GMA 13 Technical Memorandum 16-03
Draft 1GAM Predictive Scenarios 13 and 14
Simulations to Analyze Threshold Values for Saturated Thickness in
Carrizo Aquifer and Wilcox Aquifer Outcrop Areas

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

March 28, 2016

Table of Contents
1.0

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

1.1
Background ................................................................................................................................. 1
1.2
Outcrop Areas of the Carrizo and Wilcox Aquifers in GMA 13 ............................................ 1
2.0
Outcrop Area Classification ........................................................................................................... 3
2.1
3.0
3.1
3.2
4.0
4.1
4.2
4.3

Results from Previous Scenarios ............................................................................................... 3
Scenario 13 ....................................................................................................................................... 5
Pumping Reductions from Scenario 9 Pumping ...................................................................... 5
Results .......................................................................................................................................... 6
Scenario 14 ....................................................................................................................................... 8
Pumping Reductions from Scenario 9 Pumping ...................................................................... 8
Results .......................................................................................................................................... 9
Discussion of Results ................................................................................................................... 9

List of Tables
Table 1. Summary of Outcrop Area Classification .................................................................................. 3
Table 2. Outcrop Saturated Thickness Results for Scenarios 3, 4, 8, and 9 ......................................... 4
Table 3. Scenario 13 Model Runs – Specific Strategy Reductions......................................................... 5
Table 4. Scenario 13 Model Runs – Overall Outcrop Reductions ......................................................... 6
Table 5. Scenario 13 Results ..................................................................................................................... 7
Table 6. Summary of Pumping Reductions in Scenario 14 .................................................................... 8
Table 7. Scenario 14 Results ..................................................................................................................... 9
Table 8. Summary of Saturated Thickness Minima for Scenario 9 and Scenario 14 ........................ 11

AppendicesA – Outcrop and Downdip Pumping (Scenario 9 and Reductions of 20%, 40% and 60%)
B – Scenario 9 Outcrop and Downdip Pumping Categorized by Water Quality
C – Maps Showing TDS by Model Layer

GAM Predictive Scenarios 13 and 14, Simulations to analyze Threshold Values for Saturated Thickness in
Carrizo Aquifer and Wilcox Aquifer Outcrop Areas
GMA 13 Technical Memorandum 16-03, Draft 1

1.0 Introduction
1.1

BackgroundAt a result of discussions at the February 25, 2016 GMA 13 meeting, and in accordance with ascope-of-work dated February 29, 2016, the outcrop analysis that was the subject of the Technical
Memorandum 16-02 has been extended to include simulations that were designed to maintain
threshold values of saturated thickness in the outcrop areas of the Carrizo and Wilcox aquifers in
GMA 13. The initial pumping for these simulations were the same as the values in Scenario 9(please see Technical Memoranda 16-01 and 16-02 for more details).
During the discussion at the February 25, 2016 GMA 13 meeting, a simulation that maintained a
2070 saturated thickness of 75 percent of 2011 saturated thickness was discussed. As part of thescope of work dated February 29, 2016, there was a plan to also evaluate 70 percent and 65 percent
threshold values.
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 issue of accuracy of the GAM in the outcrop area is acknowledged in thisanalysis. Comparative analyses are more appropriate given the limitations of the GAM, and strictreliance on the results is not recommended.

1.2

Outcrop Areas of the Carrizo and Wilcox Aquifers in GMA 13

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 square mile). Layers 6, 7, and 8 of themodel represent the Wilcox Aquifer.
The analysis considered the 2011 saturated thickness in each model cell (the initial condition of
the simulations), the saturated thickness in 2070 of each model cell for each of the simulations, thedrawdown in each cell between 2011 and 2070, and the percentage of storage remaining in eachcell by dividing the saturated thickness in 2070 by the saturated thickness in 2011 and multiplying
the result by 100.
Scenario 13 consisted of 22 separate model runs. Eighteen of these runs included varying
reductions in individual water management strategies from Scenario 9 (i.e. the reductions weremade one-by-one to evaluate the sensitivity of each strategy to changes in outcrop area saturated
thickness). The results showed that the saturated thickness in the outcrop area was insensitive to1

