Estimating Groundwater Recharge CH1 - Healy, Richard, Scanlon, Bridget

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Estimating Groundwater Recharge

Understanding groundwater recharge is essential for the successful management of water resources and modeling fluid and contaminant transport within the subsurface. This book provides a crit-ical evaluation of the theory and assumptions that underlie methods for estimating rates of ground-water recharge. Detailed explanations of the meth-ods are provided – allowing readers to apply many of the techniques themselves without needing to consult additional references. Numerous practical examples highlight the benefits and limitations of each method and provide guidance on the selec-tion and application of methods under both ideal and less-than-ideal conditions. More than 800 ref-erences allow advanced practitioners to pursue additional information on any method.

For the first time, theoretical and practical considerations for selecting and applying methods for estimating groundwater recharge are covered in a single volume with uniform presentation. Hydrogeologists, water-resource specialists, civil and agricultural engineers, earth and environ-mental scientists, and agronomists will benefit from this informative and practical book, which

is also a useful adjunct text for advanced courses in groundwater or hydrogeology.

For more than 30 years, Rick Healy has been con-ducting research for the US Geological Survey on groundwater recharge, water budgets of natural and human-impacted hydrologic systems, and fluid and contaminant transport through soils. He has taught numerous short courses on unsat-urated zone flow and transport, and groundwater flow modeling. He first presented a short course on methods for estimating recharge in 1994, and over the intervening 15 years the course has been presented to several hundred professionals and students. The material in that course has been expanded and refined over the years and forms the basis of Estimating Groundwater Recharge. Rick has authored more than 60 scientific publications and developed the VS2DI suite of models for sim-ulating water, solute, and heat transport through variably saturated porous media. He is a member of the Soil Science Society of America, the Amer-ican Geophysical Union, and the Geological Soci-ety of America.

Estimating Groundwater Recharge

Richard W. HealyUS Geological SurveyLakewood, Colorado

With contributions by

Bridget R. ScanlonBureau of Economic GeologyJackson School of GeosciencesUniversity of Texas, Austin

CAMBRIDGE UNIVERSITY PRESS

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Cambridge University Press

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ISBN-13 978-0-521-86396-4

ISBN-13 978-0-511-79610-4

© Richard W. Healy 2010

2010

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Contents

Preface page ixAcknowledgments x

1 Groundwater recharge 1

1.1 Introduction 11.2 Terminology 31.3 Overview of the text 41.4 Developing a conceptual model of recharge processes 5

1.4.1 Spatial and temporal variability in recharge 6

1.4.2 Climate 7

1.4.3 Soils and geology 8

1.4.4 Surface topography 9

1.4.5 Hydrology 9

1.4.6 Vegetation and land use 9

1.4.7 Integration of multiple factors 11

1.4.8 Use of existing data 11

1.4.9 Intersite comparison 11

1.5 Challenges in estimating recharge 121.5.1 Uncertainty in recharge estimates 12

1.5.2 Spatial and temporal scales of recharge estimates 12

1.5.3 Expense 13

1.6 Discussion 13

2 Water-budget methods 15

2.1 Introduction 152.2 Water budgets 16

2.2.1 Uncertainty in water budgets 19

2.3 Local-scale application 212.3.1 Precipitation 21

2.3.2 Evapotranspiration 23

2.3.3 Change in storage 24

2.3.4 Surface flow 27

2.3.5 Subsurface flow 28

2.4 Mesoscale application 312.4.1 Precipitation 32



2.4.4 Surface flow 34

2.4.5 Subsurface flow 35

2.5 Macroscale application 382.5.1 Precipitation 40



VI CONTENTS

2.5.4 Indirect use of remotely sensed data 41

2.6 Discussion 42

3 Modeling methods 43

3.1 Introduction 433.1.1 Data sources 44

3.2 Model calibration and inverse modeling 453.3 Unsaturated zone water-budget models 47

3.3.1 Soil water-budget models 47

3.3.2 Models based on the Richards equation 50

3.4 Watershed models 523.4.1 Precipitation runoff modeling system (PRMS) 54

3.5 Groundwater-flow models 573.6 Combined watershed/groundwater-flow models 633.7 Upscaling of recharge estimates 66

3.7.1 Simple empirical models 66

3.7.2 Regression techniques 67

3.7.3 Geostatistical techniques 69

3.7.4 Geographical information systems 69

3.8 Aquifer vulnerability analysis 703.9 Discussion 72

4 Methods based on surface-water data 74

4.1 Introduction 744.1.1 Groundwater/surface-water exchange 74

4.1.2 Base flow 76

4.2 Stream water-budget methods 774.3 Streambed seepage determination 79

4.3.1 Seepage meters 79

4.3.2 Darcy method 82

4.3.3 Analytical step-response function 82

4.4 Streamflow duration curves 824.5 Physical streamflow hydrograph analysis 85

4.5.1 Empirical hydrograph separation methods 85

4.5.2 Recession-curve displacement analysis 87

4.6 Chemical and isotopic streamflow hydrograph analysis 914.6.1 End-member mixing analysis 91

4.6.2 Tracer-injection method 93

4.7 Discussion 94

5 Physical methods: unsaturated zone 97

5.1 Introduction 975.2 Measurement of unsaturated-zone physical properties 97

5.2.1 Soil-water content 97

5.2.2 Pressure head 99

5.2.3 Water-retention and hydraulic conductivity curves 100

CONTENTS VII

5.3 Zero-flux plane method 1025.4 Darcy methods 1075.5 Lysimetry 1105.6 Discussion 116

6 Physical methods: saturated zone 117

6.1 Introduction 1176.1.1 Groundwater-level data 117

6.2 Water-table fluctuation method 1186.2.1 Causes of water-table fluctuations in unconfined aquifers 119

6.2.2 Specific yield 122

6.2.3 Fractured-rock systems 127

6.3 Methods based on the Darcy equation 1326.3.1 Theis (1937) 132

6.3.2 Hantush (1956) 132

6.3.3 Flow nets 132

6.4 Time-series analyses 1336.5 Other methods 1346.6 Discussion 135

7 Chemical tracer methods (by Bridget R. Scanlon) 136

7.1 Introduction 1367.2 Tracers in the unsaturated zone 138

7.2.1 Tracer sampling: unsaturated zone 141

7.2.2 Natural environmental tracers 142

7.2.3 Historical tracers 147

7.2.4 Applied tracers 150

7.3 Groundwater tracers 1527.3.1 Age-dating methods 153

7.3.2 Natural environmental tracers 156

7.3.3 Historical tracers 158

7.4 Discussion 164

8 Heat tracer methods 166

8.1 Introduction 1668.2 Subsurface heat flow 166

8.2.1 Temperature measurements 169

8.3 Diffuse recharge 1698.3.1 Diffuse drainage in the geothermal

zone 169

8.3.2 Diffuse drainage in the surficial zone 172

8.4 Focused recharge 1738.5 Discussion 178

VIII CONTENTS

9 Linking estimation methods to conceptual models of groundwater recharge 180

9.1 Introduction 1809.2 Considerations in selecting methods for estimating

recharge 1809.3 Comparison of methods 1829.4 Recharge characteristics of groundwater regions

