Rhoads Fork Spring

Lead

Custer

Spearfish

Rapid City

Madison aquifer, Hot Springs

Black Hills region

Black

Hills

Spearfish Creek

Austin

San Antonio

Comal Springs

Barton Springs

Boerne

Edwards aquifer,

Balcones Escarpment region

B a l c o n e s E s c a r p m e n tB a l c o n e s E s c a r p m e n t

Bexar County

index well

N

Edwards

Plateau

Black Hills

region

Balcones 

Escarpment

region

EXPLANATION

Edwards aquifer surface-recharge area

Edwards aquifer below land surface (confined)

Madison aquifer surface-recharge area

Hydrogeologic unit

Minnelusa aquifer surface recharge area

Madison aquifer below land surface (confined)

Streamgage and identifier

Spring or spring complex 

 and identifier

Well and identifier

Weather station and 

 identifier

Prepared in cooperation with the Department of Interior South-Central Climate Science Center 

Effects of Projected Climate (2011–50) on Karst Hydrology and Species 

Vulnerability—Edwards Aquifer, South-Central Texas, and Madison 

Aquifer, Western South Dakota

Karst aquifers—formed by the dissolution of soluble rocks 

such as limestone—are critical groundwater resources in North 

America, and karst springs, caves, and streams provide habitat 

for unique flora and fauna. Springflow and groundwater levels 

in karst terrane can change greatly over short time scales, and 

therefore are likely to respond rapidly to climate change. How 

might the biological communities and ecosystems associated with karst respond to climate change and accompanying 

changes in groundwater levels 

and springflow?

Sites in two central U.S. regions—the Balcones Escarpment 

of south-central Texas and the Black Hills of western South 

Dakota (fig. 1)—were selected to study climate change and its 

potential effects on the local karst hydrology and ecosystem. 

The ecosystems associated with the Edwards aquifer (Balcones 

Escarpment region) and Madison aquifer (Black Hills region) 

support federally listed endangered and threatened species and 

numerous State-listed species of concern, including amphibians, 

birds, insects, and plants. Full results are provided in Stamm 

and others (2015), and are summarized in this fact sheet.

Figure 1. Vulnerability of karst-related species to climate change was 

investigated for the Edwards aquifer, Balcones Escarpment region, central 

Texas, and the Madison aquifer, Black Hills region, South Dakota.

Photos, top to bottom: American 

dipper, by Dave Menke, Fish and 

Wildlife Service; autumn willow, by 

Tab Tannery; lesser yellow lady’s 

slipper, courtesy of Smithsonian 

Institution, Richard A. Howard 

Image Collection.

Photos, top to bottom: fountain 

darter, by Gregg Eckhardt; 

Barton Springs salamander, by 

Lisa O’Donnell, City of Austin; 

Comal Springs riffle beetle, by 

Mike Quinn, TexasEnto.net.

Highlights

• Linking global and regional climate models, a hydrologic 

model, and a species vulnerability index allowed evaluation 

of the vulnerability of karst-related flora and fauna to climate 

change.

• An upward trend in air temperature is projected for weather 

stations near Edwards and Madison aquifer sites during 

2011–50; springflow is projected to decrease during 2011–50 

at Edwards aquifer sites but not at Madison aquifer sites.

• Of 16 species evaluated for the Edwards aquifer, 1 salamander is projected to be highly vulnerable to climate change, 

and 9 additional species are projected to be moderately 

vulnerable.

• Of 8 species evaluated for the Madison aquifer, 4 were predicted to be moderately vulnerable to climate change.

• Inclusion of hydrologic factors that the species depend on 

was key to evaluating the vulnerability of karst species to 

climate change.

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2014–3046 

September 2015
Linked global

and regional

climate models

Hydrologic model

(RRAWFLOW)

Climate change

vulnerability

index

Temperature

Precipitation

Groundwater level

Springflow

Stream base flow

Temperature

Precipitation

Species vulnerability to

climate change 2011–50

Figure 2. Global and regional climate models, a hydrologic 

model, and a species vulnerability index were linked to evaluate 

the vulnerability of selected karst-related species to projected 

climate change.