GAM Predictive Scenarios 13 and 14, Simulations to analyze Threshold Values for Saturated Thickness in
Carrizo Aquifer and Wilcox Aquifer Outcrop Areas
GMA 13 Technical Memorandum 16-03, Draft 1

individual reductions. The other four runs were to test the sensitivity of overall reductions in theoutcrop areas of the Carrizo and Wilcox aquifers separately.
Scenario 14 consisted of 16 separate model runs. Pumping reductions were applied to: 1) Carrizo
Aquifer outcrop area, 2) Carrizo Aquifer downdip area, 3) Wilcox Aquifer outcrop area, and 4)
Wilcox Aquifer downdip area.
In order to better interpret the results, the saturated thickness analysis was also applied to Scenarios
3, 4 and 8, and these results were compared to the results of the 16 runs of Scenario 14.
Before the discussion of the model runs, the method used to classify different areas within theoutcrop area is discussed below.

2

GAM Predictive Scenarios 13 and 14, Simulations to analyze Threshold Values for Saturated Thickness in
Carrizo Aquifer and Wilcox Aquifer Outcrop Areas
GMA 13 Technical Memorandum 16-03, Draft 1

2.0 Outcrop Area Classification
The outcrop area of the Carrizo and Wilcox aquifers was subdivided based on estimated 2011
saturated thickness. Table 1 summarizes the classification and the number of cells within eachclass.
Table 1. Summary of Outcrop Area Classification

Aquifer

2011 Saturated
Thickness (ft)Number ofModel Cells in
GMA 13Carrizo
Carrizo
Carrizo
Carrizo
Carrizo
Wilcox
Wilcox
Wilcox
Wilcox
Wilcox0 to 50
50 to 100
100 to 250
250 to 500> 500
0 to 50
50 to 100
100 to 250
250 to 500
> 500

475
131
260
121
12
860
168
291
177
62

Model output was processed to calculate the saturated thickness in 2070 for each of these classes.
The 2070 saturated thickness was then divided by the saturated thickness in 2011 (Table 1) andmultiplied by 100 to develop an estimate of the saturated thickness remaining in 2070 as apercentage of the saturated thickness in 2011 for each class.

2.1

Results from Previous Scenarios

At the October 9, 2013 GMA 13 meeting, GMA 13 reviewed the initial seven GAM simulations.
Scenarios 1 to 7 were developed based on the previous DFC and MAG. Scenario 4 was a “basecase” which used the pumping in the previous MAG and specific input from SAWS and GCDs.
Scenarios 1, 2 and 3 represented reductions from the base case, and Scenarios 5, 6, and 7
represented increases from the base case to provide an overall understanding of the changes indrawdown to changes in pumping. In terms of total pumping, Scenario 3 was most similar to thecurrent MAG.
Scenario 8 was completed and presented at the March 13, 2014 GMA 13 meeting. Scenario 8 wasdeveloped on specific changes to Scenario 4 by individual groundwater conservation districts.

3

GAM Predictive Scenarios 13 and 14, Simulations to analyze Threshold Values for Saturated Thickness in
Carrizo Aquifer and Wilcox Aquifer Outcrop Areas
GMA 13 Technical Memorandum 16-03, Draft 1

Scenario 9 was completed and presented at the January 22, 2016 GMA 13 meeting, and was
documented in Technical Memorandum 16-01. This scenario included all Region L water
management strategies (recommended and alternative).
To provide context to the results of Scenarios 13 and 14 below, the results of Scenarios 3, 4, 8, and9 are summarized in Table 2.
Table 2. Outcrop Saturated Thickness Results for Scenarios 3, 4, 8, and 9

Please note that even under Scenario 3 (the lowest pumping of the scenarios presented), the 75
percent threshold cannot be maintained in all areas. A threshold of 70 percent can be maintainedin Scenarios 3 and 8, but not in Scenario 4 or 9. A threshold of 65 percent can be maintained in
Scenarios 3, 4, and 9, but not in Scenario 9. Scenario 9 results in maintaining 65 percent in all but
one class (Carrizo 250 to 500 feet).

