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Figure
1. Barton Springs model region. (Click on picture for details.)
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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).
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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: |
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stream
gauge data upstream of the outcrop areas for recharge estimation |
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detailed
pumping records reported by individual users of large wells
collected by the BSEACD since 1989 |
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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 |
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detailed
synoptic water level maps developed by BSEACD for low and high
flow periods for comparison with model simulations |
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| 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. |
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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.
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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] |
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July
2004
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