Model Linkage Provides the Bridge

Linking historical weather-station records, global and 

regional climate models, and a hydrologic model provides a 

bridge from global to regional to site-specific scales and provides the metrics needed to assess vulnerability of karst-related 

species to climate change (fig. 2). Climate variability was 

derived from historical weather-station observations and climate 

projections at the regional scale from the Advanced Research 

version of the Weather Research and Forecasting (WRF) model 

(Skamarock and others, 2008) downscaled from simulations of 

global climate from the Community Climate System Model, 

version 3.0 (CCSM3) (Collins and others, 2004). The RainfallResponse Aquifer and Watershed Flow (RRAWFLOW) model 

(Long and Mahler, 2013) was used to simulate springflow or 

water-table level at sites. Historical and projected climate and 

hydrologic variability at specific sites was used to quantify species vulnerability using the Climate Change Vulnerability Index 

(CCVI) (Young and others, 2012).

Climate—from Global to Regional Scales

The WRF model was used to simulate the projected climate of North America at a 36-kilometer grid spacing, using 

boundary and initial conditions from the CCSM3 and an “A2” 

emissions scenario, which is based on the assumption of a world 

of independently operating nations, continuously increasing 

population, and regionally oriented economic development 

(Nakićenović and Swart, 2000). Daily precipitation and mean, 

maximum, and minimum daily air temperature (at 2-meter 

height) were extracted for the locations of the Boerne weather 

station in Texas and the Custer and Lead weather stations in 

South Dakota (fig. 1). 

The linked CCSM3 and WRF models projected (2011–50) a 

significant upward trend in air temperature for weather stations 

in both regions, but a downward trend in precipitation for only 

the weather station in the Balcones Escarpment (Boerne). The 

distribution of mean monthly maximum and minimum air temperatures for all three weather stations showed a general upward 

shift in air temperature from 1943–2010 to 2011–50, but not 

for all months. Monthly anomalies were as large as increases 

of about 4.9 degrees Celsius (°C) (anomaly for maximum air 

temperature in May at the Lead weather station) and decreases 

of as much as 1.1 °C (anomaly for maximum air temperature in 

October at the Custer weather station and in December at the 

Boerne weather station).

Hydrology is a Key Variable

The response of local hydrology to climate change was 

explored for two springs and a well fed by the Edwards aquifer 

(Barton Springs, Comal Springs, and the Bexar County Index 

Well), and a spring-fed creek and a spring fed by the Madison 

aquifer (Spearfish Creek and Rhoads Fork Spring) (fig. 1). 

The RRAWFLOW lumped-parameter model uses input of air 

temperature and precipitation to project hydrologic output of 

groundwater level or springflow or streamflow at specific sites 

(Long and Mahler, 2013). 

Consistent with the projection of a downward trend in 

precipitation for the Balcones Escarpment, the RRAWFLOW 

model projected a significant downward trend in mean annual 

springflow or groundwater level for the three Edwards aquifer 

sites for 2011–50. Projected mean monthly springflow had 

downward trends for all months at all three Edwards aquifer 

sites, and water levels at the Bexar County Index well fell below 

that observed during the dry years of the 1950s (fig. 3). Consistent with the projection of no significant trend in precipitation at 

the Lead and Custer weather stations, the RRAWFLOW model 

projected no significant trend in springflow or streamflow for 

the two Madison aquifer sites.