4GAM Predictive Scenarios 13 and 14, Simulations to analyze Threshold Values for Saturated Thickness in
Carrizo Aquifer and Wilcox Aquifer Outcrop Areas
GMA 13 Technical Memorandum 16-03, Draft 1

3.0 Scenario 13
3.1

Pumping Reductions from Scenario 9 Pumping

Scenario 13 consisted of 18 runs that tested the sensitivity of individual reductions in specific water
management strategies that were included in Scenario 9. A summary of these runs are presented
in Table 3.
Table 3. Scenario 13 Model Runs – Specific Strategy Reductions
Model
Run

Water Management Strategy

Aquifer

Pumping
Reduction from
Scenario 9 (%)

13-1
13-2
13-3
13-4
13-5
13-6
13-7
13-8
13-9
13-10
13-11
13-12
13-13
13-14
13-15
13-16
13-17
13-18

SSLGC Expanded Carrizo Project
CVLGC Carrizo Project
CRWA Wells Ranch - Phase 2
SAWS Expanded Local Carrizo
Hays/Caldwell PUA Project
TWA Carrizo Project
SSLGC Expanded Carrizo Project
CVLGC Carrizo Project
CRWA Wells Ranch - Phase 2
SAWS Expanded Local Carrizo
Hays/Caldwell PUA Project
TWA Carrizo Project
SSLGC Brackish Wilcox
Brackish Wilcox for SS WSC
Brackish Wilcox Groundwater for CRWA
SSLGC Brackish Wilcox
Brackish Wilcox for SS WSC
Brackish Wilcox Groundwater for CRWA

Carrizo
Carrizo
Carrizo
Carrizo
Carrizo
Carrizo
Carrizo
Carrizo
Carrizo
Carrizo
Carrizo
Carrizo
Wilcox
Wilcox
Wilcox
Wilcox
Wilcox
Wilcox

10
10
10
10
10
10
20
20
20
20
20
20
10
10
10
20
20
20

The final four runs of Scenario 13 included the overall reduction in pumping in the outcrop areas of the
Carrizo and Wilcox aquifers. Table 4 summarizes the pumping reductions for these runs.

5

GAM Predictive Scenarios 13 and 14, Simulations to analyze Threshold Values for Saturated Thickness in
Carrizo Aquifer and Wilcox Aquifer Outcrop Areas
GMA 13 Technical Memorandum 16-03, Draft 1

Table 4. Scenario 13 Model Runs – Overall Outcrop Reductions

3.2

Pumping
Reduction from
Scenario 9 (%)

Model
Run

Description

13-19
13-20
13-21
13-22

Reduction in Carrizo Aquifer Outcrop Area Pumping
Reduction in Carrizo Aquifer Outcrop Area Pumping
Reduction in Wilcox Aquifer Outcrop Area Pumping
Reduction in Wilcox Aquifer Outcrop Area Pumping

10
20
10
20

Results

Results from the runs are summarized in Table 5, which presents that saturated thickness in 2070
as a percentage of the saturated thickness for each sub area of the outcrop. The results demonstrate
the overall insensitivity to reducing pumping in individual water management strategies. Thisappears to be due to the relatively small reductions in outcrop area pumping by considering any
individual strategy. For reference purposes, the results of Scenario 9 are also presented.
Please note that as presented in Table 5, the areas with less than 50 feet of saturated thickness in
2070 are higher than in 2011. This is likely due to less pumping due to the general lack of saturated
thickness, and the overall increase in recharge from 2011 (a relatively dry year) to the assumed
average recharge simulated from 2012 to 2070.
Also in Table 5, it can be seen that the thicker portion of the outcrop of the Wilcox are above the75 percent threshold value discussed at the February 25, 2016 GMA 13 meeting. In contrast, thethicker portions of the Carrizo outcrop are less than 70 percent in all runs.
Based on these results, Scenario 14 was developed to provide results that would be useful to thegroundwater conservation districts in GMA 13 to evaluate the balance between maintaining thesaturated thickness of the outcrop area above a certain threshold and the development ofgroundwater that satisfies all recommended and alternative strategies in the most recent Regional
Water Plan.

