The Myth of Groundwater Resource Evaluation
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The Myth of Groundwater

Resource Evaluation

Victor M. Ponce and Bavya Vuppalapati


10 November 2015


ABSTRACT. The myth of groundwater resource evaluation is examined herein. Conventional groundwater resource evaluation is based on a water budget, with the following premise: There is this amount of recharge and, therefore, we can pump so much groundwater. Actually, the situation is not that simple. We reckon that groundwater is not a volume, but a flow. Under natural equilibrium conditions, recharge to a control volume is coupled with a corresponding and equal discharge from the same volume. Under developed equilibrium conditions, pumping imposes an external anthropogenic demand which draws from both recharge and discharge, increasing the former and reducing the later. Therefore, it is incorrect to base the evaluation of safe yield solely on recharge. This approach has been widely discredited over the past 20 years. The new paradigm seeks to consider both the increase in recharge and the decrease in discharge in groundwater resource evaluations. The focus has now shifted to the assessment of the effect of reduced discharge on the rest of the hydrologic system, the related ecosystem, and on society at-large. The issue is seen to reach beyond the realm of hydrogeology, to encompass the hydrological, ecohydrological, socioeconomic, institutional and legal aspects of groundwater utilization. This approach is bound to give a fresh new meaning to the concept of sustainability.


1.  INTRODUCTION

Groundwater lies below the ground surface at depths that vary with the prevailing climate, from very close to the surface in humid regions, to distances exceeding hundreds of meters in some very arid regions. Traditionally, human societies have relied on surface water to satisfy their varying need for water. Increasingly, however, in the past 100 years, societies have relied on groundwater by pumping it out of the ground.

Although groundwater and surface water are ultimately connected, with one becoming the other and vice versa, in practice they are quite distinct. Surface water replenishes readily, with a global recycling time averaging 11 days (L'vovich, 1979). In contrast, groundwater takes much longer to replenish. Recycling times for groundwater vary widely, from days, to years, to centuries, to millennia, depending on aquifer location, type, depth, and properties (Fig. 1).

U.S. Geological Survey

Fig. 1  Age of groundwaters.

The average time for renewal of groundwater is 1,400 years (World Water Balance, 1978; Ponce, 2006a). This fact alone would imply that the use of groundwater is a double-edged sword: Depletion typically follows any development that fails to pay heed to the timescale factor. Yet, exploitation of groundwater continues to this date, in a wide-ranging experiment that is dominated more by economics and convenience than by physical reality and reason.

The prevailing wisdom is that groundwater exists in apparently large quantities, relatively close to the surface, and that we should use it if and when surface water becomes limiting or it is already all committed. Little attention, if any, is paid to the connectivity issue, i.e., to the fact that most groundwaters are bound to eventually become surface water. Even less attention is paid to the salinity concern: It is well known that groundwater quality deteriorates with depth, becoming more saline as it is pumped from ever increasing depths (Chebotarev, 1955; Ponce, 2012a).

Notwithstanding these recurring issues, the past century has seen the groundwater utilization industry thrive, albeit primarily in developed societies. Groundwater resource evaluations continue to be performed to this date in the traditional or conventional way. Purportedly, the aim is to find how much groundwater can or may be pumped safely. In line with current societal trends, the dated concept of safe yield (Lee, 1915; Todd, 1959) has now given way to the more timely sustainable yield (Alley et al., 1999; Maimone, 2004). Nevertheless, the resource question still remains. It may be encapsulated as follows:

  1. In light of the fact that most all groundwater is connected, can a control volume for analysis be reasonably determined for a particular application (Bredehoeft, 1997; Ponce, 2012b)?

  2. The consumptive use of groundwater is a capture; therefore, it is bound to affect the rights of downstream users, both natural and anthropogenic (Seward et al., 2006; Ponce, 2006b).

  3. Given that groundwater quality deteriorates with depth, it follows that the pumping of saline groundwater is likely to exacerbate the problem of salt disposal at the surface, particularly at locations that are far from the ocean.

  4. In arid regions, spring-fed upland vegetation is largely supported by groundwater (Fig. 2). Thus, lowering the groundwater table through pumping could lead to the drying of springs, placing at risk the vegetation that relies primarily on this resource, further increasing the carbon footprint and compounding global warming (Ponce, 2014).

