Climate Change, Karst Springs and Rare Species: Edwards and Madison Aquifers, 2011–50

Karst aquifers form where water dissolves soluble rock such as limestone. They are vital groundwater sources in North America, and their springs, caves and streams are home to unique plants and animals. Because springflow and groundwater levels in karst can change sharply over short periods, they are likely to respond quickly to climate change. How will the life that depends on them fare?

The U.S. Geological Survey, with the Interior Department's South-Central Climate Science Center, studied two regions: the Balcones Escarpment of south-central Texas, fed by the Edwards aquifer, and the Black Hills of western South Dakota, fed by the Madison aquifer. Both support federally listed endangered and threatened species and many state species of concern — amphibians, birds, insects and plants.

Highlights

• Linking global and regional climate models, a hydrologic model and a species vulnerability index made it possible to judge how karst species will fare.
• Air temperature is projected to rise near both aquifers in 2011–50; springflow is projected to fall at the Edwards sites but not at the Madison sites.
• Of 16 Edwards species, one salamander is projected to be highly vulnerable and nine more moderately vulnerable.
• Of 8 Madison species, four are projected to be moderately vulnerable.
• Including the water conditions species depend on was key.

From global climate to a single spring

Historical weather records were combined with the Community Climate System Model (CCSM3) downscaled to a 36-kilometre grid over North America by the Weather Research and Forecasting (WRF) model, under an "A2" emissions scenario — a world of independent nations, steadily growing population and regional economic development. The RRAWFLOW model turned air temperature and precipitation into springflow or water levels at particular sites, and the Climate Change Vulnerability Index (CCVI) scored each species.

Climate. The models projected a significant warming trend at the weather stations in both regions for 2011–50, but a drying trend only at Boerne, on the Balcones Escarpment. Compared with 1943–2010, monthly maximum and minimum temperatures generally shifted upward, though not in every month — from about 4.9 °C warmer (May maximums at Lead) to as much as 1.1 °C cooler (October at Custer and December at Boerne).

Water. At the Edwards sites — Barton Springs, Comal Springs and the Bexar County Index Well — the models projected a significant decline in springflow and groundwater level, in every month, with water in the index well falling below the levels of the 1950s drought. At the Madison sites — Spearfish Creek and Rhoads Fork Spring — they projected no significant trend.

The species

The CCVI combines exposure to climate change, sensitivity and adaptive capacity, scoring 6 measures of exposure and 16 of sensitivity and adaptability. Species from a range of groups were chosen: 16 from Comal or Barton springs and 8 from the Black Hills.

In Texas, every species scored moderately or highly vulnerable lives in caves or springs. Many suffer when very low flows cut dissolved oxygen and warm the water, or when very high flows wash away habitat — so projected springflow was central to their scores. The endangered fountain darter of Comal Springs, for instance, has a narrow temperature tolerance and scored higher where falling springflow meant warmer water.

In the Black Hills, species sensitive to moisture are balanced by little projected change in moisture, and moderate warming eases their heat sensitivity. The exception is the American dipper, which tolerates heat poorly: more days above its threshold of 36 °C raised its vulnerability.

Why the regions differ

First, the Edwards aquifer has one of the world's most diverse and unique karst groundwater and cave faunas, with a high level of endemism: of 233 species considered, 75 percent were found nowhere else, against 1 of 25 (4 percent) in the Madison area. Endemic species tend to have narrow ranges, habitats and temperature tolerances. Second, the models projected harsher hydrologic effects for the Balcones Escarpment. The water projections raised the vulnerability scores of 12 of the 16 Texas species.

What the approach adds

Coupling the climate and hydrologic models gave the vulnerability index far more detailed information. The WRF model's fine resolution matters in regions like these, where climate changes sharply over short distances, and adding water projections was critical for judging aquatic species.

Sources

• Mahler, B.J., Stamm, J.F., Poteet, M.F., Symstad, A.J., Musgrove, M., Long, A.J., and Norton, P.A., 2015, Effects of projected climate (2011–50) on karst hydrology and species vulnerability — Edwards aquifer, south-central Texas, and Madison aquifer, western South Dakota: U.S. Geological Survey Fact Sheet 2014–3046. https://pubs.usgs.gov/publication/fs20143046
• Full results: Stamm and others (2015).
• Figures: U.S. Geological Survey. The species photographs framing the sheet's map are mostly by outside photographers and institutions and are left out, and the map is cropped to exclude them.
• Rewritten in hubnx's own words.