of the United States 1899.4.1 Western Mountain Ranges 191

9.4.2 Alluvial Basins 192

9.4.3 Columbia Lava Plateau 194

9.4.4 Colorado Plateau 195

9.4.5 High Plains 195

9.4.6 Unglaciated Central Region 197

9.4.7 Glaciated Central Region 199

9.4.8 Unglaciated Appalachians Region 200

9.4.9 Glaciated Appalachians Region 200

9.4.10 Atlantic and Gulf Coastal Plain 201

9.4.11 Recharge in urban settings 202

9.5 Final thoughts 203

References 205Index 238

Preface

Groundwater is an integral part of natu-ral hydrologic systems. Humans have used groundwater for thousands of years. Its use has increased greatly over time, but only in the last few decades has our appreciation of the limita-tions of its supply and its vulnerability to con-tamination grown to the point where steps are being taken to protect this valuable resource. One of the most important components in any assessment of groundwater supply or aquifer vulnerability is the rate at which water in the system is replenished – the rate of recharge.

A number of textbooks are devoted to hydrogeology, groundwater flow, and contam-inant transport (e.g. Freeze and Cherry, 1979; Domenico and Schwartz, 1998; Todd and Mays, 2005). The importance of recharge is cited in all of these textbooks, but only limited information is provided on the description and analysis of techniques for estimating recharge. Similarly, undergraduate and graduate courses on hydro-geology, groundwater flow, and contaminant transport are offered at many universities, but we know of no university level courses specif-ically devoted to groundwater recharge. This book attempts to fill these gaps by providing a systematic and comprehensive analysis of methods for estimating recharge.

The book is aimed at practicing hydrogeolo-gists who are actively involved in groundwa-ter studies. The material contained in the text should also be useful to water-resource special-ists, civil and agricultural engineers, geologists, geochemists, environmental scientists, soil physicists, agriculturalists, irrigators, and sci-entists from other fields that have an elemen-tal understanding of hydrologic processes. The book can be used as an adjunct text or refer-ence in an advanced undergraduate or graduate groundwater or hydrogeology course; it can also serve as a primary text in courses on ground-water recharge. Theoretical as well as practical considerations for selecting and applying tech-niques are discussed. Theoretical analysis of the

methods allows the evaluation of assumptions inherent in each method. Practical examples of applications provide guidance for readers in applying methods in their own studies.

Over the years, hydrology has become a diverse field with the development of many new topic areas. Few hydrologists can claim expertise in all areas of hydrology; specializa-tion in groundwater, surface water, unsaturat-ed-zone flow and transport, geochemistry, or other subfields has become more the norm. We anticipate that most readers will have a back-ground in groundwater hydrology. However, application of many of the methods described herein (e.g. streamflow hydrograph separation, the zero-flux plane method, and watershed modeling) requires knowledge of areas outside of groundwater hydrology. A challenge in writ-ing this text was to bring together a number of methods that are drawn from fields outside of groundwater hydrology, fields such as surface-water hydrology, flow and transport through the unsaturated zone, geophysics, remote sens-ing, and water chemistry. Unsaturated-zone processes, in particular, are described in some detail. Many methods for estimating recharge require assumptions about the mechanisms by which water moves through the unsaturated zone; insight into unsaturated-zone processes provides a basis for evaluating the validity of those assumptions.

Acknowledgments

This text was largely derived from lecture notes for short courses on groundwater recharge that the authors have presented over the last 15 years. Many thanks are due to the following individuals who reviewed one or more parts of the book; unless otherwise noted, these indi-viduals are with the US Geological Survey: Kyle Blasch, Jim Bartolino, J. K. Böhlke, Alissa Coes, John Czarnecki, Geoff Delin, Keith Halford, Randy Hanson, Bill Herkelrath, Randy Hunt, Eve Kuniansky, Steve Loheide (University of Wisconsin), Andy Manning, Dennis Risser, Don Rosenberry, Marios Sophocleous (Kansas Geological Survey), Dave Stannard, Katie Walton-Day, and Tom Winter. Special thanks are owed to Stan Leake and Ed Weeks who were kind enough to provide reviews of the entire text. Finally, we would like to express our gratitude to the US Geological Survey and the Bureau of Economic Geology, University of Texas, Austin, for allowing us to invest time in this endeavor.

Groundwater recharge

1

1.1 Introduction

Groundwater is a critical source of fresh water throughout the world. Comprehensive statistics on groundwater abstraction and use are not available, but it is estimated that more than 1.5 billion people worldwide rely on ground-water for potable water (Clarke et al., 1996). Other than water stored in icecaps and glaciers, groundwater accounts for approximately 97% of fresh water on Earth (Nace, 1967; Shiklomanov and Rodda, 2003). As the world population con-tinues to grow, more people will come to rely on groundwater sources, particularly in arid and semiarid areas (Simmers, 1990). Long-term availability of groundwater supplies for burgeoning populations can be ensured only if effective management schemes are devel-oped and put into practice. Quantification of natural rates of groundwater recharge (i.e. the rates at which aquifer waters are replenished) is imperative for efficient groundwater man-agement (Simmers, 1990). Although it is one of the most important components in ground-water studies, recharge is also one of the least understood, largely because recharge rates vary widely in space and time, and rates are difficult to directly measure.

The rate, timing, and location of recharge are important issues in areas of groundwater contamination as well as groundwater sup-ply. In general, the likelihood for contami-nant movement to the water table increases

as the rate of recharge increases. Areas of high recharge are often equated with areas of high aquifer vulnerability to contamination (ASTM, 2008; US National Research Council, 1993). Locations for subsurface waste-disposal facili-ties often are selected on the basis of relative rates of recharge, with ideal locations being those with low aquifer vulnerability so as to minimize the amount of moving water com-ing into contact with waste (e.g. US Nuclear Regulatory Commission, 1993). A high profile example of the importance of susceptibility to contamination is the study for the proposed high-level radioactive-waste repository at Yucca Mountain, Nevada. Tens of millions of dollars were invested over the course of two decades in efforts to determine recharge rates at the site (Flint et al., 2001a).

