Karst Hydrology and Ecosystem Research

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 Ccontributing Zzone 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 Water-Quality 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 soil-moisture 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. William Asquith of the U.S. Geological Survey, Dr. Marcyi Litvak of the UT Department of Integrative Biology provided vital expertise and assistance in setting up and operating the evapotranspiration station.

References

[DRR1]

BEG, 1999

BS/EACD, 2001

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

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 Aquifer-Barton Springs Segment, Travis and Hays Counties, Texas, Austin Geological Society Guidebook 6, pp. 36-42. C. Woodruff and R. Slade coordinators.Woodruff, 1984

Veni, George, 2000, Hydrogeologic Assessment of Flint Ridge Cave, Travis County, Texas: consulting report prepared for the City of Austin, 55 p.

[DRR1]I only changed the format of this page because with the added references, it would have added another page.