GMA 13 Technical Memorandum 16-02
Draft 1GAM Predictive Scenarios 9 to 12
Supplemental Analyses in Outcrop Areas of Carrizo and Wilcox
Aquifers

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

February 23, 2016

Table of Contents
1.0

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

2.0

Tabular Summary of Averages ...................................................................................................... 2

2.0

Cross-Plots of Results ..................................................................................................................... 4

2.1
2.2

2011 Saturated Thickness vs. Drawdown ................................................................................. 4
Saturated Thickness vs. Storage Remaining ............................................................................ 6

3.0

Histograms of Drawdown ............................................................................................................... 84.0

Maps of Saturated Thickness ....................................................................................................... 10

List of Tables
Table 1. Tabular Averages – Carrizo Aquifer ........................................................................................ 2
Table 2. Tabular Averages – Wilcox Aquifer .......................................................................................... 3

List of Figures
Figure 1. Carrizo Aquifer, 2011 Saturated Thickness vs. Scenario Drawdown................................... 4
Figure 2. Wilcox Aquifer, 2011 Saturated Thickness vs. Scenario Drawdown .................................... 5Figure 3. 2011 Saturated Thickness vs 2070 Saturated Thickness – Carrizo Aquifer ........................ 6Figure 4. 2011 Saturated Thickness vs 2070 Saturated Thickness - Wilcox Aquifer .......................... 7Figure 5. Histogram of Drawdown - Carrizo Aquifer ............................................................................ 8Figure 6. Histogram of Drawdown - Wilcox Aquifer ............................................................................. 9Figure 7. 2011 Saturated Thickness of the Outcrop Area of the Carrizo Aquifer ............................. 10Figure 8. Simulated Saturated Thickness of the Outcrop Area of the Carrizo Aquifer in 2070
(Scenario 9) ................................................................................................................................................ 11
Figure 9. 2011 Saturated Thickness of the Outcrop Area of the Wilcox Aquifer .............................. 12
Figure 10. Simulated Saturated Thickness of the Outcrop Area of the Wilcox Aquifer in 2070
(Scenario 9) ................................................................................................................................................ 13

GAM Predictive Scenarios 9 to 12, Supplemental Analyses in Outcrop Areas of Carrizo and Wilcox Aquifers
GMA 13 Technical Memorandum 16-02, Draft 1

1.0 Introduction
At the request of Greg Sengelmann, the model output of Scenarios 9 to 12 (covered in Technical
Memorandum 16-01) were processed further to obtain more detailed results on impacts to theoutcrop areas of the Carrizo and Wilcox aquifers.
Impacts to the outcrop area are important to consider for two reasons: 1) the shallow domestic
wells that are completed in the outcrop are more sensitive to drawdown than wells completed in
the confined portions of the aquifers, and 2) any impacts to surface water flow would occur in theoutcrop areas.
The outcrop areas of the model were identified as those cells where overlying layers were inactive(i.e. if layers 1 to 4 in a particular row and column were inactive, but layer 5 was active, it was
assumed that layer 5 outcropped at this location).
In GMA 13, there are 999 cells in the outcrop area of the Carrizo Aquifer (each cell is one squaremile). Layer 5 of the model represents the Carrizo Aquifer.
In GMA 13, there are 1,553 cells in the outcrop area of the Wilcox Aquifer (each cell is one squaremile). Layers 6, 7, and 8 of the model represent the Wilcox Aquifer.
The analysis considered the 2011 saturated thickness in each model cell (the initial condition ofthe simulations), the saturated thickness in 2070 of each model cell for each of the four simulations(Scenario 9 to 12), the drawdown in each cell between 2011 and 2070, and the percentage ofstorage remaining in each cell by dividing the saturated thickness in 2070 by the saturated
thickness in 2011 and multiplying the result by 100.
The results are summarized in the form of tables, cross-plots, histograms, and maps in thistechnical memorandum.

