
4.
Discussion
[35] The difference in response to sea-level rise between
topography- and recharge-limited coastal groundwater systems highlights the importance of this distinction in assessing vulnerability, as defined by the rate and magnitude of
aquifer salinization and changes in groundwater discharge to
the sea. In the first part of this study, we analyzed changes in
idealized 2-D hydrogeologic systems with a range of hydrogeologic characteristics (horizontal and vertical permeability)
and hydrologic settings (recharge rates). We focused on seawater flux into the aquifer (salinization extent and rate) as well
groundwater flux to the sea (changes in fresh and saline SGD)
over a 200 year management timescale for an instantaneous 1
m rise in sea level. In all cases, recharge-limited systems experienced nearly negligible changes in response to sea-level rise.
Only topography-limited systems changed measurably.
[36] Our GIS analyses show that under present and future
scenarios, over half of the world’s coastlines may be topography-limited and thus vulnerable to sea-level rise. There is
no dominant characteristic that alone indicates whether a
system is topography-limited or recharge-limited. Instead,
it is the combination of hydrologic and physical features
that determines vulnerability. For example, areas with high
recharge rates or high fresh discharge rates may be more
likely to be topography-limited, but high gradients may
cause offshore salinity interfaces, which are much less vulnerable to salinization inland. Similarly, while low-permeability systems may exhibit lower rates of salinization and
SGD changes, they are also more likely to be topographylimited for a given recharge rate.
[37] This analysis indicates that the 15.5% of coastlines
that are consistently recharge-limited for all values of permeability and divide distance, for current and future sea level,
and all recharge scenarios are robust with respect to vulnerability. Other areas may have more uncertain responses to
effects of climate change. The 14.1% of coastlines that are
topography-limited for all scenarios and should be considered most vulnerable to climate change. While recharge rates
and their changes are fairly homogeneous on a regional scale
in most areas, aquifer permeability can vary by orders of
magnitude over short distances. This means that locally, aquifer characteristics may be most important to characterize
for vulnerability assessment and that vulnerability varies
locally to a much greater extent than indicated by Figures 5
and 6. The very small difference in the extent of topography-limited coastlines between current and future recharge
and sea-level scenarios also indicates that local hydrogeologic characteristics are more critical in vulnerability assessment than uncertainties in climate predictions.
[38] Although topography-limited areas have the potential
to be greatly affected by sea-level rise, the magnitude of system response and its implications depend on the hydrogeologic setting. The timescale over which salinization occurs is
highly variable: some conditions would produce significant
salinization at equilibrium, but salinization occurs so slowly
that it is no longer relevant for management. These are systems that may not be in equilibrium with present-day sea
level; thus, managing to minimize impacts of pumping on
salinization, rather than sea-level rise, is a greater priority
[i.e., Yu et al., 2010]. Additionally, some aquifers are less
likely to be used for water supply, particularly those with low
permeability; thus, groundwater salinization is not a primary
resource concern in those areas.
4.1. Assumptions and Limitations
[39] The aim of this work is to improve understanding of
basic controls on coastal aquifer vulnerability to aspects of
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MICHAEL ET AL.: GLOBAL ASSESSMENT OF VULNERABILITY TO SEA-LEVEL RISE
Figure 6. Map of type classification sensitivity. Coastlines are designated consistently topographylimited (red), consistently recharge-limited (blue), and variable across a 1 m sea-level rise and (a) permeability values of 1014 m2 to 1010 m2, (b) hydraulic divide from 1 to 50 km, and (c) nonvaried parameters are base-case: permeability distribution from Gleeson et al. [2011b], recharge from Döll [2009], and
a 10 km distance to hydraulic divide.
changes in climate and to assess the global distribution of
areas at risk. In order to gain such general insights, we have
considered very simple systems and avoided site-specific
complexity. We considered homogeneous hydrogeologic parameters within 2-D shore-perpendicular cross-sectional
models and over world coastlines. This approach allows illustration of overall effects of changes in parameters but ignores
the true spatial distribution of these properties and 3-D
effects. In the vulnerability analysis, we consider only one
aquifer geometry. However, effects of geometric variations
may be inferred from the analysis. Shorter aquifers will
respond similarly to long aquifers with lower recharge rates
for this model setup, and thinner aquifers will respond similarly to thick, lower-permeability aquifers (analogous to a
reduction in transmissivity).