Computer models of groundwater-flow are perhaps the most useful tools available for groundwater-resource management. Models are applied in both water-supply and aquifer-vulnerability studies. We expect that many readers of this book will be modelers seeking recharge estimates for use in groundwater-flow models or for evaluating model results.

The primary objective of this text is to pro-vide a critical evaluation of the theory and assumptions that underlie methods for estimat-ing rates of groundwater recharge. A complete understanding of theory and assumptions is fun-damental to proper application of any method. Good practice dictates that recharge estimation techniques be matched to conceptual models of

2 GROUNDWATER RECHARGE

recharge processes at individual sites to ensure that assumptions underlying the techniques are consistent with conceptual models. As such, the text should serve as a resource to which hydrol-ogists can refer for making informed decisions on the selection and application of methods. A thorough understanding of methods also pro-vides a framework for the analysis of implica-tions of modifying methods or applying them under less-than-ideal conditions.

A conceptual model of recharge processes attempts to answer the questions of where, when, and why recharge occurs. The model will thus identify the prominent recharge mechanisms, perhaps provide initial estimates of recharge rates, and serve as a guide for the selection of methods and for deciding on loca-tions and time frames for data collection. The importance of matching methods for estimat-ing recharge with conceptual models cannot be overemphasized. Development of a sound con-ceptual model is imperative for selecting proper methods and obtaining meaningful recharge estimates, but this process can be difficult, com-plicated by both natural and anthropogenic fac-tors. A conceptual model often evolves over time as data are collected and interpreted; there may be a dynamic feedback effect – recharge esti-mates may support revision of the conceptual model or suggest the application of alternative methods.

Nature is complex, and each study site is unique. Although conceptual models of recharge processes are important, the development of a conceptual model is not the main focus of this book. Because of the great complexity and lim-itless variability in hydrologic systems, it is beyond the scope of this text to provide more than general guidelines for developing a con-ceptual model of recharge processes. It is sim-ply not practical to describe or examine every scenario under which a method will be applied. Section 1.4 provides a general review of critical components of a conceptual model. For illus-trative purposes, typical recharge processes in groundwater regions of the United States are briefly discussed in the final chapter.

This text is not intended as a cookbook that provides a recipe for estimating recharge

for any and all situations; application of any method requires some hydrologic analysis. However, many of the methods described are simple enough that all the details required for their application are contained herein. Other methods, such as the use of complex models, require training that is beyond the scope of this text. Information is provided on these methods to assist the reader in deciding whether the cost of such training will be balanced by the benefits gained from applying the methods. Applications are illustrated with examples to highlight ben-efits and limitations. Many references are pro-vided to allow the interested reader to pursue more details on any of the methods discussed.

Most of the discussion in this text is directed toward quantifying rates of natural recharge; however, many methods can and have been used to estimate recharge from artificial recharge operations, irrigated areas, and human-made drainage features, such as canals and urban water-delivery systems. In addition, many of the methods can be used to provide qualitative information on recharge rates (i.e. identifying areas of high and low relative recharge rates) for purposes of determining aquifer vulnerabil-ity to contamination from surface sources.

Numerous journal articles and reports describe the theory and details of the vari-ous techniques for estimating recharge. Applications of methods are discussed in many other papers. Given the importance of the sub-ject matter, the paucity of textbooks devoted to this topic is surprising. Lerner et al. (1990) is the most thorough of these publications in terms of method descriptions. That text pro-vides generic descriptions of physical controls that influence recharge in different hydrogeo-logical provinces and discussion of techniques based on source of recharge water (i.e. precipi-tation, rivers, irrigation, and urbanization). Wilson (1980), Simmers (1997), and Kinzelbach et al. (2002) provide informative discussions on recharge processes in arid and semiarid regions and the techniques that are applicable in those regions. Simmers (1988) is a compendium of papers associated with a conference devoted to groundwater recharge. Hogan et al. (2004) and Stonestrom et al. (2007) each comprise a series

1. 2 TERMINOLOGY 3

of papers on recharge processes and case stud-ies of recharge in arid and semiarid regions of the southwestern United States.

1.2 Terminology

Recharge is defined, herein, as the downward flow of water reaching the water table, adding to groundwater storage. This definition is simi-lar to those given by Meinzer (1923), Freeze and Cherry (1979), and Lerner et al. (1990). Strictly speaking, this definition does not include water flow to an aquifer from an adjoining ground-water system (such as water movement from an unconfined aquifer across a confining bed to an underlying aquifer); we refer to this flow as interaquifer flow. Others include this flow in their definition of recharge. Interaquifer flow has also been referred to as groundwater underflow. Regardless of terminology, methods for estimating interaquifer flow are included in this text. Recharge is usually expressed as a volumetric flow, in terms of volume per unit time (L3/T), such as m3/d, or as a flux, in terms of volume per unit surface area per unit time (L/T), such as mm/yr.

Recharge occurs through diffuse and focused mechanisms (Figure 1.1). Diffuse recharge is recharge that is distributed over large areas in response to precipitation infil-trating the soil surface and percolating through

the unsaturated zone to the water table; dif-fuse recharge is sometimes referred to as local recharge (Allison, 1987) or direct recharge (Simmers, 1997). Focused recharge is the move-ment of water from surface-water bodies, such as streams, canals, or lakes, to an underlying aquifer. Focused recharge generally varies more in space than diffuse recharge. A distinction between different types of focused recharge has been proposed by Lerner et al. (1990), with local-ized recharge defined as concentrated recharge from small depressions, joints, or cracks, and indirect recharge defined as recharge from map-pable features such as rivers, canals, and lakes. Groundwater systems receive both diffuse and focused recharge, but the importance of each mechanism varies from region to region and even from site to site within a region. Generally, diffuse recharge dominates in humid settings; as the degree of aridity increases, the impor-tance of focused recharge in terms of total aqui-fer replenishment also tends to increase (Lerner et al., 1990). Some methods addressed in this book are designed to estimate diffuse recharge; others are specific to focused recharge.

Infiltration is the entry of water into the sub-surface. Infiltrating water can be viewed as potential recharge; it may become recharge, but it may instead be returned to the atmosphere by evapotranspiration, or it may simply remain in storage in the unsaturated zone for some period of time. The zero-flux plane (ZFP) is the horizontal

Figure 1.1 Vertical cross section showing infiltration at land surface, drainage through the unsaturated zone, diffuse and focused recharge to an unconfined aquifer, flow between the unconfined aquifer and an underlying confined aquifer (interaquifer flow), and the zero-flux plane.