Focus on the Species—Vulnerability to Projected Climate and 

Hydrologic Response

Species vulnerability to climate change was assessed using 

the CCVI, which is based on three components: exposure 

(character, magnitude, and rate of change in climatic parameters), sensitivity (physiological, biological, and ecological 

factors particular to a species), and adaptive capacity (the ability 

of the species to cope with climate change effects). For each 

species, 1 or more categorical scores are assigned to 6 measures of exposure and 16 measures of sensitivity and adaptive 

capacity. Sixteen species from the Balcones Escarpment and 

eight species from the Black Hills that span the range of broad 

taxonomic groups (such as mammal, bird, plant) were selected 

for assessment.

The 16 Balcones Escarpment species assessed occurred in 

either Comal or Barton Springs. All of the species that were 

scored as moderately to highly vulnerable are aquatic cave or 

spring species (table 1). Many of these species are negatively 

affected by extremely low flows that lead to decreased dissolved oxygen concentrations and increased water temperature (Woods and others, 2010; Gillespie, 2011; BIO-WEST, 

Inc., 2011; RECON Environmental, Inc., and others, 2012), 

or by extremely high flows that lead to habitat loss (RECON 

Environmental, Inc., and others, 2012). As a result, the use of 

projected springflow and water-table level was key for scoring 
C V

Observed air temperature

Observed precipitation

Simulated springflow or 

water-table level

Observed springflow or 

water-table level

Simulated air temperature

Simulated precipitation

EXPLANATION

A

B

1900191019201930194019501960197019801990200020102020203020402050

Year

Air temperature, 

in degrees Celsius

Water-table level, in meters

above NAVD 88

Precipitation, in millimeters

Calibration period

Validation period

C

V

190

192

194

196

198

200

202

204

206

208

210

212

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

12

13

14

15

16

17

18

19

20

21

22

23

24

Figure 3. Climate and hydrologic projections are exemplified for the Bexar County Index Well, Texas. A, The climate model for 

2011–50 projects that air temperature in the area will increase and precipitation will decrease. B, Using the projected air temperature 

and precipitation, the hydrologic model projects an overall decrease in water level at the well from 2011 through 2050. The ability of 

the RRAWFLOW model to simulate observed hydrologic response is indicated by the close match of the solid and dashed lines.

vulnerability of these species. For example, species with narrow 

thermal tolerance—such as Etheostoma fonticola, the endangered fountain darter in Comal Spring (U.S. Fish and Wildlife 

Service, 2013)—were scored with higher vulnerability where 

decreasing springflow was linked to higher water temperature.

Of the eight species assessed in the Black Hills region, four 

were scored as not vulnerable and four as moderately vulnerable to effects related to climate change (table 1). For species 

in this area, biological factors that make them sensitive to 

moisture-related climate changes are generally balanced by 

little projected change in moisture, and moderate air temperature changes ameliorate the physiological sensitivity of the 

species to heat. However, for one species with a low tolerance 

for high air temperatures—American dipper (Cinclus mexicanus)—the increasing frequency of days with a maximum air 

temperature greater than a thermal threshold (36 °C) increased 

its vulnerability.

The vulnerability of species in the two regions differs for 

at least two reasons. First, the Edwards aquifer has one of the 

most diverse and unique karst groundwater and cave faunas 

in the world (Longley, 1986) with a high level of endemism 

(the ecological state of being unique to a defined geographic 

location); of the 233 species considered for inclusion in this 

study, 75 percent were endemic. In contrast, of the 25 species 

in the Madison aquifer area considered for inclusion in this 

assessment, only 1 species (4 percent) was endemic. Endemic 

species tend to have highly restricted range, habitat, and thermal 

tolerances. Second, the hydrologic effects of climate change 

were projected by the climate models to be more severe for the 

Balcones Escarpment region than for the Black Hills region. 

The hydrologic data produced by the RRAWFLOW model 

led to increased vulnerability scores for 12 of the 16 Balcones 

Escarpment species scored. 

