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Karst Hydrology and Ecosystem Research
Headquarter Flat Sink Research Site on City of Austin J-17 Tract
April 27 and 29, 2004
Field Trip Leaders:
Dr. Marcy Litvak, University of Texas Dept. of Integrative Biology
Nico M. Hauwert, City of Austin Watershed Protection and Development Review Department
Carbonate aquifers, such as the Edwards Aquifer, typically develop solution cavities, because the
limestone is dissolved by slightly acidic waters. The most common acid is carbonic acid,
typically forms by adding carbon dioxide to water. Carbon dioxide is naturally produced by
bacteria as they consume organic debris. As a carbonate aquifer matures, more of its surface
runoff is directed underground, leaving irregular and poorly defined surface drainages and a very
efficient internal drainage system. In many areas of the recharge zone, large and overlapping
internal drainage sinkholes insure that little surface runoff contributes to the major creeks, except
during unusually heavy rain events. In the Edwards Aquifer, solution cavity development seems
to be enhanced both along more permeable stratigraphic layers and along structural faults and
fractures. Within the aquifer, the development of conduits is sufficient for some recharging
water from the HQ Flat Sink area to travel 8 miles to Barton Springs within a day or two
(BSEACD, 2002).
A water balance of the Barton Springs segment estimates that 85% of the recharge infiltrates
within the main channels of Barton, Williamson, Slaughter, Bear, Little Bear and Onion creeks.
The remaining 15% of the recharge is believed to infiltrate in the upland tributaries and
intervening area of the recharge zone (Woodruff, 1984). This water balance was based on three
years of upstream and downstream flow measurement on most of the six major creeks (Slade,
Dorsey, and Stewart, 1986). Using this gross equal-area estimation, Veni (2000) calculated that
only 3% of the rainfall falling into a sinkhole basin would contribute recharge to the aquifer.
It is not surprising that most of the recharge to the Barton Springs segment occurs within the
major creek channels. Creek flow generated within the relatively large, upstream Contributing
Zone must flow to the recharge zone through the major creek channels. In addition, much of the
intervening upland area of the recharge zone also generates flow to the major creek channels.
However, because the 1980’s water budget was based on limited data, the recharge contribution
from the upland areas may be underestimated. A recent geochemical study by the USGS of the
Edwards Aquifer recharge sources in Bexar County found that only 44% of the recharge
originated from stream channel losses, and the remaining 56% originated as direct infiltration on
upland areas (Ockerman, 2002).
Evaporation of water and transpiration of water by plants back to the atmosphere is by far the fate
of most fallen rainfall, and with new technology both can now be directly measured. Karst
landscapes have efficiently developed internal drainage compared to other aquifers, and provide
less opportunity for extended evapotranspiration near the surface (Jennings, 1985). Furthermore,
in the upland areas of the recharge zone, infiltration occurs only during or shortly after rain
events, when the humidity is high and ET rates are low. In the karst portion of the Mammoth
Cave area, Hess and White (1974) found that 19% less evapotranspiration normally occurred
than in nonkarst portions of the same river basin due to greater recharge. The water balance can
also be improved today since recent groundwater tracing tests have better delineated the source
areas for Barton Springs and others such as Cold Springs on the south bank of the Colorado
River.
The Headquarter Flat Sinkhole Research station is located on the “J-17” City of Austin WaterQuality Protection Land. The sinkhole catchment basin is 36 acres in size and is a typical
internal drainage basin in that no surface drainages ever contributes to nearby creeks. In late
June 2002, a flow station was completed, and it now measures flow to Headquarter Flat Cave
from its major drainage channel to the west. A weir structure was used during the first year to
meter the flow, was coupled with a bubbler flow meter to record water upstream of the weir. In
summer 2003, the weir was replaced with a flume structure. Based on three quarters of data
collection, roughly 18% of the rainfall was measured to flow directly into the cave
In order to measure the amount of additional recharge that occurs within the sinkhole basin, the
evaporation and transpiration (evapotranspiration) is measured continuously using eddy
covariance. Eddy correlation relies on turbulent transfer of heat and water vapor to directly
measure both sensible and latent heat flux above the canopy. During the day, updrafts
influenced by the ground surface should be warmer and have higher water vapor concentration
(due to evaporation and transpiration from plant canopy) than downdrafts. We use instruments
placed above the canopy that can rapidly resolve very small changes in vertical wind velocity and
water vapor concentration, and calculate ecosystem evapotranspiration fluxes as the covariance
between these two variables every 30 minutes. Similarly, 30-minute sensible heat fluxes are
calculated as the covariance between vertical wind velocity and temperature derived from the
speed of sound.