Aquifer

Landsurface

Flow to or from aquifer(Interaquifer flow)

Focusedrecharge

Stream

Drainage

InfiltrationZero-flux plane

Evapotranspiration

Diffuse recharge

Water table

Precipitation


plane at some depth within the unsaturated zone that separates upward and downward moving water; the ZFP is sometimes equated with the bottom of the root zone (Figure 1.1). Water above the ZFP moves upward in response to evapotranspiration demand; water beneath the ZFP drains downward, eventually arriving at the water table. The depth of the ZFP changes in response to infiltration and evapotranspira-tion, ranging from land surface (for the case of downward water movement throughout the unsaturated zone) to some depth beneath the water table (for the case of groundwater eva-potranspiration). Water draining beneath the ZFP in the unsaturated zone is referred to as drainage, percolation, or net infiltration; it becomes actual recharge when it arrives at the water table. Some techniques described in this book provide estimates of potential recharge; others provide estimates of drainage; and some meth-ods provide estimates of actual recharge.

For clarity, we use the term groundwater to refer to water beneath the water table (within the saturated zone) and the term pore water to refer to water above the water table (within the unsaturated zone). A point estimate pertains to recharge at a specific point in space or time, whereas an integrated estimate refers to a value of recharge that is averaged over some larger space or time scale.

Different climatic regions are referred to throughout the text. Climatic regions are clas-sified on the basis of annual precipitation. An arid climate is one with annual precipitation of less than 250 mm; a semiarid region has pre-cipitation rates between 250 and 500 mm/yr; a subhumid climate refers to precipitation rates between 500 and 1000 mm/yr; and humid cli-mates have annual precipitation rates that exceed 1000 mm.

1.3 Overview of the text

This text is organized by methods, which are grouped on the basis of types of required or avail-able data (e.g. methods based on water budgets, or on data obtained from the unsaturated zone, or on streamflow data). Our approach differs

from that of Lerner et al. (1990) and Wilson (1980), who chose to organize methods on the basis of source of recharge (precipitation, riv-ers, etc.). While there is perhaps no ideal format for this presentation, the format used in this text has proved workable within the classroom over the decade and a half that we have taught this material. Examples are given to show how methods can be applied for different sources of recharge water.

This first chapter provides an introduction to the book, emphasizing the importance of developing a conceptual model of recharge processes for the area of interest. Chapters 2 through 8 are the heart of the book. They pro-vide in-depth analysis of methods for estimat-ing recharge. The format for each presentation is similar: discussion of theory and assump-tions, advantages and limitations of the meth-ods, and description of example case studies. Each chapter is devoted to a particular family of methods. Water-budget methods (Chapter 2) are presented first to emphasize the importance of water budgets in all studies.

Water-budget methods are widely used; indeed, most methods for estimating recharge could be classified as water-budget methods. To avoid making Chapter 2 too long, its content is limited to the use of the residual water-budget method, whereby a water-budget equation is derived for a control volume, such as a water-shed or an aquifer. All components within that equation, except for recharge, are measured or estimated; recharge is then set equal to the residual in the equation. Other methods that can be categorized as water-budget methods (e.g. the water-table fluctuation method, the zero-flux plane method, and modeling methods) are described in other chapters. Remote-sensing tools are described in Chapter 2, although they may be useful in other methods as well.

Discussion in Chapter 3 is devoted to the use of models for estimating recharge. A general approach to modeling, applicable to all models, is presented first; a brief description of inverse techniques is included. Unsaturated zone water-budget models, watershed models, groundwater flow models, and integrated surface- and subsur-face-flow models are then discussed. Because of

1.4 A CONCEPTUAL MODEL OF RECHARGE PROCESSES 5

the complexities of some models, detailed model descriptions are avoided. Instead, examples are used to highlight capabilities of complex mod-els, resources required for model application, and benefits and limitations of using models to generate estimates of recharge. Empirical equations, which are widely used for predict-ing recharge, are also described, as are regres-sion and geostatistical techniques for upscaling point estimates of recharge to obtain average values for an aquifer or watershed.

Chapter 4 addresses physical methods that are based on surface-water data. Included are stream water-budget methods, seepage meters, streamflow-duration curves, stream-flow hydrograph analysis (hydrograph sepa-ration), and chemical or isotopic hydrograph separation.

Chapter 5 describes physical methods that can be applied on the basis of data collected in the unsaturated zone. These methods include the zero-flux plane, the Darcy method, and the use of lysimeters. Physical methods based on data collected in the saturated zone form the basis of Chapter 6. The primary method in this group is the water-table fluctuation method. The Darcy method and methods based on time series of measured groundwater levels are also discussed.

Chapters 7 and 8 are devoted to the use of tracers for estimating recharge. Chemical and isotopic tracer methods are described in Chapter 7. Tracers can be naturally occurring (e.g. chlo-ride and isotopes of carbon and hydrogen), can occur as an indirect outcome of anthropogenic activity (e.g. tritium, chlorine-36, and chlo-rofluorocarbon gases), or can be intentionally applied to the surface or subsurface for experi-mental purposes (e.g. bromide, fluorescent dyes). Tracers can be used to study water from any source. Use of heat as a tracer for estimat-ing recharge is described in Chapter 8.

The final chapter, Chapter 9, attempts to link conceptual models of recharge processes with estimation methods. It begins with a dis-cussion of considerations important in selecting methods. Figures and tables are presented to compare methods in terms of spatial and tem-poral scales of applicability. Typical recharge

processes and methods that have been used to study these processes are described for ground-water regions of the United States. This dis-cussion is not an attempt at a comprehensive summary of recharge processes and studies; such an attempt is neither practical nor feasible. Rather, the idea is to illustrate how conceptual models of recharge processes can be formed and used to select appropriate methods. The closing section presents some final thoughts on good practices for any recharge study.

1.4 Developing a conceptual model of recharge processes

The development of a conceptual model of recharge processes (Figure 1.2) is an import-ant step in any recharge study. The conceptual model should be developed at the beginning of a study; it can be revised and adjusted as add-itional data and analyses provide new insights to the hydrologic system (Zheng and Bennett, 2002; Bredehoeft, 2005). Although this book is focused on methods, the reader should bear in mind the importance of a conceptual model when reviewing various methods. This section provides some discussion on factors that can influence a conceptual model – climate, geol-ogy, topography, hydrology, vegetation, and land use. The contents of this section are by no means comprehensive; the intent is to illustrate some of the factors that can help to shape a con-ceptual model.

Water budgets are fundamental compo-nents of any conceptual model of a hydrologic system, providing a link between recharge processes and other processes in the hydrologic cycle. Water-budget equations can be derived for one or more control volumes, such as an aquifer, a watershed, a stream, or even a col-umn of soil (Healy et al., 2007). A water-budget equation allows consideration of the entire hydrology of the system under study, providing information not only on recharge, but also on interrelationships among recharge, discharge, and change in storage. Preliminary water budg-ets can be readily constructed with existing data and refined as various measurements and


Figure 1.2 Schematic showing iterative process for developing a conceptual model of recharge processes.