[40] The global analysis does not consider variability in
system response : heterogeneous rates of seawater intrusion
and SGD due to both geologic and geomorphic heterogeneity. Terrestrial drains such as rivers and lakes that occur on
smaller scales than the DEM resolution are also not
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MICHAEL ET AL.: GLOBAL ASSESSMENT OF VULNERABILITY TO SEA-LEVEL RISE
represented. Finer-scale representation would likely increase
the proportion of topography-limited regions because the
topographic lows are determining factors in the maximum
water-table height. Clearly higher resolution regional- to
local-scale efforts are essential to assess true vulnerability of
coastal aquifer systems.
[41] The analysis also assumes that armored shorelines
prevent transgression and overtopping that would cause
salinization from the aquifer top rather than lateral intrusion. In cases where transgression occurs, overtopping and
resulting vertical infiltration would accelerate the salinization modeled in this work. Results of the SGD analysis
would also change, since discharge onshore of an armored
coastline would occur offshore under transgression. Overtopping by storm surge inundation is also not considered,
but has the potential to cause groundwater salinization.
Under some conditions, this salinization may be more pronounced in recharge-limited compared to topography-limited systems because of greater unsaturated zone thickness.
[42] This analysis is limited to unconfined aquifer systems, which may be more vulnerable than deeper systems,
but which also may not be the primary groundwater
resource in some areas. It can be difficult to assess the vulnerability of deep fresh resources, which may not be in
equilibrium with present-day sea level [e.g., Kooi and
Groen, 2001; Person et al., 2003; Yu et al., 2010], suggesting that future sea-level rise may not have a great impact
on the slow salinization rate. Though deep, confined
groundwater resources may be less vulnerable to sea-level
rise, the cost of deep drilling may be prohibitive, and so
domestic pumping in many regions of the world and irrigation pumping may commonly occur from unconfined
aquifers.
[43] Though groundwater pumping has been shown to be
an important factor contributing to salinization and changes
in land-ocean fluxes [Ferguson and Gleeson, 2012; Loaiciga et al., 2012; Yu et al., 2010], we consider only natural
hydrogeologic factors in this analysis in order to isolate
effects of climate change. If considered in a distributed
manner, pumping effectively reduces the net recharge by
an amount less than the total amount extracted (since irrigation return flows may occur and pumping may induce more
recharge). This would shift our estimates toward lower
recharge rates and greater vulnerability. The importance of
pumping in driving salinization would depend on the relative rates of recharge to and pumping from the unconfined
aquifer.
4.2. Implications
4.2.1. Salinization
[44] The primary implications of aquifer salinization modeled in this work are associated with human uses: reduction
in the sustainable rate of pumping, deterioration of drinking
water quality, and agricultural soil salinization by applied
irrigation water. These threats are particularly acute in
highly populated areas, which comprise a substantial portion
of world coastlines. A secondary threat of this salinization,
particularly in the near surface, is mobilization of land-based
anthropogenic contaminants from soils in urban and agricultural areas, industrial sites, and waste disposal facilities,
many of which are located along highly populated coastlines
[Danielopol et al., 2003; Pope et al., 2011].
4.2.2. Changes in Saline SGD
[45] In many cases considered in this analysis, the freshwater-saltwater interface moved from an offshore or partially offshore position to a new location inland of the
shoreline, despite the assumption that the coastline would
remain stationary with sea-level rise (as by construction of
a sea wall, for example). The onshore discharge of saline
water across the constant-head boundary is equivalent to
discharge into surface water bodies: the increase in on-land
saline groundwater discharge is nearly equivalent to the
reduction in saline SGD reported in section 3.1. Increased
flow of brackish or saline groundwater into previously fresh
surface water bodies can have severe ecological effects.
Even in the absence of surface flooding, freshwater wetlands may convert to salt marsh, salinity-sensitive species
may die, and aquatic ecosystems hosting freshwater species
may be permanently altered [e.g., Baldwin and Mendelssohn, 1998; James et al., 2003].