6

GAM Predictive Scenarios 13 and 14, Simulations to analyze Threshold Values for Saturated Thickness in
Carrizo Aquifer and Wilcox Aquifer Outcrop Areas
GMA 13 Technical Memorandum 16-03, Draft 1

Table 5. Scenario 13 Results

7

GAM Predictive Scenarios 13 and 14, Simulations to analyze Threshold Values for Saturated Thickness in
Carrizo Aquifer and Wilcox Aquifer Outcrop Areas
GMA 13 Technical Memorandum 16-03, Draft 1

4.0 Scenario 14
4.1

Pumping Reductions from Scenario 9 Pumping

Based on the results of Scenario 13, Scenario 14 was designed to simulate the effects of pumping
reductions of a larger scale and over a wider area than those in Scenario 13. Scenario 14 consistedof 16 runs of the model with pumping reductions in the Carrizo outcrop, Carrizo downdip, Wilcox
outcrop, and Wilcox downdip areas as summarized in Table 6. As with Scenario 13, initial
pumping was Scenario 9 pumping.
Table 6. Summary of Pumping Reductions in Scenario 14

Model
Run
14-1
14-2
14-3
14-4
14-5
14-6
14-7
14-8
14-9
14-10
14-11
14-12
14-13
14-14
14-15
14-16

Pumping Reduction from Scenario 9 (percent)Carrizo
Carrizo
Wilcox
WilcoxOutcrop
Downdip
Outcrop
Downdip
20
0
0
0
40
0
0
0
60
0
0
0
0
20
0
0
0
40
0
0
0
60
0
0
0
0
20
0
0
0
40
0
0
0
60
0
0
0
0
20
0
0
0
40
0
0
0
60
20
20
0
0
40
40
0
0
0
0
20
20
0
0
40
40

The actual pumping amounts by county and model layer for Scenario 9, and broken out by outcrop
and downdip area are presented in Appendix A. Appendix A also includes the pumping with 20
percent, 40 percent and 60 percent reductions.
Appendix B presents the outcrop and downdip pumping totals for Scenario 9 classified by water
quality (<1,000 mg/l TDS, between 1,000 and 3,000 mg/l TDS, and > 3,000 mg/l TDS) by county
and model layer. Maps of the water quality were previously developed and presented at the GMA
13 meeting of November 19, 2015, and are reproduced in Appendix C.
8GAM Predictive Scenarios 13 and 14, Simulations to analyze Threshold Values for Saturated Thickness in
Carrizo Aquifer and Wilcox Aquifer Outcrop Areas
GMA 13 Technical Memorandum 16-03, Draft 1

4.2

Results

The results of the 16 runs of Scenario 14 are summarized in Table 7, which presents that saturated
thickness in 2070 as a percentage of the saturated thickness for each sub area of the outcrop.
Scenario 9 results are also included for comparison.
Table 7. Scenario 14 Results

Please note that none of the runs result in 75 percent or greater saturated thickness remaining in
all areas. However, there are five runs (14-10, 14-11, 14-12, 14-15, and 14-16) where the saturated
thickness is 70 percent or greater. All the runs result in 65 percent or greater saturated thickness.
In contrast, the results of Scenario 9 show that in the Carrizo outcrop area with a saturated thicknessof between 250 and 500 feet, the 2070 saturated thickness would be about 61 percent of the 2011
saturated thickness. All other areas (classes) would have greater than 65 percent.