Initial Steps

Formulate Conceptual Model

Consider allmethods

Selectappropriate

methods

Importance of focused vs. diffuse recharge.Where, when, why does recharge occur?

At what rate?Construct water budget for aquifer, watershed.Apply numerical model of aquifer, watershed.

Review previous studiesAccumulate and analyze existing data

Analyzedata-measurement

errorsspatial/temporal

variabilities

Generateestimate ofrecharge or

drainage

Assess uncertaintiesand sensitivities

Examinespatial/temporal

variabilities

Compare resultsfrom different

methods

Collectdata

Determine- What data to collect?- Where?- For how long?

recharge estimates are obtained. As noted by Lerner et al. (1990), a good method for estimat-ing recharge provides not only an estimate of how much water becomes recharge, but also explains the fate of the remaining water that does not become recharge. Water-budget ana-lyses serve that purpose. In addition, a water-budget equation provides a convenient context for the analysis of assumptions inherent in vari-ous estimation techniques.

Although recharge is important in water-supply studies, recharge rates are sometimes incorrectly equated with the sustainable yield of an aquifer (Meinzer, 1923; Bredehoeft et al., 1982; Bredehoeft, 2002; Alley and Leake, 2004). The term sustainable yield or safe yield refers to the rate at which water can be withdrawn from an aquifer without causing adverse impacts. Those impacts could be in the form of decreased discharge to streams and wetlands, land subsidence, or induced contamination of groundwater, for example, by seawater intru-sion. The notion that recharge is equivalent to sustainable yield is based on an incomplete or incorrect conceptual model of a hydrologic sys-tem. Knowledge of recharge rates is important

for determining sustainable yields in many groundwater systems (Sophocleous et al., 2004; Devlin and Sophocleous, 2005), but recharge rates by themselves are not sufficient for deter-mining sustainability (Bredehoeft et al., 1982; Bredehoeft, 2002). The effects of changes in groundwater levels on groundwater discharge rates and aquifer storage must also be consid-ered. From a hydrologic perspective, sustain-able yield is best studied within the context of the entire hydrologic system of which the aqui-fer is a part, but decisions as to what constitutes a sustainable yield often involve more than just hydrologic considerations. Ecological, cultural, economic, and other considerations should help to determine the acceptability of any effects related to groundwater development (Alley and Leake, 2004).

1.4.1 Spatial and temporal variability in recharge

Recharge rates vary in space in both system-atic and random fashions. This is true for both focused and diffuse recharge. Systematic trends often are associated with climatic trends, but land use and geology are also important. Statewide maps of estimated annual recharge for Texas (Figure 1.3; Keese et al., 2005) and Minnesota (Lorenz and Delin, 2007) both


display trends similar to those in statewide maps of annual precipitation. The concept of recharge rates increasing with increasing pre-cipitation rates is certainly intuitive – recharge cannot occur if water is not available. The random factor in recharge variability can be viewed as local-scale variability that can be attributed, for example, to natural heterogen-eity in permeability in surface soils or variabil-ity in vegetation. Any of the factors addressed below can contribute to apparent random vari-ability. Delin et al. (2000) found that annual recharge varied by more than 50% within what appeared to be a uniform 2.7-hectare agricul-tural field simply because of slight differences in surface topography; the total relief in the field was less than 1.5 m. It could be argued that this difference in topography was not random; indeed, distinguishing between systematic and random patterns of recharge is sometimes a matter of scale. In the context of the entire upper Mississippi River valley, the topographic

differences in this field are minute, apparently random; to someone standing in the field dur-ing a rain storm, the systematic pattern in recharge is obvious.

Recharge also varies temporally. Seasonal, multiyear, or even long-term trends in climate affect recharge patterns. Because of its close link to climate, temporal variability of recharge is addressed more thoroughly in Section 1.4.2. Changes in land use or in vegetation type and density can also result in large changes in recharge rates over time.

The importance of spatial and temporal vari-ability of recharge must be considered within the context of study objectives. Spatial variabil-ity may not be critical for groundwater resource evaluation if an average rate of recharge can be determined for an entire aquifer. Spatial variability is important, though, for assessing aquifer vulnerability to contamination; there-fore, methods that provide point estimates of recharge may be appropriate. Historically, many groundwater-flow models were devel-oped under the assumption that recharge was constant in time. Current model applications typically allow recharge to vary over time but hold it constant for periods of months or years. Recent advances in incorporating land-scape features into combined surface-water and groundwater flow (Section 3.6) will allow impacts of climate, land-use, and vegetation change on water resources to be examined at unprecedented levels of temporal and spatial variability.

1.4.2 ClimateClimate variability is often the most important factor affecting variability in recharge rates. Precipitation, the source of natural recharge, is the dominant component in the water budget for most watersheds. The relation between spa-tial trends of precipitation and recharge has been noted in Section 1.4.1. Temporal variabil-ity in precipitation also is important. Seasonal, year-to-year, and longer-term trends in pre-cipitation, as well as frequency, duration, and intensity of individual precipitation events also affect recharge processes. Conditions are most favorable for water drainage through

Figure 1.3 Map of average annual recharge rate for the state of Texas (Keese et al., 2005).

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the unsaturated zone to the water table when precipitation rates exceed evapotranspiration rates. In regions outside of the tropics, eva-potranspiration rates follow a seasonal trend, with highest rates occurring during summer months and lowest rates in winter months. If precipitation rates are fairly uniform through-out the year, the most likely time of the year for drainage to occur is winter through spring, when precipitation rates exceed evapotran-spiration rates. At a site in the eastern United States, Rasmussen and Andreasen (1959) esti-mated that 62% of recharge over a 2-year period occurred in the months of November through March (Figure 1.4); precipitation was relatively uniform throughout the year, but evapotranspiration rates were lowest during these months.

Duration and intensity of individual pre-cipitation events can have a large influence on recharge in some settings. On the humid, wind-ward side of the Hawaiian Islands, precipita-tion and evapotranspiration rates are relatively uniform throughout the year. Recharge occurs at any time of the year in response to intense

rain storms, when the total precipitation for a day exceeds the daily evapotranspiration rate (Ahuja and El-Swaify, 1979).

