5/27/2011

1

Groundwater budget myth, safe yield and

sustainability

Yangxiao Zhou

UNESCO-IHE Institute for Water Education, The Netherlands

Outline of presentation

Introduction Groundwater budget myth Safe yield Sustainability Demonstration case Conclusions

5/27/2011

2

Characteristics of groundwater

• Renewable resources• Large storage• Stable quality • Slow movement and long residence time• Wider availability in any location at any time• An important source for ecosystems• Large scale development for water supply

Effects of groundwater exploitation

• Continuous decline of groundwater levels• Depletion of groundwater storage• Deterioration in groundwater quality• Seawater intrusion • Dry rivers and springs• Degradation of wetlands and ecosystems• Land subsidence

5/27/2011

3

Groundwater depletion in High Plains aquifer

Satellite image of crops irrigated by groundwater from the High Plains aquifer in Kansas, USA. Groundwater is being depleted faster than it is being renewed. Image courtesy of NASA/GSFC/METI/ERSDAC/JAROS and the US/Japan ASTER Science Team

Gleeson, Zhou, et al. 2010, Nature Geoscince, 3

R0 D0

Groundwater budget myth• Water balance equation

– Natural condition

00 DR

– Pumping condition

∆R0: increased recharge;∆D0: decreased discharge (positive);dV/dt: change of groundwater storage.

dtdVP)DD()RR( 0000

R0 + ∆R0 D0 - ∆D0

P dV/dt

5/27/2011

4

• Sustainable pumping rate

∆R0 + ∆D0 is called the capture (Lohman, 1972). Sustainable pumping rate equals the capture rate.

Bredehoeft (1997, 2002) has drawn the conclusion: sustainable groundwater development has nothing to do with the natural recharge. The idea that knowing the natural recharge is important in determining the size of a sustainable groundwater development is a myth. Water Budget Myth

Groundwater budget myth

• Importance of natural recharge (Zhou, 2009, J hydrology 370)

By defining residual discharge equal to natural discharge minus captured discharge by pumping, we obtain:

Sustainable groundwater development depends on natural recharge. The truth is that natural discharge after all is originated from natural recharge. Capturing natural discharge really means to capture natural recharge.

Sustainable pumping rate is the natural recharge plus induced recharge by pumping and minus residual discharge

DRR0 + ∆R0

Ps

Groundwater budget myth

5/27/2011

5

Sources of water to pumping wells• Theis (1940) stated already that the pumping must be balanced

by: 1) A reduction in natural discharge; or2) An increase in natural recharge; or3) A loss in groundwater storage; or4) A combination of these.

• Dynamic response of a aquifer to pumping depends on: 1) Distance of pumping wells to, and character of, recharge;2) Distance of pumping wells to natural discharge;3) Character of the cone of depression.

• Alley et al (1999) showed that– Some groundwater must be removed from storage before the

system can be brought into a new equilibrium.– The time that is required to bring a hydrologic system into

equilibrium depends on the rate at which the discharge can be captured.

– The rate at which discharge can be captured is a function of the characteristics of the aquifer system and the placement of pumping wells.

– Equilibrium is reached only when pumping is balanced by capture.

– It usually takes a long periods of time before even an approximate equilibrium condition can be reached.

Sources of water to pumping wells

5/27/2011

6

• In conclusion, sustainable pumping means that the groundwater system will reach a new equilibrium state which depends on:

– Natural recharge;

– Pumping rate and pattern;

– Dynamic response of the aquifer to pumping.

Natural recharge along can’t determine the sustainable pumping. Assessment of dynamic response of the aquifer to pumping is also very important!

Sources of water to pumping wells

Concept of safe yield• Todd (1959) defined “safe yield” of a groundwater basin

as “the amount of water which can be withdrawn from it annually without producing an undesired result”. The undesirable results may include:– Depletion of groundwater storage;– Intrusion of bad quality water;– Contravention of existing water right;– Increase of costs of pumping;– Excess depletion of stream flow;– Land subsidence.

