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    SUSTAINABLE YIELD OF GROUNDWATER



    Victor M. Ponce



    Department of Civil and Environmental Engineering
    San Diego State University
    San Diego, California


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    INTRODUCTION

  • Surface water can become ground water through infiltration, while ground water can become surface water through exfiltration.

  • Therefore, surface water and ground water are inextricably connected; one cannot be considered or evaluated without regard to the other.

  • This study examines the historical development of groundwater use and of the limits placed thereon throughout the years.

  • The traditional concept of safe yield, which equates safe yield to annual recharge, is shown to be flawed because of its narrow focus.

  • Sustainable yield extends beyond the conventional boundaries of hydrogeology, to encompass surface water hydrology, ecology, and other related subjects.
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    BACKGROUND

  • Attempts to limit groundwater pumping have been commonly based on the concept of safe yield.

  • Safe yield is defined as the maintenance of a long-term balance between the annual amount of ground water withdrawn by pumping and the annual amount of recharge.

  • This definition is too narrow because it does not take into account the rights of groundwater-fed surface water and groundwater-dependent ecosystems.

  • Sustainable yield reserves a fraction of the so-called "safe yield" for the benefit of the surface waters.

  • There is a lack of consensus as to what percentage of safe yield should constitute sustainable yield.
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  • A distinction is necessary between pristine and non-pristine groundwater reservoirs.

  • Pristine reservoirs are those that have not been subject to human intervention.

  • In pristine reservoirs, average annual natural recharge is equal to average annual natural discharge, which feeds springs, streams, wetlands, lakes, and groundwater-dependent ecosystems.

  • Thus, net recharge, i.e., average annual recharge minus average annual discharge, is zero.
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  • Three groundwater scenarios are possible:

    1. a pristine groundwater system, in equilibrium or steady state, in the absence of pumping;

    2. a developed groundwater system, in equilibrium or steady state, with moderate pumping at a fixed depth; and

    3. a depleted groundwater system, in nonequilibrium or unsteady state, with heavy pumping at an ever increasing depth.

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  • In the pristine groundwater system, natural recharge is equal to natural discharge, net recharge is zero, and pumping is zero (Fig. 1 a).

  • In the developed groundwater system, pumping is equal to net recharge, i.e., capture (Fig. 1 b).

  • In the depleted groundwater system, pumping is equal to net recharge plus captured storage (Fig. 1 c).

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  • The greater the level of development, the greater the amounts of captured recharge and captured discharge, and, in the case of a depleted system, captured storage.

  • The greater the captured discharge, the smaller the residual discharge.

  • Since all aquifer discharge feeds surface water and evapotranspiration, intensive groundwater development can substantially affect local, subregional, or regional groundwater-fed surface water bodies and groundwater-dependent ecosystems.

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Fig. 2 Dead riparian trees, Ash Creek, New Harmony, Utah.
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  • Sustainable development must meet the needs of the present without compromising the ability of future generations to meet their own needs.

  • Alley (1999) has defined groundwater sustainability as the development of ground water in a manner that can be maintained for an indefinite time without causing unacceptable environmental, economic, or social consequences.

  • Sophocleous (2000a) has pointed out that the traditional concept of safe yield ignores the fact that, over the long term, natural recharge is balanced by discharge from the aquifers.

  • Unlike natural recharge, which tends to be a constant for a given basin, capture is a function of the level of development; the greater the pumping, the greater the capture.

  • There is concern about the long-term effects of groundwater development on the health of springs, wetlands, lakes, streams, and estuaries.
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The concepts may be summarized as follows:

  • All groundwater is in transit from a place of recharge to a place of discharge.

  • Determinations of sustainable yield must subtract from recharge the fraction that can be shown to fulfill the needs of surface water and related ecosystems.

  • Assessments of sustainable yield must encompass the interdisplinary synthesis of surface water hydrology, ecology, geology, and climatology.

  • The sustainable yield dilemma is how to reconcile ecology and economics with groundwater development.
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  • Essentially, the goal is to be able to determine an appropriate yield-to-recharge percentage.

  • What are typical values of the yield-to-recharge percentage?

  • Solley et al.(1998) have estimated that the pumpage of fresh ground water in the United States in 1995 was 8.6% of the natural recharge to the Nation's groundwater systems.

  • Limited experience suggests that workable yield-to-recharge percentages are likely to be somewhat higher.
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CASE STUDIES

  • In Wales, Great Britain, Miles and Chambet (1995) calculated yield-to-recharge percentages, when baseflow was reduced to zero, which is clearly the least conservative assumption, varying in the range 50-78%.

  • In Korea, sustainable yield has been defined as the average rate of pumping that can be maintained without endangering either the quantity or quality of pumped water. Sustainable yield was estimated at a yield-to-recharge percentage of 42%.

  • The Chester County Water Resource Authority, in Pennsylvania, has recently selected stream baseflow as the standard against which to measure groundwater pumping.

  • The more conservative lower management target would use up to 50% of the 1-day 25-yr low flow.

  • The less conservative upper target would use up 100% of the 1-day 25-yr low flow.
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SYNTHESIS

  • A pristine groundwater reservoir is in steady state, with inflows equal to ouflows.

  • All pumping comes from capture, and all capture is due to pumping.

  • Capture comes from decreases in natural discharge and increases in recharge, the latter coming either from increased ground surface recharge or from the surrounding areas.

  • In depletion cases, capture is augmented with decreased storage, i.e., with a permanent lowering of the water table.

  • Sustainability studies will require a balance of the entire hydrologic system, not just of the aquifer.
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  • Current practice notwithstanding, sustainable yield does not depend on the aquifer's natural recharge, because the natural recharge has already been appropriated by the natural discharge.

  • Sustainable yield depends on the amount of capture, and whether this amount is socially acceptable as a reasonable compromise between little or no use, on one extreme, and the sequestration of all natural discharge, on the other extreme.

  • In practice, sustainable yield may be taken as a suitable percentage of precipitation.

  • A reasonably conservative estimate would take up all the deep percolation amount as sustainable yield.

  • On a global basis, deep percolation amounts to about 2% of precipitation.
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  • In the absence of basin-specific studies, the figure of 2% deep precolation may be used as a point-of-start on which to base sustainable yield assessments.

  • Sustainable yield can also be expressed as a percentage of recharge.

  • Globally, if recharge can be assumed to be approximately 20% of precipitation, then deep percolation would be about 10% of recharge.

  • Thus, a reasonably conservative estimate of sustainable yield would be 10% of recharge.

  • Limited experience indicates that average values of this percentage may be around 40%, while less conservative percentages may exceed 70%.
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CONCLUSIONS

  • The traditional concept of safe yield, which equates safe yield with natural recharge, is flawed and has been widely discredited.

  • Since 1987, the concept of sustainable yield has emerged, seeking to provide a reasonable compromise between the rights of established ground water users, and the rights of downstream ecosystems and surface water users.

  • The ideal solution appears to be to conserve all ground water, excluding deep percolation, for the benefit of the surface waters.

  • However, this solution may prove to be too harsh, and probably socioeconomically not viable in places where ground water usage has become, over the years, a way of life.
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    • Safe or sustainable yield can no longer be taken as equal to natural recharge.

    • A suitable compromise is to consider sustainable yield as a fraction of natural recharge.

    • Baseflow conservation is emerging as the standard against which groundwater pumping will be increasingly measured in the future.

    • In the absence of detailed holistic studies, a reference value of sustainable yield may be taken as the global average for deep percolation, estimated as 2% of precipitation.

    • Detailed local and regional studies will determine whether this value may be increased on a case-by-case basis.

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