1GAM Predictive Scenarios 9 to 12, Supplemental Analyses in Outcrop Areas of Carrizo and Wilcox Aquifers
GMA 13 Technical Memorandum 16-02, Draft 1

2.0 Tabular Summary of Averages
The average saturated thicknesses, drawdowns, and storage remaining for the Carrizo Aquifer arepresented in Table 1. Please note that there are two columns in the table, one for all cells, and onefor cells with a 2011 saturated thickness of greater than 50 feet.
Table 1. Tabular Averages – Carrizo Aquifer

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GAM Predictive Scenarios 9 to 12, Supplemental Analyses in Outcrop Areas of Carrizo and Wilcox AquifersGMA 13 Technical Memorandum 16-02, Draft 1

The average saturated thicknesses, drawdowns, and storage remaining for the Wilcox Aquifer arepresented in Table 2. Please note that there are two columns in the table, one for all cells, and one
for cells with a 2011 saturated thickness of greater than 50 feet.
Table 2. Tabular Averages – Wilcox Aquifer

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GAM Predictive Scenarios 9 to 12, Supplemental Analyses in Outcrop Areas of Carrizo and Wilcox Aquifers
GMA 13 Technical Memorandum 16-02, Draft 1

2.0 Cross-Plots of Results
2.1

2011 Saturated Thickness vs. Drawdown

Figure 1 presents the cross-plot of 2011 saturated thickness for each cell of the Carrizo Aquifer vsthe drawdown in the Carrizo Aquifer for Scenarios 9 to 12. Please note that the in many areas,
there is no substantial difference between the scenarios, and in other areas, the drawdown isgreatest for Scenario 9 and the least for Scenario 12 even though the changes in the scenarios arein the pumping in the Wilcox Aquifer.

Figure 1. Carrizo Aquifer, 2011 Saturated Thickness vs. Scenario Drawdown

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GAM Predictive Scenarios 9 to 12, Supplemental Analyses in Outcrop Areas of Carrizo and Wilcox Aquifers
GMA 13 Technical Memorandum 16-02, Draft 1

Figure 2 presents the cross-plot of 2011 saturated thickness for each cell of the Wilcox Aquifer vsthe drawdown in the Wilcox Aquifer for Scenarios 9 to 12. Note that there are areas where thedrawdown is substantially less in Scenario 12 as compared to Scenario 9 due to difference inWilcox Aquifer pumping. For example, there is a collection of five cells with a 2011 saturated
thickness of about 400 feet. Scenario 9 drawdown is about 125 feet, Scenario 10 drawdown isabout 100 feet, Scenario 11 drawdown is about 75 feet, and Scenario 12 drawdown is about 50feet. There are also many areas where the drawdown is substantially the same in all scenarios.

Figure 2. Wilcox Aquifer, 2011 Saturated Thickness vs. Scenario Drawdown

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GAM Predictive Scenarios 9 to 12, Supplemental Analyses in Outcrop Areas of Carrizo and Wilcox Aquifers
GMA 13 Technical Memorandum 16-02, Draft 1

2.2

Saturated Thickness vs. Storage Remaining

Figure 3 presents the cross-plot of 2011 saturated thickness vs the storage remaining in the Carrizo
Aquifer in 2070 for Scenarios 9 to 12. Please note that this plot is limited to cells where the 2011
saturated thickness is at least 50 feet. Also, note that the differences between the scenarios isminor.

Figure 3. 2011 Saturated Thickness vs 2070 Saturated Thickness – Carrizo Aquifer

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GAM Predictive Scenarios 9 to 12, Supplemental Analyses in Outcrop Areas of Carrizo and Wilcox Aquifers
GMA 13 Technical Memorandum 16-02, Draft 1

Figure 4 presents the cross-plot of 2011 saturated thickness vs the storage remaining in the Wilcox
Aquifer in 2070 for Scenarios 9 to 12. Please note that this plot is limited to cells where the 2011
saturated thickness is at least 50 feet. Note that the five cells that were discussed from Figure 2also show up in Figure 4. There are five cells with 2011 saturated thickness of about 400 feet. In2070, Scenario 9 results show that 2070 saturated would be about 70 percent of 2011 saturated
thickness, but in Scenario 12, 2070 saturated thickness would be nearly 90 percent of 2011
saturated thickness. This is the result of differences in Wilcox pumping.