Evaluation of the Approach

The CCVI species assessment was enhanced by the detailed 

climate and hydrologic information produced by the coupling of 

the CCSM3, WRF, and RRAWFLOW models. The high spatial 

resolution of the WRF model climate projections is particularly 

important for regions, like the Balcones Escarpment and the 

Black Hills, that have strong climatic gradients over relatively 

small distances. The inclusion of hydrologic projections in the 

vulnerability assessment added critical information for assessing the effects of climate change on aquatic species.

References Cited

BIO-WEST, Inc., 2011, Comprehensive and critical period monitoring program to evaluate the effects of variable flow on biological 

resources in the Comal Springs/River aquatic ecosystem, final 

2010 annual report: accessed August 7, 2013, at http://www.

edwardsaquifer.org/scientific-research-and-reports/scientificreports-document-library.

Collins, W.D., Rasch, P.J., Boville, B.A., Hack, J.J., McCaa, J.R., 

Williamson, D.L., Kiehl, J.T., Briegleb, B., Bitz, C., Lin, S-J., 

Zhang, M., and Dia, Y., 2004, Description of the NCAR Community 

Atmosphere Model (CAM 3.0): NCAR Technical Note NCAR/TN464+STR, 214 p., accessed August 29, 2013, at http://www.cesm.

ucar.edu/models/atm-cam/docs/description/description.pdf, model 

output downloaded June 7, 2012, at http://www.earthsystemgrid.org.

Gillespie, J.H., 2011, Ecology and conservation of the endangered Barton Springs salamander (Eurycea sosorum): Austin, Tex., University 

of Texas at Austin, unpublished Ph.D. dissertation, 143 p.

Long, A.J., and Mahler, B.J., 2013, Prediction, time variance, and 

classification of hydraulic response to recharge in two karst aquifers: 

Hydrology and Earth System Sciences, v. 17, p. 281–294. [Also 

available at http://dx.doi.org/10.5194/hess-17-281-2013.]
Longley, G., 1986, The biota of the Edwards 

aquifer and the implications for paleozoogeography, in Abbott, P.L., and Woodruff, 

C.M., Jr., eds.,The Balcones escarpmentgeology, hydrology, ecology and social 

development in central Texas: Field Trip 

Guidebook., 1986 Geological Society of 

America Annual Meeting, San Antonio, 

Texas, p. 51–54.

Nakićenović, N., and Swart, R., eds., 2000, 

Special report on emission scenarios—A 

special report of Working Group III of 

the Intergovernmental Panel on Climate 

Change: Cambridge, United Kingdom, 

Cambridge University Press, 570 p.

RECON Environmental, Inc., Hicks and 

Co., Zara Environmental LLC, and 

BIO-WEST, 2012, Edwards Aquifer 

Recovery Implementation Program—

Habitat conservation plan: Prepared for 

Edwards Aquifer Recovery Implementation Program, Edwards Aquifer Authority, 

accessed May 21, 2013, at http://www.

fws.gov/southwest/es/Documents/R2ES/

EARIP_HCP_Final_Nov_2012.pdf.

Skamarock, W.C., Klemp, J.B., Dudhia, 

Jimy, Gill, D.O., Barker, D.M., Duda, 

M.G., Huang, X.-Y., Wang, Wei, and 

Powers, J.G., 2008, A description of the 

Advanced Research WRF version 3: 

NCAR Tech Note NCAR/TN-475+STR, 

113 p., accessed August 26, 2013, at 

http://www.mmm.ucar.edu/wrf/users/docs/

arw_v3.pdf.

Stamm, J.F., Poteet, M.F., Symstad, A.J., 

Musgrove, M., Long, A.J., Mahler, B.J., 

and Norton, P.A., 2015, Historical and 

Projected Climate (1901–2050) and 

Hydrologic Response of Karst Aquifers, 

and Species Vulnerability in SouthCentral Texas and Western South Dakota: 

U.S. Geological Survey Scientific Investigations Report 2014–5089, 59 p.

U.S. Fish and Wildlife Service, 2013, Species 

Profile: accessed February 13, 2013, at 

http://www.fws.gov/endangered/.