4.3

Discussion of Results

As noted above, maintaining a 75 percent threshold was not maintained in any of the scenarios,even the one that is closest to the current MAG (Scenario 3). Although it is possible to craft apumping scenario that could meet the 75 percent threshold, due to model limitations, it would notnecessarily be useful to the development of desired future conditions.
9GAM Predictive Scenarios 13 and 14, Simulations to analyze Threshold Values for Saturated Thickness in
Carrizo Aquifer and Wilcox Aquifer Outcrop Areas
GMA 13 Technical Memorandum 16-03, Draft 1

Attention needs to be focused on the range of changes in saturated thickness in each of theclassifications. As noted previously, the shallow portions of the outcrop area (where 2011
saturated thickness is less than 50 feet) shows a consistent rise in all scenarios. As discussed
earlier, this is likely due to increased recharge from 2011 (a dry year) to the average recharge
conditions from 2012 to 2070 that were assumed in the predictive simulations. Due to the lack ofsaturated thickness, this is also an area with minimal pumping.
In contrast, the thicker areas (greater than 50 feet in 2011) show various ranges of saturated
thickness remaining. In the thicker portions of the Wilcox Aquifer outcrop (greater than 250 feet),the results of Scenario 14 show that in 2070, over 85 percent of that saturated thickness will remain,even in Scenario 9.
Based on these scenarios, Scenario 9 is consistent with a minimum threshold value of 60 percent.
Scenario 9 is also consistent with a minimum threshold value of 60 percent in the Carrizo outcrop
and a minimum threshold value of 65 percent in the Wilcox outcrop.
The minimum values of saturated thickness in 2070 from Table 7 are summarized in Table 8. The
minimum of all Carrizo outcrops is in the column labeled Carrizo, the minimum of all Wilcox
outcrops is in the column labeled Wilcox, and the overall minimum is in the column Carrizo andWilcox.
Note that, in runs 14-1, 14-2 and 14-3 (reductions in Carrizo outcrop pumping) the impact tosaturated thickness is less than reductions in Carrizo downdip pumping (runs 14-4, 14-5 and 146). Although the percentage reductions were the same (20, 40 and 60 percent), the total pumping
reductions were larger in the downdip scenarios due to the larger pumping. A similar observation
is made for the Wilcox Aquifer (runs 14-7 to 14-9 versus 14-10 to 14-12).
Based on the results of Scenario 14, if the DFC were to be set to maintain a minimum outcrop
saturated thickness in 2070 of 60 percent of 2011 saturated thickness, Scenario 9 would be
acceptable.
Based on the results of Scenario 14, if the DFC were to be set to maintain a minimum outcrop
saturated thickness of 2070 of 70 percent of 2011 saturated thickness, Scenario 9 pumping in theoutcrop area of the Carrizo and the Wilcox would have to be reduced by at least 20 percent, or
pumping in the downdip area would have to be reduced by at least 20 percent.

10GAM Predictive Scenarios 13 and 14, Simulations to analyze Threshold Values for Saturated Thickness in
Carrizo Aquifer and Wilcox Aquifer Outcrop Areas
GMA 13 Technical Memorandum 16-03, Draft 1

Table 8. Summary of Saturated Thickness Minima for Scenario 9 and Scenario 14

Run

9
14-1
14-2
14-3
14-4
14-5
14-6
14-7
14-8
14-9
14-10
14-11
14-12
14-13
14-14
14-15
14-16

Minimum Outcrop Saturated Thickness in 2070(% of 2011 Saturated Thickness)
Carrizo

