GMA 13 Technical Memorandum 16-04
Draft 2

Yegua-Jackson Aquifer: GAM Predictive Simulations

Prepared for:

Groundwater Management Area 13

Prepared by:William R. Hutchison, Ph.D., P.E., P.G.
Independent Groundwater Consultant
9305 Jamaica Beach
Jamaica Beach, TX 77554
512-745-0599
billhutch@texasgw.com

May 9, 2016

Table of Contents
1.0

Introduction ..................................................................................................................................... 1

2.0

Simulation Results .......................................................................................................................... 3

3.0

References ........................................................................................................................................ 4

List of Tables
Table 1. Estimates of Historic Pumping and Simulated Pumping from Oliver (2010)........................ 1
Table 2. Assumed Pumping for Updated Simulation ............................................................................. 2
Table 3. Summary of Average Drawdown and Output Pumping ......................................................... 3

Yegua-Jackson Aquifer: GAM Predictive Simulations
GMA 13 Technical Memorandum 16-04, Draft 2

1.0 Introduction
The Yegua-Jackson Aquifer is a minor aquifer within the boundaries of eight counties of GMA
13. In 2010, GMA 13 adopted desired future conditions for the Yegua-Jackson after considering
the results of ten alternative GAM simulations using the Groundwater Availability Model (GAM)
for the Yegua-Jackson Aquifer. The GAM is documented in Deeds and others (2010), and the ten
alternative simulations are documented in Oliver (2010).
As part of its process to consider updated proposed desired future conditions, this technicalmemorandum supplements the analysis of Oliver (2010) by modifying the pumping in Gonzales
and Karnes counties to account for increased use by the oil and gas industry, and extending thetime period of the simulations to 2070.
Table 1 presents the 2012 estimated pumping (from the TWDB pumping data base) organized bycounty, and the simulated pumping for each county used in Scenario 4.0 of Oliver (2010). The
simulated pumping became the basis for the modeled available groundwater (MAG) for the YeguaJackson Aquifer in GMA 13. It should be noted that TWDB’s estimated pumping in 2012 assumed
no pumping in the mining category (i.e. no groundwater pumping associated with oil and gas).
Evergreen UWCD and Gonzales UWCD provided updated oil and gas pumping estimates for2012, 2013 and 2014.
Table 1. Estimates of Historic Pumping and Simulated Pumping from Oliver (2010)

County2012
EstimatedPumping(TWDB)

SimulatedPumpingin 2060(Oliver,2010)

AtascosaGonzalesKarnes
La SalleMcMullenWebb
WilsonZapata

396
687
271
54294177
159

856
975
776
92
180
19,999840
8,000

In Gonzales County, groundwater pumping from the Yegua-Jackson Aquifer for oil and gas
activities peaked at 2,500 AF/yr. Oil and gas related groundwater pumping in Karnes Countypeaked at 1,741 AF/yr according to data provided by the UWCD.

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Yegua-Jackson Aquifer: GAM Predictive Simulations
GMA 13 Technical Memorandum 16-04, Draft 2

After reviewing monitoring well data and initial model runs, Gonzales County UWCD requested
that pumping be increased to achieve a drawdown of 3 feet. This required adding an additional
1,500 AF/yr of pumping in Gonzales County (above the 2,500 AF/yr historic oil and gas relatedpumping). These amounts were added to the TWDB estimated historic use pumping estimates toupdate the simulation in Oliver (2010).
Table 2 summarizes the pumping that was used for the simulation described in this technicalmemorandum.
Table 2. Assumed Pumping for Updated Simulation

CountyAssumed Pumpingfor Updated
Simulation (AF/yr)

AtascosaGonzales
Karnes
La Salle
McMullenWebbWilsonZapata

856
4,687
2,01292180
19,999840
8,000

The model input file from Oliver (2010) for his Scenario 4.0 was used to develop the pumping
input file for this simulation by increasing pumping in Gonzales County by a factor of 4.81, andincreasing pumping in Karnes County by a factor of 2.59. In addition, an additional 10 years wasadded to the simulation to extend it to 2070.

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Yegua-Jackson Aquifer: GAM Predictive Simulations
GMA 13 Technical Memorandum 16-04, Draft 2

2.0 Simulation Results
The simulation results were processed and average drawdown was calculated for each county, andtotal pumping was obtained from the model output file (i.e. the cbb file). The Yegua-Jackson
GAM is somewhat unique in that the assignment of aquifer does not necessarily conform tospecific model layers like most GAM’s in Texas. The model’s grid file was used to define areaused in the drawdown calculations.
The model grid file includes a specification labeled “ib”, which designates the aquifer unit for aparticular model cell (designated by its layer, row, column) as follows:0 = Inactive cell
1 = Catahoula
2 = Upper Jackson
3 = Lower Jackson
4 = Upper Yegua
5 = Lower Yegua
6 = Conduit cells

The conduit cells (and the inactive cells) were not included in the calculation of drawdown.
Drawdowns in all cells labeled 1 to 5 in the ib array were summed for each county and divided by
the total number of cells labeled 1 to 5 in the ib array for each county. Pumping was also summed
in a similar fashion (i.e. summation of all calls labeled 1 to 5 in the ib array for each county).
The results are summarized in Table 3. The current DFC is also shown in Table 3.
Table 3. Summary of Average Drawdown and Output Pumping

CountyCurrent DFC(from 2010 to2060)

Atascosa
GonzalesKarnes
La Salle
McMullen
Webb
WilsonZapata

0
1
100
31
3

Average
Drawdown(ft) from2010 to 2070
03100
4133Pumping(AF/yr)
856
4,710
2,057
92
179
19,986827
7,982

Yegua-Jackson Aquifer: GAM Predictive Simulations
GMA 13 Technical Memorandum 16-04, Draft 2

3.0 References
Deeds, N.E., Yan, T., Singh, A., Jones, T., Kelley, V.A., Knox, P.R., and Young, S.C., 2010. Final
Report, Groundwater Availability Model for the Yegua-Jackson Aquifer. Prepared for the Texas
Water Development Board, March 2010, 582p.
Oliver, W., 2010, GAM Task 10-012 Model Run Report. Texas Water Development Board,
Groundwater Availability Modeling Section, August 9, 2010, 48p.

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