Completed Study
Groundwater Availability Modeling of the Barton Springs Segment of the Edwards Aquifer
Figure 1. Barton Springs model region. (Click on picture for details.)

Bridget Scanlon, principal investigator

The Bureau of Economic Geology and subcontractor Barton Springs Edwards Aquifer Conservation District developed a groundwater flow model for the Barton Springs segment of the Edwards Aquifer to evaluate the effects of future pumpage and potential future droughts on groundwater availability (Fig. 1). The model covers an area of ~ 260 square miles and uses a 14,400 node grid (7,043 active nodes) with rectangular cells (500 ft x 1000 ft) to simulate the potentiometric surface and spring discharge in response to past, present, and future pumpage and potential future droughts. Dr. Robert Mace of the Texas Water Development Board ((TWDB) was involved in the early development of this model. The model was calibrated using a combination of trial and error and an inverse optimization code (UCODE).

The Barton Springs aquifer constitutes the sole source of water to about 45,000 residents. Barton Springs pool also serves as a municipal swimming pool in Zilker Park, downtown Austin. Increased population growth and recent droughts (1996) have focused attention on groundwater resources and sustainability of spring flow. The primary management issue for this aquifer is maintaining spring flow during drought periods and assessing current and future pumpage effects on spring flow. Maintaining spring flow is a critical objective because the spring outlets are the sole habitat of the Barton Springs salamander, which is listed as an endangered species.
Numerical modeling of groundwater flow in the Barton Springs Edwards aquifer presented many challenges because of the complex geology resulting from numerous faults with large vertical offsets, and the extremely dynamic nature of the aquifer as a result of large conduits. Unique aspects of the model that made it particularly suitable for modeling include:
·
stream gauge data upstream of the outcrop areas for recharge estimation
·
detailed pumping records reported by individual users of large wells collected by the BSEACD since 1989
·
daily spring flow records for ³ 100 yr for Barton Springs and water level hydrographs distributed throughout the aquifer for variable time periods up to 10 yr for comparison with simulated values
·
detailed synoptic water level maps developed by BSEACD for low and high flow periods for comparison with model simulations
No other aquifer in the state has such detailed records for model input and evaluation of model results. The information provided by these data was critical in developing a conceptual model of flow for this Barton Springs Edwards Aquifer.
Model calibration was particularly challenging because of the complex geologic structure in the model area and the dynamic nature of flow. Model parameterization included a trial and error approach based on the distribution of hydraulic heads followed by automated inverse modeling to further reduce the root mean square (RMS) error between simulated and measured heads. Steep head gradients in the outcrop area were assigned low values of hydraulic conductivity and shallow head gradients in the central and eastern part of the model were assigned high hydraulic conductivities. Highest hydraulic conductivities were assigned to a zone surrounding Barton Springs because this area should reflect the convergence of flow paths. This approach of parameterising hydraulic conductivity resulted in a parsimonious distribution of hydraulic conductivity and low RMS errors.
The transient model was run using data from 1989 through 1998 and generally reproduced the temporal variability in spring discharge with no calibration. The challenge with the model was to accurately simulate low spring discharges because these discharges are critical for using the model to predict the effect of increased pumpage and potential future droughts on spring discharge. By varying the specific yield in the outcrop area, the low spring discharges could be simulated fairly accurately. High spring discharges were generally overestimated because the model did not include reported ungauged springs that start flowing at high discharges.
To assess the impact of future pumpage and potential future droughts on groundwater availability, transient simulations were conducted using extrapolated pumpage for 10-yr periods (2001 through 2050) and average recharge for a 3-yr period and recharge from the 1950's drought for the remaining 7 yr. Results of these simulations were compared with those using average recharge and future pumpage. Simulated spring discharge in response to future pumpage under average recharge decreased proportionally to future pumpage (2 cfs per decade), whereas spring discharge decreased to 0 cfs in response to future pumpage under drought-of-record conditions. Management of water resources under potential future drought conditions should consider enhanced recharge and conservation measures.
In addition, simulations based on the distributed parameter modeling of the Barton Springs Edwards Aquifer using MODFLOW were compared to those based on a much simpler lumped parameter model developed by Barrett and Charbeneau, 1987. Results from this comparison indicated that both the distributed and lumped parameter models could adequately simulate spring discharge (Fig. 2) and well hydrographs, but that the distributed parameter model is required to simulate the potentiometric surface and to evaluate aquifer response to future pumpage.

Figure 2. Simulated and measured discharge at Barton Springs.

The results of this study demonstrate the ability of equivalent porous media distributed and lumped parameter models to simulate regional groundwater flow, which is critical for managing water resources in karst aquifers and predicting the impact of future pumping and potential future drought conditions on spring flow.

Scanlon, B. R., R. E. Mace, M. E. Barrett, and B. Smith. 2002. Can we simulate regional groundwater flow in a karst system using equivalent porous media models? Case study, Barton Springs Edwards Aquifer, USA. J. Hydrol. 276:137-158. [PDF]

July 2004