Technical Commentary/

Also Consider the Rechargeby Daniel B. Stephens

Quantifying recharge is a common objective in manyapplications in ground water hydrology and is especiallyrelevant to discussions about ground water sustainability.John Bredehoeft’s (2007) recent guest editorial, ‘‘It is thedischarge,’’ is an important reminder for hydrogeologists toconsider multiple techniques to determine basin recharge,including taking into account the basin discharge, when thesystem is in a condition of dynamic equilibrium.

In a condition of dynamic equilibrium, basin rechargeapproximately equals basin discharge. Dr. Bredehoeftrightly notes that where ground water is visibly dis-charged—such as at springs, in base flow to streams, andvia phreatophytes in a desert environment—it should bequantified. One reason he tells us ‘‘it is the discharge’’ isthat pumping in desert ground water basins is more likely toreach a new dynamic equilibrium by capturing anddiminishing ground water discharge than by capturingrejected recharge. He notes that ‘‘basin discharge is of muchmore pragmatic concern than recharge.’’ Thus, to understandbasin dynamics and pumping impacts better, he encouragesus to spend more effort on quantifying the discharge.

Dr. Bredehoeft also states that ‘‘the recharge is themost difficult component of the ground water system toquantify.’’ Because of this, he feels that ‘‘recharge is bet-ter understood through the discharge’’ and that in lieu ofresearch to understand recharge processes, ‘‘it is morefruitful to study the discharge.’’ It is these points on quan-tifying recharge through measurements of ground waterdischarge that I want to address in this commentary.

In my experience, there are instances where measur-ing basin discharge to estimate recharge is not easy, reli-able, appropriate, or feasible. In many cases, determiningnet infiltration below the root zone (i.e., deep percolationthat later becomes recharge) and determining rechargefrom physical and chemical data collected in aquifers canbe alternatives that are just as good, if not better, thanusing basin discharge as a surrogate.

In some instances, basin discharge may be fairly easyto quantify, for example, by measuring spring flow or, wherethe stream is fed by ground water discharge, by measuringthe gain in streamflow. However, estimation of these compo-nents of discharge may underestimate the total recharge insome basins because discharge by deep ground water flow-ing beneath near-surface discharge zones is neglected.

If basin discharge is by evapotranspiration, quantifyingdischarge using site-specific data may be quite challenging.Although there are methods to estimate evapotranspirationfrom the literature and perhaps from available site infor-mation, measuring actual evapotranspiration may requireconstructing large-scale lysimeters from which measure-ments are collected for a year or more. Calculating actualevapotranspiration may require obtaining meteorologicaldata and assessing seasonal variations of plant activity, aswell as assessing vegetation density, root distributions,soil properties, and other parameters. Incidentally, thesesame lysimeter and meteorological measurements madein the recharge areas can be applied in soil-water balancecalculations to quantify deep percolation that maybecome recharge.

To support his theme of ‘‘it is the discharge,’’ Dr.Bredehoeft cites the widely used Maxey-Eakin methodfor estimating recharge. The Maxey-Eakin method isa convenient but poorly documented empirical relation-ship between mean precipitation in Nevada as of 1936and basin discharge, e.g., by phreatophytes, with the lat-ter used as a surrogate for recharge. The discharge fromselected Nevada basins was based partly on evapotranspi-ration rates obtained from earlier publications on OwensValley, California, and Escalante Valley, Utah. The Max-ey-Eakin method has been updated recently by Nichols(2000) using more rigorous and site-specific analysis ofevapotranspiration and basin underflow.

Some ground water basins may have no surfaceexpression of discharge at all, and in these cases, onemay need to calculate discharge using Darcy’s law. To dothis requires assessing subsurface geologic structure andstratigraphy, determining the width and saturated thick-ness of the aquifer, installing wells for establishing thehydraulic head gradient, and conducting aquifer tests.Although this standard hydrogeological information

Daniel B. Stephens & Associates Inc., 6020 Academy RoadNE, Suite 100, Albuquerque, NM 87109; (505) 822-9400; fax:(505) 821-2313; dan.stephens@dbstephens.com

Copyright ª 2008 The Author(s)Journal compilationª2008NationalGroundWater Association.doi: 10.1111/j.1745-6584.2008.00476.x

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should be collected as part of an evaluation of any groundwater basin, depending on the degree of characterizationavailable, primarily in the hydraulic conductivity of theaquifer, the uncertainty in computing discharge this waycould be considerable, especially where there may beflow through multiple heterogeneous or fractured bedrockformations. Moreover, in deep basins, the cost of explora-tion and testing could be substantial.