4.2.3. Changes in Fresh SGD
[46] Changes in direct offshore discharge of fresh groundwater can have important effects, both positive and negative.
Reduction in fresh SGD due to sea-level rise can result from
an increase in rejected recharge or landward movement of
the freshwater-saltwater interface causing increased discharge into onshore surface water bodies, as discussed previously. This means that if onshore discharge is occurring to
streams connected to coastal surface water bodies, the volume of water that eventually reaches the coast is the same
before and after sea-level rise. However, the discharge path
(via surface water, wetlands, or directly by groundwater) can
impact processes that attenuate nutrients or immobilize contaminants. Therefore, changing distributions of groundwater
discharge to streams, wetlands, and offshore water bodies
are likely to impact chemical fluxes to estuaries and the
ocean.
4.2.4. Rising Water Tables
[47] Topography-limited systems are assumed in this
work to experience no change in elevation of the water table. In reality, there is likely to be some depth of unsaturated zone, at least in some areas during some of the year.
These previously aerated areas would be subject to more
frequent flooding or poor drainage with rising sea level or
increases in recharge. Problems associated with rising
water tables may be more pronounced in recharge-limited
systems, despite their relative resistance to salinization. In
areas with water tables sufficiently below land surface,
storage or disposal of wastes in the unsaturated zone is
common. Proper functioning of these systems, such as septic tanks, land-based wastewater disposal sites, and landfills, requires, often by law, a minimum thickness of the
unsaturated zone. A systematic rise in the water table may
render this infrastructure ineffective or illegal. Moreover,
saturation of soils previously under oxic conditions may
cause a change in redox state to more reducing conditions.
These geochemical changes have the potential to mobilize
contaminants, such as phosphate or toxic trace metals bound
to iron or manganese oxides [e.g., Borch et al., 2010].
5.
Conclusion
[48] This two-part analysis establishes the relative vulnerability of recharge-limited and topography-limited coastal
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MICHAEL ET AL.: GLOBAL ASSESSMENT OF VULNERABILITY TO SEA-LEVEL RISE
hydrogeologic systems to sea-level rise and explores the
prevalence and spatial distribution of highly vulnerable world
coastlines. A series of 2-D variable-density groundwater flow
and salt transport simulations establish that topography-limited
systems are more vulnerable than recharge-limited systems to
salinization rates, salinized aquifer volume, and changes in
groundwater flow to the sea. This is true for a wide range of
two critical hydrogeologic factors, aquifer permeability and
recharge rate. Global analysis indicates that more than half of
world coastlines are topography-limited. Local hydrogeologic
properties (permeability, recharge, and aquifer geometry) are
the primary determinant: the proportion of highly vulnerable
coastlines for current conditions ranges from 15.7% to 77.9%
over values of hydrogeologic parameters typical of natural systems. This range is much greater than the uncertainty due to
estimates of climate change induced sea-level rise and changes
in recharge; differences between an estimated current and
predicted future scenario differ by less than 8.3%. Thus,
local hydrogeologic characteristics are the primary indicators of vulnerability of coastal groundwater systems to sealevel rise and should be prioritized over reduction of uncertainty in predicted changes for adaptation, management,
and planning purposes. Because the response of coastal
groundwater systems to sea-level rise depends strongly on
system type, classification of topography-limited and
recharge-limited systems may be an important first indicator of vulnerability. Improved assessment of this indicator
can be accomplished on local and regional scales by incorporating site-specific hydrogeologic characteristics.
[49] Acknowledgments. This work was funded in part by the University of Delaware Research Foundation and the National Science Foundation (EAR-0910756 and EAR-1151733). The authors thank Petra Döll and
Tom Gleeson for groundwater recharge and permeability data sets, respectively. The authors also thank Clifford Voss for helpful discussions and
comments on this manuscript, Lawrence Feinson for assistance with analysis of model results, and Tracy DeLiberty and Luc Claessens for assistance
with the GIS analysis. The helpful comments of Daniel Fernandez-Garcia,
Tom Gleeson, Adrian Werner, Diana Allen, and an anonymous reviewer
substantially improved this manuscript.
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