Fig. 2  Desert upland spring, McCain Ranch, Boulevard, California.

These issues point to the myth of groundwater resource evaluation (Bredehoeft, 1997). There is an urgent need to broaden the resource analysis to include all other aspects that have been neglected in the past. These propositions are further examined in this paper.


2.  GROUNDWATER RESOURCE EVALUATION

Conventional groundwater resource evaluation is based on a water budget, with the following premise: There is this amount of recharge and, therefore, we can pump so much groundwater; thus, the focus is on the recharge. Actually, the situation is not that simple. We reckon that groundwater is not a volume, but a flow. Under natural equilibrium conditions, recharge to a control volume is coupled with a corresponding and equal discharge from the same volume. Under developed equilibrium conditions, pumping imposes an external anthropogenic demand which draws from both recharge and discharge, increasing the former and reducing the later. Under developed non-equilibrium conditions, depletion sets in, with pumping capturing an additional volume from aquifer storage.

These relations are portrayed in Fig. 3. The following statements apply:

  • Under pristine conditions, natural recharge equals natural discharge;

  • In a developed groundwater system, pumping increases the amount of recharge, while reducing at the same time the discharge; and

  • In a groundwater system undergoing depletion, a fraction of the capture draws from aquifer storage.

Fig. 3  Pristine, developed, and depleted groundwater systems.

The situation is patently clear in the case of an unconfined aquifer. Pumping from this type of aquifer lowers the groundwater levels near the well, forming a cone of depression (Fig. 4). The land surface overlying the cone is known as the area of influence of the well. Pumping changes the natural direction and amount of groundwater flow within the area of influence. The result is more recharge to the control volume and less discharge from it.

Edad de las aguas subterráneas
U.S. Geological Survey

Fig. 4  Cone of depression produced by groundwater pumping in an unconfined aquifer.

The traditional groundwater resource evaluation has sought to calculate the recharge and to limit the amount of allowable pumping to not exceed this amount. This practice has been referred to as the determination of the "safe yield." The approach, however, is flawed, because it completely disregards the existence of discharge (Sophocleous, 1997). If pursued, it will end up drying up neighboring springs and wetlands, eventually reducing baseflow in streams in the vicinity. Therefore, it is not sustainable (Alley et al., 1999).

To illustrate the effect of "safe-yield" pumping on baseflow, Sophocleous (2000) presented two timed maps of perennial streams in Kansas, within the High Plains regional aquifer (Fig. 5). The right-hand map (1994) shows a marked decrease in total length of streamflows in the western third of the state, within the elapsed period (1961-1994), showing the impact of groundwater depletion on surface-water resources.

Major perennial streams in Kansas: 1961 vs 1994
Kansas Geological Survey

Fig. 5   Major perennial streams in Kansas: 1961 vs 1994 (Sophocleous, 2000).


3.  THE CONTROL VOLUME

In addition to ignoring the discharge, the traditional groundwater resource evaluation suffers from a decided quandary: The evaluation hinges upon the definition of a control volume, but the latter can seldom be readily ascertained. What should be the control volume for analysis? In other words: What is the volume upon which recharge and discharge are evaluated?

The usual practice is to take the area of the surface water basin as delimiting the control volume, for lack of anything better or more obvious. This choice, however, is flawed, because the limits of surface water and groundwater are generally (i.e., in most practical cases) not the same. Surface water is limited by watershed area and delimited by its boundary, which can be precisely determined. In sharp contrast, there is no such strict limit in groundwater flow.

Groundwater flows follow the prevailing hydraulic gradients, which may ignore surface boundaries and therefore, defy precise characterization. In fact, pumping may actually change the natural pattern of groundwater flow, even reversing the flow direction to conform with the new gradients imposed by the pumping. Thus, matching the groundwater flow boundary with the surface water boundary may be convenient, but it is not necessarily the correct approach in every case.