Eddy flux system
Ten Hz measurements of wind speed and acoustic temperature are measured with a sonic
anemometer (CSAT-3, Campbell Scientific), while 10 Hz measurements of water vapor
concentration are made with with an open path infra-red gas analyzer (LI-7500, LI-COR). The
anemometer and gas analyzer are controlled by a micrologger (CR23X, Campbell Scientific) and
whole system is powered by solar power.
Meteorological Measurements - Air temperature and humidity is currently measured with a
temperature/humidity probe (HMP45C, Vaisala). Net radiation is measured with a net
radiometer (Q7.1, REBS). Global irradiance and photosynthetic photon flux density (PPFD) will
be measured with pyranometers and quantum sensors (LI-200SA and LI-190SA, respectively)
(we will have these instruments in 2-3 months). Wind speed and direction are measured with the
sonic anemometer and rainfall is measured with a tipping bucket rain gauge.
Soil Measurements - Most of these measurements we expect to come online in the next two
months. Soil temperatures will be measured continuously at depths of 2.5 and 10 cm using
thermocouple probes. Soil heat flux will be measured using the combination method at three
locations as well. Heat flux plates (HFT3, REBS) will be installed at a depth of 5 cm, and
storage heat flux in the 0-5 cm layer will be calculated from soil temperature measurements at
2.5 cm depth and estimates of heat capacity (Kimball and Jackson, 1979). Continuous
measurements of water content at 2.5 cm will be made with TDR probes (Campbell Scientific)
installed horizontally.
The recharge occurring within the sinkhole basin can be calculated by the following:
Discrete flow to cave + Other diffuse recharge within basin =
= Sinkhole basin rainfall - Basin evapotranspiration + Changes in soil storage
Some water is temporarily stored in the soil of the sinkhole basin, and may eventually
evapotranspirate or recharge. This change in storage can be roughly measured using soilmoisture probes. Over long periods of measurement, this change in soil-moisture storage
becomes less important in the overall budget.
This research station required the support of several groups, including the City of Austin
Watershed Protection and Development Review Department, the University of Texas
Department of Geological Sciences, the City of Austin Water/Wastewater Utility, the Barton
Springs/Edwards Aquifer Conservation District, the Texas Cave Management Association, and
Lady Bird Johnson Wildflower Center. Dr. Marcy Litvak of the UT Department of Integrative
Biology provided vital expertise and assistance in setting up and operating the evapotranspiration
station.
References
Barton Springs/Edwards Aquifer Conservation District, 2002, Summary of Groundwater Dye
Tracing Studies (1996-2002), Barton Springs Segment of the Edwards Aquifer, Texas: 6
p. Available free online at
http://www.bseacd.org/graphics/Report_Summary_of_Dye_Trace.pdf
Hess, J.W., Jr., and W.B. White, 1974, Hydrograph analysis of carbonate aquifers: Institute of
Land and Water Resources, The Pennsylvania State University Res. Pub. No. 83.
Jennings, J.N.,1985, Karst Geomorphology. Basil Blackwell, Oxford.
Ockerman, Darwin, 2002, Simulation of runoff and recharge and estimation of constituent loads
in runoff, Edwards Aquifer recharge zone (outcrop) and catchment area, Bexar County,
Texas: USGS WRI 02-4241. 31 p.
Slade, R. M., Jr., Dorsey, M. E., and Stewart, S. L., 1986, Hydrology and Water Quality of the
Edwards Aquifer Associated with Barton Springs in the Austin area, Texas: U.S.
Geological Survey Water-Resources Investigations Report 86-4036. 117 p.
Woodruff, Charles, 1984a, Water Budget Analysis for the Area Contributing Recharge to the
Edwards Aquifer, Barton Springs Segment: from Hydrogeology of the Edwards AquiferBarton Springs Segment, Travis and Hays Counties, Texas, Austin Geological Society
Guidebook 6, pp. 36-42. C. Woodruff and R. Slade coordinators.
Veni, George, 2000, Hydrogeologic Assessment of Flint Ridge Cave, Travis County, Texas:
consulting report prepared for the City of Austin, 55 p.