This document discusses estimated recoverable storage (ERS), which is defined as the estimated amount of groundwater that could potentially be recovered from an aquifer between 25-75% of the aquifer's volume. It notes that ERS does not necessarily indicate how much water is available for production or can be withdrawn without harming the aquifer. While ERS indicates the total water in an aquifer, the document cautions that actual recoverability depends on aquifer type, water quality impacts, subsidence risks, and other factors. It concludes that ERS should be understood in context of local aquifer characteristics to determine its relevance for groundwater planning and management.

1. Estimated Recoverable Storage
What it does, doesn’t and might mean for planning
Wade A. Oliver, P.G.
Texas Alliance of Groundwater Districts
Quarterly Meeting
February 26, 2014
Disclaimer: The materials presented here
were developed for informational
purposes, are considered preliminary, and
should not be used without consulting the
author.

2. The “9 Factors” Districts Shall Consider When
Adopting Desired Future Conditions
Paraphrased Factors in Texas Water Code Sec. 36.108(d) :
1. Aquifer uses or conditions…
2. Water supply needs and management strategies…
3. Hydrological conditions, including for each aquifer in the
management area the total estimated recoverable storage
as provided by the executive administrator [of TWDB]…
4. Other environmental impacts
5. Impact on subsidence
6. Socioeconomic impacts
7. Impact on private property rights
8. Feasibility of achieving the DFC
9. Any other relevant information

3. Definition
Total Estimated Recoverable Storage—The estimated
amount of groundwater within an aquifer that accounts
for recovery scenarios that range between 25% and
75% of the porosity-adjusted aquifer volume
Texas Administrative Code Sec. 356.10

4. Estimated Recoverable Storage in the News
From the Austin American Statesman
In his remarks, [the county judge] cited the Texas Water
Development Board’s recent survey indicating the Simsboro
Aquifer contains some 50 million acre-feet of water, 25 percent
of which, according to the board, is easily rechargeable and
available for export without harming the aquifer.
“There being more than 11 million exportable acre-feet of water
available means the 45,000 acre-feet reserved by Hays County is
but a small percentage — 0.04 percent — of the available water
in Lee and Bastrop counties,” [he] said.

5. Estimated Recoverable Storage in the News
GAM Task 13-035

6. Types of Aquifers
Unconfined Aquifer
Confining Unit
Confined Aquifer
Confining Unit

7. One Aquifer – Two Types
Unconfined
Confined
Northwest
Southeast
Measured
Water Level

8. Unconfined vs. Confined Storage
Specific Yield
>
Storativity
Takeaway: Each foot of drawdown yields much more water
when an aquifer is unconfined than when it is confined.
From Heath (1983)

9. Guidance from
TWDB
Typical County Example
30 miles x 30 miles
900 square miles
500 feet
576,000 acres
15%

10. Guidance from
TWDB
Typical County Example
30 miles x 30 miles
900 square miles
500 feet
576,000 acres
15%
=
43.2 million
acre-feet

11. Guidance from
TWDB
Typical County Example
30 miles x 30 miles
900 square miles
500 feet
15% =
576,000 acres
1000 feet 0.0001 =
Unconfined Portion:
Specific Yield
Confined Portion:
Storativity
43.2 million
acre-feet
0.06 million
acre-feet

12. Guidance from TWDB
Total Estimated Recoverable Storage and Modeled
Available Groundwater, Why They Are Different
Presentation by TWDB available at:
http://www.twdb.texas.gov/groundwater/docs/TotalEst
imatedRecoverableStorage.pdf