5/27/2011

7

• Problems with the safe yield:– How much is the annual amount of the safe yield?– No standard method of determining safe yield;– What defines an undesired result?– No standard method of assessing undesired results;– In practice, safe yield was calculated as a percentage of the

natural recharge.A misconception was that groundwater development is “safe”, if pumping rate does not exceed the rate of the natural recharge. Implementation of this safe yield policy has caused many negative effects: such as stream flow depletion and loss of wetlands and riparian ecosystems.

Concept of safe yield

• Why “safe yield” is not safe?– Pumping always create a cone of depression which may cause

intrusion of bad quality water and land subsidence;– Induced recharge may deplete stream flow;– Decreased discharge may cease spring discharge and base

flow;– Residual discharge may be not sufficient to maintain

groundwater dependent ecosystems.

Call for abandoning the use of safe yield concept (Thomas, 1955; Kazman, 1956; Sophocleous, 2000).

Concept of safe yield

5/27/2011

8

Concept of sustainability

• Alley et. al (1999) defined groundwater sustainability as:– “the development and use of groundwater resources in a manner

that can be maintained for an infinite time without causing unacceptable environmental, economic, or social consequences”.

– How much groundwater is available for use depends on how changes in recharge and discharge affect the surrounding environment and the acceptable trade-off between groundwater use and these changes.

Sustainable yield

• Kaf and Woolley (2005) defined:– Basin sustainable yield:

– Maximum basin sustainable yield:

These definitions may cause the same confusion as the safe yield.Ps and Pd are, in fact, sustainable pumping rate and maximum sustainable pumping rate.

5/27/2011

9

• Sustainable yield must satisfy:– A sustainable pumping rate which can be sustained by the capture;

– Environmental constraints: Pumping capture should not cause the excessive depletion of surface water and reduction of groundwater discharge to springs, rivers and wetlands. The cone of depression should not cause the intrusion of bad quality water, land subsidence and damage of terrestrial ecosystems.

– Economic constraints: maximise economical benefit of groundwater use and minimise pumping costs.

– Social constraints: equitable allocation of shared groundwater resources and no interference of existing water user rights.

• Sustainable yield is not a single value; is a compromised development plan acceptable by all stakeholders.

Sustainable yield

Time scale difference: groundwater mean residence time is much longer than any planning period of current groundwater policy.

(Gleeson, Zhou, et al. 2011, accepted by Groundwater)

How to achieve groundwater sustainability?

5/27/2011

10

Set multiple generation goals and backcasting for actions:

(A) The mean residence time is much greater than groundwater policy horizons; (B) The mean residence time is less than or similar to groundwater policy horizons.

(Gleeson, Zhou, et al. 2011, accepted by Groundwater)

How to achieve groundwater sustainability?

Groundwater model is useful to simulate impacts and develop adaptive management measures

Simulated impacts of groundwater withdrawal scenarios for the Chateauguay River aquifers (avigne et al. 2010)

How to achieve groundwater sustainability?

5/27/2011

11

Demonstration natural case

Unconfined aquiferSaturated thickness: 60mDimension: 5500x5500mRiver stage: 100mDrain elevation: 95mRecharge: 1mm/dayK=20 m/dSpecific yield: 0.25

River

Drainage

River Water divide Drainage

Water Balance (m3/day)

Recharge Precipitation 30250

Discharge Total 30250

Drain 17282

River 12968

Natural recharge=

Natural discharge

R0=30250 D0=30250

Demonstration natural case

5/27/2011

12

Pumping rate:

24000 m3/day,

79% of natural

recharge;

12 pumping

wells in a circle

pattern;

Demonstration pumping case

Cone of depressionCone of depression

Water Balance (m3/day)

Recharge Precipitation 30250

Discharge Total 30250

Well 24000

Drainage 5151

River 1099

Sustainable pumping maintained by natural recharge with drastic decrease of discharges: Reduction of drainage: 70%Reduction of discharge to river: 91.5%

DR=6250 R0 =30250

Ps =24000

∆R0=0

Demonstration pumping case

5/27/2011

13

Pumping rate:

36000 m3/day,

119% of natural

recharge;

12 pumping

wells in a circle

pattern;

Cone of depression

Demonstration pumping case

Water Balance (m3/day)

Recharge Precipitation 30250

River 5750

Total 36000

Discharge Well 36000

Drainage 0

River 0

Sustainable pumping maintained by the natural recharge and Induced recharge from river (19% of natural recharge);Natural discharge ceased.