In arid regions, focused recharge from ephemeral streams and playas is often the dominant form of recharge. The frequency and duration of streamflow play important roles in the recharge process. The frequency of stream-flow in Rillito Creek in Tucson, Arizona, coin-cides with the frequency of recharge events. Pool (2005) showed that interannual variabil-ity in recharge from the creek is linked to the El Niño/Southern Oscillation climate trend. Years dominated by El Niño conditions (high winter precipitation rates) produced signifi-cantly higher streamflow and recharge rates than years dominated by La Niña conditions.

1.4.3 Soils and geologyPermeabilities of surface and subsurface materials can greatly affect recharge proc-esses. Recharge is more likely to occur in areas that have coarse-grained, high-permea-bility soils as opposed to areas of fine-grained, low-permeability soils. Coarse-grained soils have a relatively high permeability and are capable of transmitting water rapidly. The presence of these soils promotes recharge because water can quickly infiltrate and drain through the root zone before being extracted by plant roots. Finer-grained sediments are less permeable, but are capable of storing greater quantities of water. Thus, in areas of finer-grained sediments, one would expect decreased infiltration, enhanced surface runoff, increased plant extraction of water from the unsaturated zone, and decreased recharge relative to an area of coarser-grained sediments. Permeability also is important in terms of focused recharge. High-permeability streambeds facilitate the exchange of surface water and groundwater. In the Black Hills of South Dakota, most recharge to the Madison Limestone aquifer occurs at high elevations as focused flow from streams that cross rock outcrops (Swenson, 1968; Downey, 1984). In karst regions, dissolution cavities or sinkholes that have developed in the geologic material can rapidly channel streamflow directly to an

Figure 1.4 Average monthly recharge, evapotranspiration, and precipitation for the 2-year period beginning in April 1950 for the Beaverdam Creek watershed in eastern Maryland. Recharge occurs throughout the year, but most of it occurs in the months of November through March when evapotranspiration rates are low (after Rasmussen and Andreasen, 1959).

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aquifer; these cavities also facilitate ground-water discharge in the form of springs.

Subsurface geology influences discharge processes as well as recharge processes. If the rate of discharge from an aquifer is less than the recharge rate, water storage within the aquifer increases. Aquifer storage can reach a maximum at which point additional recharge cannot be accepted, regardless of the amount of precipitation. This condition typically leads to enhanced runoff.

Geophysical techniques have a wide range of uses in geologic and hydrologic studies, pro-viding information on the electrical, physical, and chemical properties of surface and sub-surface sediments. In regards to quantifying groundwater recharge, geophysical methods are most useful for determining soil-water con-tent (Section 5.2.1) and changes in subsurface water storage (Section 2.3.3). However, informa-tion obtained from the application of geophys-ical techniques is also useful in a qualitative sense. Geophysical techniques can be used to infer aquifer geometry and hydraulic proper-ties, important information for shaping con-ceptual models of hydrologic systems and for constructing computer models of groundwater flow (Robinson et al., 2008a).

1.4.4 Surface topographyLand-surface topography plays an important role for both diffuse and focused recharge. Steep slopes tend to have low infiltration rates and high runoff rates. Flat land surfaces that have poor surface drainage are more conducive to diffuse recharge; these conditions also favor flooding. Small, often subtle depressions can have a profound influence on infiltration rates. Delin et al. (2000) showed that, even with highly permeable soils, slight depressions in an appar-ently uniform agricultural field caused runoff to be focused in certain areas, with the result that infiltration (and recharge) in those areas was substantially greater than that in the rest of the field. Even with uniform surface char-acteristics, apparent infiltration rates increase in the downslope direction along a long hill slope (Dunne et al., 1991) because downslope portions of the hill are exposed to runoff from

upslope portions as well as precipitation. Local relief, orientation, and altitude of mountain ranges are additional topographic factors that can affect recharge processes (Stonestrom and Harrill, 2007).

1.4.5 HydrologyA conceptual model of recharge processes needs to consider the surface-water and groundwater flow systems and how they are linked. Are streams in the area perennial or ephemeral? Are streams gaining (receiving groundwater discharge) or losing (providing recharge)? A sin-gle stream could conceivably be losing water to an aquifer in one reach, but gaining water in another reach; the difference between ground-water and surface-water elevations, according to Darcy’s law, determines whether water is moving to or from the subsurface. These are key questions, the answers to which will help shape the conceptual model.

The depth to the water table also is import-ant. If the unsaturated zone is thin, infiltrat-ing water can quickly travel to the water table; recharge may be largely episodic, occurring in response to any large precipitation event. However, shallow water tables are also sus-ceptible to groundwater discharge by plant transpiration. Therefore, water that recharges shallow subsurface systems may only reside in the saturated zone for a short time before it is extracted by plant roots and returned to the atmosphere. Thick unsaturated zones are less likely to have episodic recharge events; recharge would be expected to be seasonal or quasi-steady because wetting fronts moving through the unsaturated zone tend to slow with depth and multiple fronts may coalesce and become indistinguishable from each other.

1.4.6 Vegetation and land useVegetation and land use can have profound effects on recharge processes. Types and densities of vegetation influence patterns of evapotranspiration. A vegetated land surface typically has a higher rate of evapotranspir-ation (and, hence, less water available for recharge) than an unvegetated land surface


under similar conditions. The depth to which plant roots extend influences the efficiency with which plants can extract water from the subsurface. Trees, for example, are capable of drawing moisture from depths of several meters or more. In contrast, shallow-rooted crops cannot access soil water that penetrates to those depths. Thus, enhanced recharge rates in areas with shallow-rooted vegetation are seen in some semiarid regions when native peren-nial vegetation is replaced by shallow-rooted crops (Allison et al, 1990; Scanlon et al., 2005; Leblanc et al, 2008). Nonirrigated agricultural crops can have higher or lower evapotranspir-ation rates than native plants; therefore, it is difficult to generalize as to whether the poten-tial for recharge will increase or decrease due to changes in vegetation alone. In most set-tings, the influence of vegetation is seasonal; in periods of senescence, the presence of plants can actually promote recharge. Decay or shrinkage of roots can expose cavities that can act as preferential flow channels and enhance infiltration. Plowing and tilling in agricultural fields can have opposing effects – breaking up surface crusts, thus increasing the potential for infiltration – and destroying preferential flow channels, thus decreasing infiltration potential. Satellite remote sensing (Section 2.5) can provide information on sur-face characteristics, such as vegetation type and percent coverage, leaf area index, and land use that can be useful in formulating a conceptual model (Brunner et al., 2007).

In the Murray Basin of Australia, native eucalyptus trees were gradually replaced with nonirrigated agricultural crops through the 1900s. Allison and Hughes (1983) estimated natural recharge rates under native vegeta-tion to be less than 0.1 mm/yr. After clearing and subsequent cropping, estimated recharge rates increased by up to two orders of magni-tude (Allison et al., 1990). Unfortunately, the increased recharge has led to increased leach-ing of salts to groundwater and subsequently to the Murray River and its tributaries.