Woods, H.A., Poteet, M.F., Hitchings, 

P.D., Brain, R.A., and Brooks, B.W., 

2010, Conservation physiology of the 

plethodontid salamanders Eurycea nana

and E. sosorum—Response to declining 

dissolved oxygen: Copeia, v. 2010, no. 4, 

p. 540–553.

Young, B.E., Hall, K.R., Byers, E., Gravuer, 

K., Hammerson, G., Redder, A., and 

Szabo, K., 2012, Rapid assessment of 

plant and animal vulnerability to climate 

change, in Brodie, J., Post, E., and Doak, 

D., eds, Wildlife Conservation in a Changing Climate: Chicago, Ill., University of 

Chicago Press, p. 129–150.

Table 1. Species assessed with the Climate Change Vulnerability Index (CCVI), the 

vulnerability determined, and the confidence in that assessment.

Aquifer Major taxonomic group Taxon Climate vulnerability 

assessment

Confidence

Edwards amphibian Eurycea sosorum Barton Springs 

salamander Highly vulnerable Moderate

Edwards amphibian Eurycea waterlooensis

Austin blind salamander Moderately vulnerable Very high

Edwards arthropod Stygobromus pecki

Peck’s Cave amphipod Moderately vulnerable Very high

Edwards arthropod Heterelmis comalensis

Comal Springs riffle beetle Moderately vulnerable Very high

Edwards fish Etheostoma fonticola

fountain darter Moderately vulnerable Very high

Edwards amphibian Eurycea rathbuni

Texas blind salamander Moderately vulnerable Moderate

Edwards amphibian Eurycea sp. 8

Comal Springs salamander Moderately vulnerable Moderate

Edwards arthropod Haideoporus texanus

Edwards Aquifer diving beetle Moderately vulnerable Moderate

Edwards arthropod Stygobromus flagellatus

Ezell’s Cave amphipod Moderately vulnerable Moderate

Edwards arthropod Stygoparnus comalensis

Comal Springs dryopid beetle Moderately vulnerable Low

Edwards vascular plant Ehretia anacua

anacua Not vulnerable Very high

Edwards vascular plant Taxodium distichum

bald cypress Not vulnerable Very high

Edwards vascular plant Cabomba caroliniana

Carolina fanwort Not vulnerable Very high

Edwards mollusk Stygopyrgus bartonensis

Barton cavesnail Not vulnerable Moderate

Edwards arthropod Calathaemon holthuisi

Purgatory Cave shrimp Not vulnerable Low

Edwards vascular plant Platanus occidentalis

American sycamore Not vulnerable Low

Madison bird Cinclus mexicanus 

American dipper Moderately vulnerable High

Madison vascular plant Cypripedium parviflorum

lesser yellow lady’s slipper Moderately vulnerable High

Madison vascular plant Equisetum scirpoides 

dwarf scouringrush Moderately vulnerable Moderate

Madison vascular plant Salix serissima 

autumn willow Moderately vulnerable Low

Madison mammal

Zapus hudsonius campestris 

Bear Lodge meadow jumping 

mouse

Not vulnerable Very high

Madison vascular plant Asplenium trichomanes-ramosum 

green spleenwort Not vulnerable Very high

Madison mollusk Oreohelix cooperi*

Black Hills mountainsnail Not vulnerable Low

Madison mammal Castor canadensis 

American beaver Not vulnerable Low

*See Stamm and others (2015) for a discussion of taxonomy of the genus Oreohelix.

By Barbara J. Mahler,1 John F. Stamm,1 Mary F. Poteet,2 Amy J. Symstad,1

MaryLynn Musgrove,1 Andrew J. Long,1 and Parker A. Norton.1

1

U.S. Geological Survey.

2

University of Texas, Austin.

For more information: 

https://nccwsc.usgs.gov/karst

ISSN 2327-6919 (print)

ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20143046