Purdue University Groundwater Scarce Resource Research Paper

  

write a minimum 1800 words scientific paper about groundwater

i will not accept a paper that doesn’t bring an in-depth analysis of the topic

use information from the attached course notes

“groundwater”

cite the book and other scholarly articles, and include an appendix at the end of your paper with charts and figures

Here are the instructions for the paper:

The term project will consist of a concise but in-depth term paper of minimum 1800 words, double-spaced text (not including title page, abstract, figures, tables, and references) based on a project of the student’s choice. Students will pick a topic of their interest from the subjects that we will cover in the Watershed Engineering and Management course. Then students have to choose three related documents (e.g., water plans documents, your book, scientific journals) to the topic of their interest, cite them properly, and if at all possible, submit the PDF of the documents along with the term paper.

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HYDROLOGY AND THE
MANAGEMENT OF WATERSHEDS
A lake scene in northern Minnesota.
Fourth Edition
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A waterfall in Guizhou Province, China, illustrates both the
beauty and power of flowing water.
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Fourth Edition
HYDROLOGY AND THE
MANAGEMENT OF WATERSHEDS
Kenneth N. Brooks
Peter F. Ffolliott
Joseph A. Magner
A John Wiley & Sons, Inc., Publication
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C 2013 by John Wiley & Sons, Inc.
This edition first published 2013
C Iowa State University Press
First edition, 1991
C Iowa State University Press
Second edition, 1997
C Blackwell Publishing
Third edition, 2003
Wiley-Blackwell is an imprint of John Wiley & Sons, formed by the merger of Wiley’s global Scientific, Technical and
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ISBN-13: 978-0-4709-6305-0/2013.
Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and
product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective
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Library of Congress Cataloging-in-Publication Data
Brooks, Kenneth N.
Hydrology and the management of watersheds / Kenneth N. Brooks, Peter F. Ffolliott,
Joseph A. Magner. – 4th ed.
p. cm.
Rev. ed. of: Hydrology and the management of watersheds / Kenneth N. Brooks, et al. 2003.
Includes bibliographical references and index.
ISBN 978-0-470-96305-0 (hardback)
1. Watershed management. 2. Watersheds. 3. Hydrology. I. Ffolliott, Peter F.
II. Magner, Joseph A. III. Hydrology and the management of watersheds. IV. Title.
TC409.H93 2012
627–dc23
2012017574
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1 2013
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Dedicated to
John L. Thames, Hans M. Gregersen, and Leonard F. DeBano
and
Students and the People Who Manage Land and Water for
Future Generations
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CONTENTS
Preface xiii
Definition of Terms xv
1 Watersheds, Hydrologic
Processes, and Pathways
1
3
Introduction 7
Overview 7
Watersheds 10
Integrated Watershed Management 12
Sustainable Use and Development of Natural Resources 14
Watersheds, Ecosystem Management, and Cumulative
Effects 20
Reconciling Watershed and Political Boundaries 21
Summary and Learning Points 24
References 24
Webliography 26
2
Hydrologic Cycle and the Water
Budget 27
Introduction 27
Properties of Water 27
The Hydrologic Cycle 30
Energy and the Hydrologic Cycle 38
Water Flow in Soil 43
Water Flow on Land and in Stream Channels 47
Summary and Learning Points 47
References 48
3
Precipitation 49
Introduction 49
Precipitation Process 50
Rainfall 53
Snowfall 63
Summary and Learning Points 78
vii
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Contents
References 78
Webliography 79
4
Evaporation, Interception, and
Transpiration 81
Introduction 81
The Evaporation Process 82
Evaporation from Water Bodies 83
Evaporation from Soil Surfaces 85
Interception 85
Transpiration 92
Potential Evapotranspiration 103
Estimating Actual Evapotranspiration 105
Summary and Learning Points 109
References 110
5
Infiltration, Pathways of Water Flow, and
Recharge 113
Introduction 113
Infiltration 113
Pathways of Water Flow 125
Summary and Learning Points 138
References 138
6
Streamflow Measurement and
Analysis 141
Introduction 141
Measurement of Streamflow 141
Methods for Estimating Streamflow Characteristics 148
Summary and Learning Points 170
References 171
Webliography 172
7
Groundwater and Groundwater–Surface
Water Exchange 173
Introduction 173
Groundwater 174
Groundwater–Surface Water Exchanges 187
Summary and Learning Points 193
References 194
Webliography 195
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Contents
ix
2 Physical, Chemical, and
Biological Linkages of Water
Flow 197
8
Soil Erosion Processes and Control 199
Introduction 199
Surface Soil Erosion 199
Erosion from Gullies and Ravines 221
Soil Mass Movement 230
Summary and Learning Points 237
References 238
Webliography 241
9
Sediment Supply, Transport, and Yield 243
Introduction 243
Sediment Supply and Transport 244
Measurement of Sediment 255