Conversion from natural savannah to nonir-rigated millet crops over large areas of south-west Niger since the 1950s has produced soil

crusting on slopes, resulting in increased run-off and focused recharge beneath ephemeral ponds that collect runoff (Leblanc et al., 2008; Favreau et al., 2009). Increased recharge rates arising from the land-use change can explain the paradoxical relationship between rising groundwater levels (about 4 m between 1963 and 2007) and decadal droughts (23% aver-age annual decline in precipitation from 1970 to 1998 relative to the previous two decades). Areally averaged recharge rates are estimated to have increased from 2 to 25 mm/yr (Favreau et al., 2009).

Irrigation can play an important role in groundwater recharge. Irrigation return flow is any excess irrigation water that drains down beneath the root zone or is captured in drain-age ditches. It constitutes a significant amount of recharge in many areas, especially in arid or semiarid regions where natural recharge rates are low. Fisher and Healy (2008) studied recharge processes at two irrigated agricul-tural fields in semiarid settings; virtually all of the annual recharge occurred during the irrigation season and was attributed to irriga-tion return flow. Faunt (2009) used a complex groundwater-flow model to show that, in add-ition to the natural recharge that occurs dur-ing winter to aquifers in California’s Central Valley, recharge also occurs during the grow-ing season as a result of irrigation return flow. Within the United States, flood irrigation has gradually been replaced with more efficient sprinkle or drip irrigation systems; conversion to these new methods has reduced return flows substantially in some areas (McMahon et al., 2003).

Urbanization brings about many landsurface changes that can have significant rami-fications for recharge processes. Roads, parking lots, and buildings all provide impervious areas that can inhibit recharge. Runoff diversions are common features in urban landscapes. Diversions may lead to surface-water bodies or to infiltration galleries. In the former case, overall recharge for the area is reduced. In the latter case, recharge may not necessarily be reduced, but at the very least it is redirected and may change from a diffuse source to a focused


source. Runoff diversions may be important in terms of aquifer vulnerability to contamination because they can quickly funnel contaminants to the subsurface.

Delivery systems for water supply and treat-ment are additional artifacts of urbanization that can affect recharge processes, both in terms of water supply and potential for contam-ination. These systems consist of open channels or water pipes and sewers. Invariably, there is leakage associated with any delivery system. This leakage, a form of potential recharge, may become actual recharge. Norin et al. (1999) esti-mated that 26% of water transmitted through water mains in Göteborg, Sweden, was lost to leakage.

1.4.7 Integration of multiple factorsA conceptual model of recharge processes is formed by integrating the above factors, and perhaps other factors as well, into hypotheses on where, when, and why recharge occurs. For example, the timing and location of recharge in high mountainous valleys is often controlled by geology, climate, and hydrology. Snowfall in the mountains from late fall through early spring is the source of recharge water. Water is stored in the snowpack until late spring, when it is released to streams as rising air tempera-tures melt snowpacks. As swollen streams flow to valleys, water seeps downward, recharging underlying aquifers. Variations of this predict-able pattern of seasonal recharge occur in many mountainous regions.

Numerical or analytical models of climatic conditions, watershed processes, surface-water flow, groundwater flow, or unsaturated-zone flow are useful tools for integrating the factors affecting the conceptual model of recharge. The suggestion of using a numerical model in the initial stages of a recharge study may seem unusual because oftentimes the specific goal of a recharge study is to develop estimates for use in groundwater-flow models or for com-parison with model results. Nonetheless, a simple numerical model can be a useful tool for identifying important mechanisms, evalu-ating hypotheses included in a conceptual model, and determining optimum locations

and timing for data collection. Application of numerical or analytical models provides ben-efits at all stages of a recharge study. The con-ceptual and numerical models are both part of an iterative process, whereby the models are continually refined and revised as new data, interpretations, and simulation results become available.

1.4.8 Use of existing dataConstruction of a conceptual model should make use of all available data for the study area and surrounding areas. Many of the recharge estimation methods described in the following chapters, including watershed and groundwa-ter-flow models, can be applied without collect-ing any new data. Careful analysis of all existing data precedes any decisions on the collection of new data. Existing databases contain climato-logical data, surface-water flow data, land-use data, groundwater levels, chemistry of surface water and groundwater, physical and hydraulic properties of soils, and land-use characteristics. Pertinent data sources are described more thor-oughly in Chapters 2 and 3.

1.4.9 Intersite comparisonAs a first estimate of recharge for a particular study area, one might use an estimate derived from a site with similar climate, land use, and other features. A review of literature for simi-lar sites is always a worthwhile endeavor. Such a review would benefit from a common clas-sification scheme for climatic/hydrologic/geo-logic provinces. Such a scheme would facilitate intersite comparisons and would also be useful in the construction of conceptual models and selection of appropriate techniques. Currently (2010), there is no such scheme in widespread use, although classification schemes suggested by Salama et al. (1994b) for hydrogeomorphic units and by Winter (2001) for hydrologic land-scapes hold promise. For discussion of generic recharge processes, we resort to the ground-water regions of the United States defined by Thomas (1952); this discussion is provided in Chapter 9 so that concepts and specific methods can be discussed in complementary fashion.


1.5 Challenges in estimating recharge

1.5.1 Uncertainty in recharge estimatesAccurate estimates of recharge are always desired; yet it is beyond our current capabilities to determine, with any degree of confidence, the uncertainty associated with any recharge estimate, let alone claim that an estimate is accurate. Actual recharge rates are unknown; therefore, there are no standards that can be used to evaluate the accuracy of recharge esti-mates. The most serious errors are those asso-ciated with an incorrect conceptual model. An incorrect conceptual model can lead to the appli-cation of inappropriate estimation techniques and meaningless recharge estimates. Any esti-mates based on an incorrect conceptual model are inherently unreliable. Additional sources of error arise from improper application of meth-ods and measurement errors.

Improper application of a method can result from lack of understanding of the method or from failure to adequately account for spatial and temporal variability. Errors related to the latter arise from an inability to measure at enough locations or failure to make measure-ments for a sufficient length of time. Spatial and temporal variabilities of recharge cannot be determined exactly, but they can be examined in some detail with numerical models. A map of the annual recharge for Texas (Figure 1.3) was developed by Keese et al. (2005) by combining a one-dimensional variably saturated water-flow model with spatially variable soil, vegeta-tion, and climate properties. Techniques for the upscaling of point estimates of recharge to large areas are discussed more fully in Chapter 3.