Even if the basin is in a condition of dynamic equilib-rium and the discharge is measured accurately, such dis-charge, like recharge, by definition should be expected tovary somewhat with time. There can be time lags of years,decades, or more between periods of significant rechargeand the corresponding discharge, depending on the rechargeprocess, basin geometry, and aquifer transmissivity andstorage. For large basins with thick vadose zones and longresidence times, variations in recharge are likely to be sohighly dampened that little variation in discharge would beexpected. For other basins, this may not be the case. Withbasins in which some development has occurred, dependingon the location of the wellfield relative to the dischargearea, it may take centuries for the new equilibrium to beachieved. Unless the temporal variability of discharge hasbeen established, how can one know when to rely on thedischarge to estimate the mean recharge for a basin?

The dynamic equilibrium concept is best understoodon the basis of water volumes of recharge and dischargeover time. However, for a particular problem, we mustdetermine whether we are seeking the volumetric rate ofrecharge (recharge) (L3/T) or the recharge rate expressedas a flux (L/T). Although the native recharge and dis-charge volumetric flow rates may be comparable, theareas of recharge and discharge within the basin are mostlikely much different. Moreover, the land surface areaover which recharge occurs can be difficult to delineateby inspection, in which case determining even a meanrecharge rate from discharge (L3/T) divided by rechargearea is sometimes problematic. Thus, the recharge rateoften cannot be readily determined from either the volu-metric discharge or the discharge rate (flux).

The recharge rate is critically important in evaluatingthe rate of contaminant migration though the vadose zone.Knowing the spatial distribution of the recharge rate alsomay be important to identifying and preserving areas ofhigh soil-water percolation to sustain the natural groundwater balance. The spatial distribution of the recharge rateis often a key input requirement in numerical models ofground water systems. The total basin discharge per se tellsus little about this flux and its spatial distribution. Althoughthe volume of recharge can be determined by measuringthe discharge for many basins in a state of dynamic equi-librium, there are also many techniques available to quan-tify recharge directly in both arid and humid climates, andin many cases, these techniques are more convenient andmuch less expensive than measuring discharge. Thesemethods, as summarized by Stephens (1996) and Scanlonet al. (2002), among others, include techniques applied tothe atmosphere-soil boundary, to the vadose zone soils, andto the underlying ground water. Recharge calculated by any

of these methods is an approximation to some degreebecause of inherent assumptions and data limitations. Addi-tionally, there will be uncertainty in characterizing themean recharge rate from point measurements in a basin orfrom short-term site data. In evaluating methods to obtainrecharge, one should consider the cost of collecting thepotentially large number of point measurements required toestimate recharge, such as from soil data in large heteroge-neous basins. There also can be large uncertainty inregional-scale recharge estimation techniques, such as soil-water balance models, especially when the field data aresparse. These and other factors should bear on the approachselected to quantify basin recharge.

To reduce uncertainty in the recharge estimate, how-ever, multiple techniques, including measuring the basindischarge, should be considered. In fact, to determinewhether a basin is in dynamic equilibrium necessitatesquantifying both the recharge and the discharge (and/orassessing the hydrograph time series of one or moreobservation wells). Even if the discharge is of greatestpragmatic importance, one should consider the rechargemethods to bolster the calculated discharge.

In summary, where the goal of the project is to quan-tify recharge or recharge rate in a basin in hydrodynamicequilibrium, measuring the discharge alone may not bea sufficient or appropriate surrogate, for reasons discussedpreviously. And where the goal is to quantify discharge ina basin in hydrodynamic equilibrium, recharge calculationsare appropriate supplements to, and perhaps in some casessuitable surrogates for, basin discharge. At present, each ofthe wide variety of methods available to quantify rechargehas either some theoretical or practical limitations for thediverse hydrogeological environments one may encounter,and the application of any one of the methods results ina prediction with some uncertainty. In my experience, thereis clearly room for improving our understanding ofrecharge processes and for developing more accurate, sim-pler, and less expensive methods for quantifying rechargedirectly. With such improvements derived from rechargeresearch and wider application of the improved methodswithin a basin, uncertainty in recharge (and potentially indischarge) no doubt can be diminished in the future.

AcknowledgmentsI want to thank the anonomous reviewers, Bridget

Scanlon, and Shlomo Neuman for their helpful commentsand for the editorial support from Ellen Torgrimson andDeb Salvato.

ReferencesBredehoeft, J. 2007. It is the discharge. Ground Water 45, no. 5: 523.Nichols, W.D. 2000. Regional ground-water evapotranspiration

and ground-water budgets, Great Basin, Nevada. USGSProfessional Paper 1628. Reston, Virginia: USGS.

Scanlon, B.R., R.W Healy, and P.G Cook. 2002. Choosingappropriate techniques for quantifying groundwaterrecharge. Hydrogeology Journal 10, no. 1: 18–39.

Stephens, D.B. 1996. Vadose Zone Hydrology. Boca Raton,Florida: CRC Press Inc.

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