This fact was shown by Prudic and Herman (1996) in their simulated groundwater development of Paradise Valley, in Humboldt County, Nevada. They focused on the evolving nature of capture with long-term groundwater development, and found that pumping 48% of the recharge for 300 years produced: (1) first, losses in aquifer storage; (2) then, reduction in evapotranspiration; (3) subsequently, decreased flow discharge; and (4) eventually, downstream flow reversal, i.e., increases in recharge coming from the neighboring downstream basin. Table 1 shows a summary of the Prudic and Herman (1996) findings.


Table 1.  The evolving nature of capture (in percentage) (Prudic and Herman, 1996).
Sources of capture, at the end
of the indicated time period
Time (yr)
1.5 25 100 300
1. Loss of aquifer storage 52.8 25.3 15.4 6.0
2. Reduction in evapotranspiration 47.2 74.5 82.9 88.3
3. Decreased flow discharge 0 0.1 0.8 1.2
4. Downstream flow reversal 0 0.1 0.9 4.5
All sources 100 100 100 100

The size of the control volume depends of the amount of capture: The greater the capture, the larger the control volume. Thus, a purely mechanistic approach to groundwater resource evaluation is bound to be inherently flawed. The greater the amount of groundwater being captured, the larger the area of influence compromised by the pumping. Thus, as Bredehoeft (1997) has adroitly pointed out, sustainable yield has almost nothing to do with recharge, which, at any rate, is difficult, it not impossible, to quantify.

Alley et al. (1999) have recommended that the assessment of sustainable yield be based, not on hydrogeologic principles, but rather on an interdisciplinary approach. Such an approach would determine the amount of groundwater that could be pumped without causing unacceptable environmental, social, or economic consequences. The definition of "unacceptable consequences" is largely subjective, involving a broad range of criteria, to include the entire hydrologic system and related natural ecosystem. Accordingly, the case is made for broadening groundwater resource evaluation to include the hitherto largely neglected interdisciplinary approach. Springs, wetlands, riparian ecosystems, upland spring-fed vegetation, baseflow, and downstream water rights would play a greater role in the renewed, sustainable approach to the evaluation.


4.  THE RESOURCE MYTH

In his early seminal paper on groundwater resource evaluation, Theis (1940) reckoned that all groundwater of economic importance is in the process of moving from a place of recharge to a place of discharge. In pristine aquifers, the average range of recharge is equal to the average rate of discharge. Under natural conditions, aquifers are in a state of approximate dynamic equilibrium. Discharge by wells, i.e., capture, is a new discharge superimposed upon a previously stable system, and it must be balanced by an increase in recharge, a decrease in discharge, a loss of storage, or a combination thereof (Theis, 1940).

Therein the myth. It is incorrect to base the evaluation of safe yield solely on recharge. This approach has been widely discredited over the past 20 years (Bredehoeft, 1997; Sophocleous, 1997). The new paradigm seeks to consider both the increase in recharge and the decrease in discharge in groundwater resource evaluations. The focus has now shifted to the assessment of the effect of reduced discharge on the rest of the hydrologic system, the related ecosystem, and on society at-large.

The issue is seen to reach beyond the realm of hydrogeology, to encompass the hydrologic, ecohydrologic, socioeconomic, institutional and legal aspects of groundwater utilization, seeking to establish a reasonable compromise between the often conflicting interests (Ponce, 2006b). This approach is bound to give a fresh new meaning to the concept of sustainability.


5.  SUSTAINABLE YIELD

Sustainable yield does not depend on the size, depth, or hydrogeologic characteristics of the aquifer. It is now abundantly clear that sustainable yield does not depend on the aquifer's natural recharge, because the natural recharge has already been appropriated by the natural discharge (Sophocleous, 2000). Instead, sustainable yield is seen to depend on the amount of capture, and whether this capture 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. Sustainable yield is to be determined only after a judicious study and appraisal of all issues regarding groundwater utilization. In addition to hydrogeology, these include hydrology, ecohydrology, socioeconomics, and the related institutional and legal aspects, to name the most relevant (Ponce, 2006b).

In practice, sustainable yield may be expressed as a percentage of recharge, even though there is no relation between them. If recharge is expressed as a fraction of precipitation, then sustainable yield could also be expressed as a percentage of precipitation. Holistic studies are needed to determine these percentages on a local and/or regional basis. As such, sustainable yield is seen to be a moving target, subject to change as more information leads to more knowledge, and societal perceptions change accordingly (Maimone, 2004).