Figure 4. 2011 Saturated Thickness vs 2070 Saturated Thickness - Wilcox Aquifer

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GAM Predictive Scenarios 9 to 12, Supplemental Analyses in Outcrop Areas of Carrizo and Wilcox Aquifers
GMA 13 Technical Memorandum 16-02, Draft 1

3.0 Histograms of Drawdown
Figure 5 presents a histogram of cell-by-cell drawdown in the Carrizo Aquifer between 2011 and2070 for all Scenarios. Please note the large number of cells for which there was a recovery ofgroundwater levels from 2011 to 2070 (negative drawdown). This is due to the relatively dry
conditions of 2011 and the use of average recharge from 2012 to 2012 for the predictivesimulations. In areas with no increase in pumping, a slight increase in groundwater level would
be expected as a result of the recharge conditions simulated.
Note that there are only minor differences in the scenarios since the only pumping changes between
the scenarios is in the underlying Wilcox Aquifer.

Figure 5. Histogram of Drawdown - Carrizo Aquifer

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GAM Predictive Scenarios 9 to 12, Supplemental Analyses in Outcrop Areas of Carrizo and Wilcox Aquifers
GMA 13 Technical Memorandum 16-02, Draft 1

Figure 6 presents a histogram of cell-by-cell drawdown in the Wilcox Aquifer between 2011 and2070 for all Scenarios. Please note the large number of cells for which there was a recovery ofgroundwater levels from 2011 to 2070 (negative drawdown). This is due to the relatively dry
conditions of 2011 and the use of average recharge from 2012 to 2012 for the predictive
simulations. In areas with no increase in pumping, a slight increase in groundwater level would
be expected as a result of the recharge conditions simulated.
Note that there are differences in drawdown between the simulations since these scenarios werebased on different pumping in the Wilcox Aquifer. There are more cells with high drawdown(greater than 50 feet) in Scenario 9 (the highest pumping scenario). However, there are more cellswith 0 to 25 feet of drawdown in Scenario 12 (the lowest pumping scenario).

Figure 6. Histogram of Drawdown - Wilcox Aquifer

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GAM Predictive Scenarios 9 to 12, Supplemental Analyses in Outcrop Areas of Carrizo and Wilcox Aquifers
GMA 13 Technical Memorandum 16-02, Draft 1

4.0 Maps of Saturated Thickness
Figure 7 presents the saturated thickness in 2011 of the outcrop area of the Carrizo Aquifer in
2011. Figure 8 presents the simulated saturated thickness of 2070 of the outcrop area of the Carrizo
Aquifer in 2070 under Scenario 9 (the highest pumping scenario). Maps of Scenarios 10 to 12 are
not presented since there are only minor differences with Scenario 9.

Figure 7. 2011 Saturated Thickness of the Outcrop Area of the Carrizo Aquifer

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GAM Predictive Scenarios 9 to 12, Supplemental Analyses in Outcrop Areas of Carrizo and Wilcox Aquifers
GMA 13 Technical Memorandum 16-02, Draft 1

Figure 8. Simulated Saturated Thickness of the Outcrop Area of the Carrizo Aquifer in2070 (Scenario 9)

Figure 9 presents the saturated thickness in 2011 of the outcrop area of the Wilcox Aquifer in
2011. Figure 10 presents the simulated saturated thickness of 2070 of the outcrop area of theWilcox Aquifer in 2070 under Scenario 9 (the highest pumping scenario). Maps of Scenarios 10to 12 are not presented since there are only minor differences with Scenario 9.

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GAM Predictive Scenarios 9 to 12, Supplemental Analyses in Outcrop Areas of Carrizo and Wilcox Aquifers
GMA 13 Technical Memorandum 16-02, Draft 1

Figure 9. 2011 Saturated Thickness of the Outcrop Area of the Wilcox Aquifer

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GAM Predictive Scenarios 9 to 12, Supplemental Analyses in Outcrop Areas of Carrizo and Wilcox Aquifers
GMA 13 Technical Memorandum 16-02, Draft 1

Figure 10. Simulated Saturated Thickness of the Outcrop Area of the Wilcox Aquifer in2070 (Scenario 9)13