Wilcox

61
71
72
74
75
80
86
69
69
70
70
71
71
77
84
70
71

66
69
69
69
69
69
70
70
71
73
71
74
78
69
69
72
77

Carrizo andWilcox
61
69
69
69
69
69
70
69
69
70
70
71
71
69
69
70
71

Due to the limitations of the GAM, it would be unreasonable to draw precise conclusions or selectprecise numerical standards for establishment of desired future conditions. The model results areuseful to demonstrate that reductions in pumping in the downdip area can result in changes to thesaturated thickness in the outcrop area. This is likely the direct result of pumping in the downdip
area that is close to the outcrop/downdip boundary. The implication of this result is that, if themaintenance of the outcrop saturated thickness is a policy goal, pumping in the outcrop area andin the downdip area near the outcrop/downdip boundary should be subject to closer review thanpumping in downdip areas well removed from the outcrop area.
The comparison of the results of Scenario 14 runs with Scenario 3 (the scenario closest to thecurrent MAG and DFC) show that a 75 percent threshold is not attainable unless further pumping
reductions to the current MAG are made. Under Scenario 9, the largest changes from Scenario 3
are in the outcrop areas of the Carrizo and Wilcox where the 2011 saturated thickness 100 to 500
feet:
11

GAM Predictive Scenarios 13 and 14, Simulations to analyze Threshold Values for Saturated Thickness in
Carrizo Aquifer and Wilcox Aquifer Outcrop Areas
GMA 13 Technical Memorandum 16-03, Draft 1In Scenario 3, in the Carrizo Aquifer outcrop, the saturated thickness in 2070 would be
about 73 percent of 2011 saturated thickness in Scenario 3 as compared to about 64 percent
in Scenario 9.
In Scenario 3, in the Wilcox Aquifer outcrop, the saturated thickness in 2070 would be
about 92 percent of the 2011 saturated thickness in Scenario 3 as compared to about 81
percent in Scenario 9.

This comparison is a key issue for the groundwater conservation districts in GMA 13. The
groundwater conservation districts in GMA 13 must balance the highest practicable use ofgroundwater and the conservation of groundwater. The districts must also consider the water needs
as expressed in the regional water plan. The limitations of the model must also be taken into
consideration when evaluating the saturated thickness reductions associated with Scenario 9, andthe improvements of saturated thickness associated with pumping reductions simulated in Scenario
14.
Groundwater conservation districts are obligated to manage to meet the desired future condition.
However, the MAG is an important component of regional water planning. Desired future
conditions are updated every five years, and it is possible that an improved Groundwater
Availability Model will be in use for the next joint planning cycle (i.e. before the next proposed
DFCs are due in 2021).
Based on the results of the comparative analysis completed, the drawdowns and saturated thickness
results from Scenario 9 could be considered for a proposed desired future condition. Simulations
that considered pumping reductions from Scenario 9 could also be used to form the basis of a DFC.
Based on the simulations, it can be seen that substantial reductions would be needed to achieve the75 percent threshold (2070 saturated thickness is 75 percent of 2011 saturated thickness in theoutcrop area). Even setting a 70 percent minimum threshold would require relatively large
reductions in pumping as compared to Scenario 9.
Given the model limitations, it is difficult to assess the quantitative accuracy the results. It is
intuitively obvious that a reduction in pumping will result in more saturated thickness in 2070. It
is also notable that reductions in downdip pumping will improve the saturated thickness conditionsof the outcrop area. However, critics of the model will suggest that the model is unreliable andover-predicts drawdown in the outcrop area (an argument that is not completely true, but isnevertheless essentially true).
If the model over-predicts the drawdown in the outcrop area, then it can be reasoned that thesaturated thickness of the outcrop area in 2070 would be more than that predicted by the model.
In Scenario 9, the model predicts a minimum of 61 percent in one class (Carrizo Aquifer, initialsaturated thickness between 250 and 500 feet), and over 65 percent in all other areas.

12Appendix A
Outcrop and Downdip Pumping(Scenario 9 and Reductions of 20%, 40% and 60%)Appendix B
Scenario 9 Outcrop and Downdip Pumping Categorized by
Water Quality

Appendix C
Maps Showing TDS by Model Layer

Layer 1

Layer 2

Layer 3

Layer 4

Layer 5

Layer 6

Layer 7

Layer 8