6.  WHERE DO WE GO FROM HERE?

The conventional hydrogeologic approach to groundwater utilization having been discredited as flawed, the question remains: Where do we go from here? How much groundwater can or may be pumped in a specific application? Unfortunately, the answer is not straightforward. Two approaches stand out for further examination:

  1. Pumping is to be limited to the extent that it negatively impacts other users, where the term "users" is broadly defined to include all users, both natural and anthropogenic. This includes springs and wetlands, riparian and spring-fed upland vegetation, baseflow conservation, and existing water rights. The decision as to the degree of negative impact would have to be assessed by a suitable institutional authority.

  2. On a watershed scale, pumping is to be limited to an appropriate percentage of precipitation or recharge, this number to be determined by holistic studies and to be revised periodically as more information becomes available. This approach recognizes that the groundwater resource is a commons, to be regulated in order to avoid a repetition of Hardin's infamous tragedy (Hardin, 1968).

In any case, it is seen that groundwater utilization must reach beyond hydrogeology as the only sure way to strive for sustainability (Fig. 6) (Ponce, 2013).

Fig. 6  A large specimen of coast live oak, estimated to be about 500 years old, currently threathened by proposed groundwater development in the vicinity,
McCain Ranch, McCain Valley, Boulevard, San Diego County, California.


REFERENCES

Alley, W. M., T. E. Reilly, and. O. E. Franke. 1999. Sustainability of groundwater resources. U.S. Geological Survey Circular 1186, Denver, Colorado, 79 p.

Bredehoeft, J. 1997. Safe yield and the water budget myth. Editorial, Ground Water, Vol. 35, No. 6, November-December, 929.

Chebotarev, I. I. 1955. Metamorphism of natural waters in the crust of weathering. Geochimica et Cosmochimica Acta, Vol. 8, 22-48, 137-170, 198-212.

Hardin, G. 1968. The Tragedy of the Commons. Science, Vol. 162, 1243-1248.

Lee, C. H. 1915. The determination of safe yield of underground reservoirs of the closed-basin type. Transactions, American Society of Civil Engineers, Vol. LXXVIII, Paper No. 1315, 148-218.

L'vovich, M. I. 1979. World water resources and their future. American Geophysical Union, Washington, D.C.

Maimone, M. 2004. Defining and managing sustainable yield. Ground Water, Vol. 42, No.6, November-December, 809-814.

Ponce, V. M. 2006a. Groundwater utilization and sustainability. Online report, March.

Ponce, V. M. 2006b. Sustainable yield of groundwater. Online report, May.

Ponce, V. M. 2012a. The salinity of groundwaters. Online report, March.

Ponce, V. M. 2012b. Thompson Creek groundwater sustainability study. Online report, May.

Ponce, V. M. 2013. Impact of Soitec solar projects on Boulevard and surrounding communities, San Diego County, California. Online report, December.

Ponce, V. M. 2014. Effect of groundwater pumping on the health of arid vegetative ecosystems. Online report, December.

Prudic, D. E., and M. E. Herman. 1996. Ground-water flow and simulated effects of development in Paradise Valley, a basin tributary to the Humboldt River, in Humboldt County, Nevada. U.S. Geological Survey Professional Paper 1409-F.

Seward, P., Y. Xu, and L. Brendock. 2006. Sustainable groundwater use, the capture principle, and adaptive management. Water SA, Vol. 32, No. 4, October, 473-482.

Sophocleous, M. 1997. Managing water resources systems: Why "safe yield" is not sustainable. Editorial, Ground Water, Vol. 35, No. 4, July-August, 561.

Sophocleous, M. 2000. From safe yield to sustainable development of water resources - The Kansas experience. Journal of Hydrology, Volume 235, Issues 1-2, August, 27-43.

Theis, C. V. 1940. The source of water derived from wells: Essential factors controlling the response of an aquifer to development. Civil Engineering, Vol. 10, No. 5, 277-280.

Todd, D. K. 1959. Ground Water Hydrology. John Wiley and Sons.

World Water Balance and Water Resources of the Earth. 1978. U.S.S.R. Committee for the International Hydrological Decade, UNESCO, Paris, France.


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