Sustainable Yield of Ground Water
By Victor M. Ponce
Department of Civil and Environmental Engineering
San Diego State University
San Diego, California

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.

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Safe yield is defined as the maintenance of a long-term balance between the annual amount of ground water withdrawn by pumping and the annual amount of recharge.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

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.

Three groundwater scenarios are possible:
a pristine groundwater system, in equilibrium or steady state, in the absence of pumping;
a developed groundwater system, in equilibrium or steady state, with moderate pumping at a fixed depth; and
a depleted groundwater system, in nonequilibrium or unsteady state, with heavy pumping at an ever increasing depth.

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).

Fig. 1 Recharge and discharge in groundwater systems.

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.

Fig. 2 Dead riparian trees, Ash Creek, New Harmony, Utah.

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.
Sustainability is seen as all-encompassing, addressing issues across the disciplines.

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.

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.

Case Studies
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%.

Case Studies (continued)
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.

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.

Fig. 3 Geometric model of a groundwater reservoir.

Synthesis (continued)
Sustainability studies will require a balance of the entire hydrologic system, not just of the aquifer.

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.

Synthesis (continued)
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.
In the absence of basin-specific studies, this figure may be used as a point-of-start on which to base sustainable yield assessments.

Synthesis (continued)
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%.

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.

Conclusions (continued)

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.