Measurement errors relate to inaccuracies in data collection. Complications arise because the magnitude of a recharge flux is generally quite small and often cannot be measured dir-ectly. Most methods presented in this text are indirect methods that rely on more readily measured parameters, such as changes in water storage or tracer concentrations, to make infer-ences on recharge rates. Measurement errors are the only type of errors that are conducive

to classical error analysis (Lerner et al., 1990). Approaches for such an analysis for water-budget methods are described in Chapter 2.

Concerns about inaccuracy in recharge esti-mates should not deter application of any rea-sonable method for estimating recharge. Simple techniques, applied with careful consideration of conceptual models, can be not only useful, but enduring. Theis (1937) used a simple appli-cation of the Darcy equation to estimate natural recharge rates of between 3 and 7 mm/yr for the southern High Plains aquifer, values that fall midway in the range of estimates generated in subsequent years with more sophisticated tech-niques (Gurdak and Roe, 2009). White (1930) used a salt tracer to estimate subsurface flow into a portion of the Mimbres River watershed in southwestern New Mexico; the groundwater-flow model of Hanson et al. (1994) corroborated that estimate.

Because much of the error associated with a recharge estimate is not quantifiable, it is wise to apply multiple methods for estimat-ing recharge in any study (Lerner et al., 1990; Simmers, 1997; Scanlon et al., 2002b). Estimates from multiple methods may not quantitatively reduce uncertainty; consistency in results, while desirable, may not be a reliable indicator of accuracy. Application of different approaches may have qualitative benefits, however; incon-sistencies in estimates may provide insight into measurement errors or the validity of assump-tions underlying a method and, thus, may provide direction for revising the conceptual model.

1.5.2 Spatial and temporal scales of recharge estimates

The concept of spatial scale is important in terms of selecting appropriate methods. Different methods provide estimates that are integrated over various spatial scales. Some methods provide essentially point estimates; these methods are useful for evaluating aqui-fer vulnerability to contamination in support of land-use decisions, for example; however, appli-cation at many points may be required to deter-mine an areally averaged recharge rate. Other

1.6 DI SCUSS ION 13

techniques provide an average estimate for an entire aquifer or watershed or for a stream reach, but they may provide little insight into recharge rates at specific locations. The user must reconcile project objectives and the spatial scale of a field site of interest with the spatial scale inherent in the techniques to be applied. The spatial scale of each method is discussed in some detail in this text.

Methods for estimating recharge are asso-ciated with temporal scales as well as spatial scales. Some methods, such as the water-table fluctuation method, can provide an estimate of recharge for each individual precipitation event. Most tracer methods, on the other hand, provide a single estimate of recharge that is averaged over the time period between tracer application and tracer sampling. That time period can extend from a few days to years for applied tracers to decades or centuries or even millennia for naturally occurring tracers. In regions with thick unsaturated zones, recharge is sometimes assumed to be constant in time (some methods for estimating recharge are based on this assumption). On the basis of the chloride mass-balance method, Scanlon (1991) determined that there has essentially been no recharge since the Pleistocene in interdrainage areas of the Chihuahuan Desert in the south-western United States. As with spatial vari-ability, the importance of identifying temporal variability can be determined only by the user. The user must take care that the methods that are selected will provide results over the time frame of interest.

Water-budget methods, Darcy methods, and other methods can be applied over a var-iety of time intervals, for example, with daily, monthly, or annual data; however, results from application of the same method over differ-ent time intervals can vary. Consider a simple water-budget approach for a watershed in a subhumid climate, where recharge can occur only when precipitation exceeds evapotrans-piration. On a monthly basis, evapotranspir-ation totals exceed precipitation totals for the months of May through August; therefore, no recharge would be predicted. Within one

of these months, though, there could be days when precipitation exceeds evapotranspiration. Water-budget calculations, when performed with daily values, could conceivably calculate recharge on those days.

1.5.3 ExpenseExpense is a common limitation for apply-ing some recharge-estimation methods. Some methods can be applied by means of a single field trip to collect and analyze soil and water samples (this is sometimes the case with use of tracers); other methods require continuous monitoring over the course of a year or more. Analytical costs for tracers such as carbon-14 may appear to be beyond the means of many recharge studies. But the costs of collecting and analyzing a single set of samples may be less than that required for continuous monitoring. It should be kept in mind that “more expensive” does not always mean “better or more accu-rate.” Methods that require large expenses or hard-to-collect data are seldom applied outside of research studies. In discussions in the follow-ing chapters, the relative cost of each method is addressed. The user must balance cost against expected improvements in recharge estimates and attempt to answer the question: how much will my knowledge of the system be improved and at what cost?

1.6 Discussion

The selection of methods for estimating recharge is largely driven by the goals of a study and the financial and time constraints placed on that study. Examples of study goals might be to:

obtain a long-term average rate of recharge for an aquifer,determine point estimates of recharge in space for assessing aquifer vulnerability to contamination,estimate recharge at specific points in time and space to serve as observations for cali-bration of a model for simulating combined surface-water/groundwater flow,


assess the effects of land-use or climate change on past and future patterns of recharge.

Satisfying these objectives requires recharge estimates that span a wide range of space and time scales; therefore, it is unlikely that a single approach would work for all studies. Spatial and temporal scales associated with each method are discussed in the following chapters to facilitate matching methods with study goals. Also discussed are expenses, in terms of data requirements and manpower, for each method, so the reader can determine whether study con-straints will permit application of a particular method.

The selection of methods also must be tied to the conceptual model of the hydrologic sys-tem under study. Throughout this first chapter, the importance of building a sound conceptual model has been expounded. The final chapter contains a more detailed discussion of concep-tual models, along with examples for typical systems in generic groundwater regions of the United States. Chapters 2 through 8 provide analysis of individual methods for estimating recharge. Each of these methods is based on a set

of assumptions. Assumptions can relate to the mechanism of recharge (diffuse or focused), the timing and the location of recharge, the impor-tance of other components in the water budget of an aquifer or watershed, the uniformity of properties (such as hydraulic conductivity), and various other aspects or features. Assumptions inherent to each method are discussed. The importance of these assumptions needs to be assessed in the context of each application, and the reader must decide whether inherent assumptions are consistent with the concep-tual model of the hydrologic system under consideration.

Estimation of recharge is an iterative proc-ess with continual refinement. The conceptual model can help guide selection of suitable meth-ods and indicate where and when the meth-ods might best be applied. Recharge estimates obtained early in the course of a study may lead to refinement of the conceptual model, which, in turn, could lead to the application of alter-native estimation techniques. Many methods described in the following chapters can be applied with a minimal amount of effort by using existing data.

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Estimating Groundwater Recharge CH1 - Healy, Richard, Scanlon, Bridget