Sediment Yield 258
Cumulative Watershed Effects on Sediment Yield 260
Summary and Learning Points 263
References 264
10
Fluvial Processes and Implications for
Stream Management 267
Introduction 267
Fluvial Geomorphology 268
Valley and Stream Evaluation and Classification 272
Stream Classification 285
Summary and Learning Points 293
References 293
Webliography 295
11
Water-Quality Characteristics 297
Introduction 297
Chemistry of Precipitation 298
Physical Characteristics of Surface Water 300
Dissolved Chemical Constituents 311
Biological Characteristics 319
Groundwater Quality 323
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Contents
Cumulative Effects 324
Summary and Learning Points 325
References 326
Webliography 328
3 Integrated Watershed
Management 329
12
Managing Wildland Watersheds 333
Introduction 333
Forests 333
Woodlands 364
Rangelands 367
Upland–Downstream Considerations 371
Cumulative Watershed Effects 377
Summary and Learning Points 379
References 380
Webliography 387
13
Managing Riparian Communities and
Wetlands 389
Introduction 389
Riparian Communities 389
Wetlands 401
Cumulative Effects 422
Summary and Learning Points 422
References 423
14
Watershed Management Issues 427
Introduction 427
Fragmentation of Watershed Landscapes 427
Water Harvesting 439
Best Management Practices 442
Regulatory Compliance 446
Climatic Variability 451
Insufficient Information for Decision Making 455
Summary and Learning Points 456
References 458
Webliography 461
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Contents
15
xi
Socioeconomic Considerations in
Integrated Watershed Management 463
Introduction 463
Policies and Policy Processes 464
Planning and Implementation 470
Economic Appraisals 475
Summary and Learning Points 486
References 487
16
Tools and Emerging Technologies 489
Introduction 489
Generalized Hydrologic Simulation Models 490
Technologically Advanced Tools 495
Using the Stable Isotopes of Hydrogen and Oxygen 500
Summary and Learning Points 507
References 508
Webliography 511
Appendix: Units Commonly Used in Hydrologic Work, USA 513
Index 517
Color plates appear between pages 512 and 513
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PREFACE
The fourth edition of this book is a major revision and restructuring of the
earlier editions. The basic concepts and fundamental aspects of hydrology and
hydrologic processes have been retained with the methods and applications
of the science of hydrology in the management of watersheds. We have eliminated the separate chapters on snow hydrology, water resources development
and engineering applications, and hydrologic methods contained in the earlier editions. The subject matter in these chapters has not been eliminated
in this edition, but rather it has been integrated into chapters that emphasize
the hydrologic processes, methods, and applications of integrated watershed
management (IWM).
Given the accessibility of information through the Internet and the rapid
advancement of technologies, we have referenced URLs throughout the book
and presented them in Webliographies at the end of the chapters. This approach
facilitates the acquisition of current methods, models, baseline data summaries,
and applications of technologies for coping with today’s challenging issues of
changing land and water use and climatic variability.
The chapters in Part 1 – Watersheds, Hydrologic Processes, and Pathways
– present an updated foundation for the study of hydrology and watershed
management. Basic properties and principles of water and energy relationships
on earth are presented that provide a basis for understanding the circulation,
water form transformations, and flow processes that occur on watersheds.
Subsequent chapters build on this basic foundation and concentrate on hydrologic processes and methods of measurement and analysis as presented in
the earlier editions. Noted changes include topics of snow measurement and
snowmelt processes covered in the chapter on precipitation; hydrologic methods of estimating streamflow characteristics are presented in the chapter on
streamflow measurement and analysis; and an expanded chapter on groundwater that examines groundwater–surface water exchanges. The chapters in
Part 2 – Physical, Chemical, and Biological Linkages of Water Flows – have
been structured to reflect the current understandings of soil erosion processes
and control of these processes; accumulation and movement of sediment in a
stream channel; and the hydrologic linkages to water quality. A new chapter emphasizing geomorphology, valley and channel forming processes, evaluation,
and classification is also included in this part of the book. Part 3 – Integrated
Watershed Management – combines and updates much of the information
presented in the third edition of the book into chapters on the management of
forests, woodlands, rangeland watersheds for maintaining and where possible
enhancing the flows of high-quality water from watersheds. The separate chapters on riparian systems and wetlands found in earlier editions of the book have
been combined in a chapter on managing these ecosystems within the context
of a watershed landscape. A chapter on the effects of fragmenting wildland
xiii