18. MAGs and Storage by GMA
Total Estimated
Total MAG over
Recoverable Storage
MAG in 2060
50 Years
Total MAG as
GMA
(million acre-feet) (million acre-feet) (million acre-feet) Percent of TERS
1
588
2.270
150
25%
2
968
1.344
94
10%
3
476
0.461
23
5%
4
160
0.207
10
6%
5
Not Defined
Not Defined
Not Defined
Not Defined
6
180
0.422
22
12%
7
447
0.648
33
7%
8
1,628
0.386
19
1%
9
33
0.096
5
15%
10
46
0.100
5
11%
11
Not Defined
0.543
27
Not Defined
12
1,380
0.337
15
1%
13
2,756
0.485
24
1%
14
Not Defined
0.907
47
Not Defined
15
443
0.488
24
6%
16
2,205
0.358
18
1%
Total
11,310
7.603
442
4%
(excluding GMAs 5,11,14)

19. Sometimes Caveats Are Important
• No consideration given to:
– Aquifer water quality
– Water levels dropping below pumps
– Land surface subsidence
– Degradation of water quality
– Changes to surface water-groundwater interaction
– Practicality/economics of development

21. Sometimes Caveats Are Important
• No consideration given to:
– Aquifer water quality
– Water levels dropping below pumps
– Land surface subsidence
– Degradation of water quality
– Changes to surface water-groundwater interaction
– Practicality/economics of development

22. Example Depletion of Confined Aquifer
Initial Water Level
500 feet of confined pressure head
Confining Unit
Fully Penetrating Well Partially Penetrating Well
500 feet of saturated thickness
and well screen
Aquifer
100-foot screen on partially penetrating well

23. Drawdown vs. Storage Volume in an
Example Confined Aquifer
Exact shape of curve is aquifer specific and depends on initial water levels, aquifer
thickness, and storage properties (storativity and specific yield). Idealized curve developed
using a 500 ft thick aquifer with 500 feet of confined head. Storativity set to
0.0001 and specific yield set to 0.15.

24. Confined vs. Unconfined Aquifer
Response to Pumping
Ogallala
Unconfined Aquifer
6,000,000 ac-ft pumping
50 – 150 feet drawdown
Trinity
Confined Aquifer
200,000 ac-ft pumping
800 – 1000 feet drawdown
Source: TWDB

25. Well Yield Decline with Aquifer Depletion
Fully penetrating well
Partially penetrating well
Removal of water
providing confining
pressure. Depleted
“available drawdown”.
Draining of aquifer
pore space.
Exact shape of curve is aquifer specific and depends on initial water levels, aquifer thickness,
and storage properties (storativity and specific yield). Idealized curve developed using a 500 ft
thick aquifer with 500 feet of confined head. Storativity set to
0.0001 and specific yield set to 0.15.

26. Sometimes Caveats Are Important
• No consideration given to:
– Aquifer water quality
– Water levels dropping below pumps
– Land surface subsidence
– Degradation of water quality
– Changes to surface water-groundwater interaction
– Practicality/economics of development

27. “Recoverable” is Aquifer Specific
• The range of 25% - 75% is not an appropriate range for
all aquifers.
– Could be 55 -75% or more for highly productive, unconfined
aquifers at the surface such as the Ogallala and Seymour.
– Likely no more than 3 - 15% for most dipping, confined
aquifers in Texas (Trinity, Carrizo-Wilcox, Gulf Coast, etc.).
Recovery of anywhere close to 75% is physically impossible
given current well depths and impacts to water levels,
quality, existing wells, well yields, surface water, and
subsidence.
– For karst aquifers, total storage is practically irrelevant to
aquifer planning and management long term (Edwards).
Total storage is relatively small and fluctuates significantly
over time due to recharge events.

28. Takeaways:
The Meaning of Total Estimated Recoverable Storage
• What it does mean:
– How much water is in the aquifer
• What it doesn’t mean:
– That the water is available for production
– That using small fractions of the total volume cannot
seriously harm the aquifer and its users
– That it is a useful tool in the planning and management
of a particular aquifer
• What it might mean:
– That you’ll need to understand it well enough to explain
to your boards, permit applicants, and the public if –
and to what extent – it is relevant in your district

29. Contact:
Wade A. Oliver, P.G.
woliver@intera.com
512-425-2058