DR=0 R0 =30250

Ps =36000

∆R0=5750

Demonstration pumping case

5/27/2011

14

Pumping rate

119% of natural

recharge;

Regional

decrease of

groundwater

level:

14 to 42 m

Demonstration pumping case

Demonstration: dynamic responseObservation well at the centre of pumping wells

50

60

70

80

90

100

110

120

0 10 20 30 40 50 60 70 80 90 100 110

Years

Gro

un

dw

ate

r le

vels

(m

)

42 m

5/27/2011

15

Observation well near the river

90

95

100

105

110

115

0 10 20 30 40 50 60 70 80 90 100 110

Years

Gro

un

dw

ate

r le

ve

ls (

m)

Inducing recharge from river

Demonstration: dynamic response

Observation well near the drain

90

95

100

105

110

115

0 10 20 30 40 50 60 70 80 90 100 110

Years

Gro

un

dw

ate

r le

ve

ls (

m)

Cease of drainage

Demonstration: dynamic response

5/27/2011

16

0

3600

7200

10800

14400

18000

21600

25200

28800

32400

36000

0 10 20 30 40 50 60 70 80 90 100 110

Years

Ca

ptu

re b

y p

um

pin

g w

ells

(m

3/d

ay

)

Decrease of discharge Induced recharge Capture

Demonstration: dynamic response

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100 110

Years

Pe

rce

nta

ge

of

wa

ter

to p

um

pin

g w

ells

Percentage of capture Percentage of storage removal from the aquifer

Demonstration: dynamic response

5/27/2011

17

Conclusions• Pumping from a groundwater basin will always cause

the decrease of groundwater levels to induce more recharge and to capture natural discharge. The dynamic development of the capture depends on groundwater system characteristics, nature of recharge and discharge, and rates and pattern of pumping wells.

• When pumping rate is larger than the total recharge, the capture is not sufficient to maintain the pumping rate. Groundwater storage will be continuously removed until it is eventually depleted.

• When pumping rate is smaller than the total, the capture is sufficient to balance the pumping rate and the groundwater system will reach a new equilibrium state. However, the capture may have already caused the unacceptable environmental, economic and social impacts.

Conclusions

5/27/2011

18

• The concept of safe yield is misused. One misconception is that the development of groundwater is considered to be “safe “ if the pumping rate does not exceed the natural recharge. Another misconception is that the natural recharge is irrelevant in determining the safe yield or sustainable yield. The truth is that the natural recharge and dynamic development of the capture and its impacts together determine the safe yield or sustainable yield.

Conclusions

• The concept of sustainability emphasises more on impact assessment. A sustainable pumping scheme with acceptable consequences by all stakeholders can be considered as a sustainable development plan. Therefore, the sustainable development can not be determined by a single value of a sustainable yield, but a compromised plan satisfying not only water balance equation, but also environmental, economic and social constraints.

Conclusions

5/27/2011

19

• Groundwater simulation model is the best available tool to simulate dynamic development of the capture and the impacts of the proposed development scenarios. The challenge is to link the predicted changes of groundwater levels and flows to the impacts on ecosystems and social benefits. This is where the active research is needed.

• Effects of groundwater development may take decades to become evident. Model predictions are always uncertain. Therefore, continuous monitoring of groundwater from a well-established regional monitoring network is essential to detect these effects.

Conclusions

• Groundwater mean residence time is much longer than any current policy horizon. Setting multi-generation goals, backcasting for actions, managing adaptively and locally, this is a feasible path for achieving long-term groundwater sustainability. Hydrogeologists can play an active role by using groundwater models to simulate effects of management polices and measures, and by monitoring the effectiveness of polices and measures in achieving the long-term sustainable goals.

Conclusions

5/27/2011

20

Thank You!

Questions and remarks