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Estimated Recoverable Storage: What it does, doesn't and might mean for planning, Wade Oliver, Intera
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.
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)
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
13.
14.
15.
16.
17.
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
20.
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.
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
In this slide I’ll point out that TERS has been in the news and this is an example of how people are interpreting and using the numbers. My point is not to pick on the county judge or the journalist because, by the nature of the numbers and most people’s limited understanding of groundwater, many intelligent, college-educated people would come to the same conclusion. I bring this up because it illustrates the confusion that these numbers can cause. The sentence in the first paragraph reads like a bad game of telephone where TWDB put out a report, it was picked up by the county judge or one of their consultants, then relayed to a journalist, who then relayed it to the public. Not the point of this slide, but the math cited in the article is wrong. 45k acre-feet is 0.4 percent of the lower range 25 percent of total volume. Over 50 years it amounts to 20.5% of the lower range 25% of total volume.
This is the TWDB report, GAM Task 13-035 referenced in the article where the total of Bastrop and Lee storage, at the 25% of total storage volume, is 11.5 million acre-feet.