Yangxiao Zhou UNESCO-IHE Institute for Water Education ... UNESCO-IHE Institute for Water Education,

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  • 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

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     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

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     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/ERSD AC/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.

    dt dVP)DD()RR( 0000  

    R0 + ∆R0 D0 - ∆D0

    P dV/dt

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    • 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

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

    by: 1) A reduction in natural discharge; or 2) An increase in natural recharge; or 3) A loss in groundwater storage; or 4) 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

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    • 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.

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    • 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

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     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.

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    • 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?

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    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?

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    Demonstration natural case

    Unconfined aquifer Saturated thickness: 60m Dimension: 5500x5500m River stage: 100m Drain elevation: 95m Recharge: 1mm/day K=20 m/d Specific yield: 0.25

    R iver

    D rainage

    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

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    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

    Drai