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Preface
watersheds into agricultural and urban areas; implementation of “best management practices”; the importance of regulatory compliance, and coping with
climatic variability are contained in this part of the book. Socioeconomic considerations of watershed management are presented in a chapter. A chapter on
tools and emerging technologies available to managers for more efficient and
responsive watershed management concludes the book.
This fourth edition of the book is intended largely for introductory college
courses in hydrology and watershed management. However, the book can also
serve as a reference for personnel of governmental and nongovernmental organizations with responsibilities for the management of land, water, and other
natural resources on watershed landscapes. The book is also suited for international audiences with examples of watershed processes and the management
of land and water extending beyond the US borders. Examples of applications
are liberally presented in the text and in boxes throughout the book to help students understand how basic principles and methods can be applied in practice.
Metric (SI) units are used in the book with the exception of where original
formulas, figures, tables, and other unit-dependent relationships were developed originally in English (customary) units and where the conversion to metric
units is awkward. A table of metric to English units is presented in an appendix
to assist the reader in making conversions if desired.
The authors thank Clara M. Schreiber once again for her dedicated work
in the preparation of this fourth edition. Clara has also been a partner in all of
the earlier editions of the book. The contributions of Hans M. Gregersen, the
late John L. Thames, and Leonard F. DeBano – collaborating authors of the
earlier editions of this book – have been integral to the evolution of this fourth
edition and are greatly appreciated. The contributions of Mark Davidson, Britta
Suppes, Linse Lahti, Mary Presnail, Peter Magner, and Dain Brooks in providing
new figures and photographs are appreciated. They have improved the visual
presentations of the book.
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DEFINITION
OF TERMS
Hydrology is the science of water concerned with the origin, circulation,
distribution, and properties of waters of the earth.
Forest Hydrology/Range Hydrology/Wildland Hydrology refer to branches of
hydrology that deal with the effects of vegetation and land management on
water quantity and quality, erosion, and sedimentation in the respective
settings.
A watershed or catchment is a topographically delineated area drained by a
stream system; that is, the total land area above some point on a stream or
river that drains past that point. A watershed is a hydrologic unit often used as
a physical-biological unit and a socioeconomic-political unit for the planning
and management of watershed resources.
River basin is similarly defined but is a larger scale. For example, the
Mississippi River Basin, the Amazon River Basin, and the Congo River Basin
include all lands that drain through those rivers and their tributaries into the
ocean.
Integrated watershed management is the process of organizing and guiding
land, water, and other natural resource use on a watershed to provide desired
goods and services to people without affecting adversely soil and water
resources. Embedded in the concept of integrated watershed management is
the recognition of the interrelationships among land use, soil, and water, and
the linkages between uplands and downstream areas.
Watershed management practices are those changes in vegetative cover,
land use, and other nonstructural and structural actions taken on a watershed
to achieve watershed management objectives.
xv
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HYDROLOGY AND THE
MANAGEMENT OF WATERSHEDS
A lake scene in northern Minnesota.
Fourth Edition
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PLATE 1.
Measuring groundwater levels in a forested wetland with a pressure transducer in a shallow well (Photograph by Chris Lenhart)
Hydrology and the Management of Watersheds, Fourth Edition. Kenneth N. Brooks, Peter F. Ffolliott and Joseph A. Magner.

C 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc.
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PLATE 2.
Forested headwater watersheds are the source of most of the streamflow
in the United States as depicted in this scene in the Northern Cascades of Washington
(Photograph by Mark Davidson)
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PLATE 3.
Students measuring streamflow with a current meter (Photograph by
Lucas Bistodeau)
PLATE 4.
The hydrologic cycle
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PLATE 5.
Unstable stream channels result in bluff erosion as depicted in the stream
in southern Minnesota, USA (Photograph by Mark Davidson)
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PLATE 6.
Soil mass erosion can provide sediment to streams as depicted in this
scene in southwestern Montana, USA (Photograph by Mark Davidson)
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PLATE 7.
A sediment splitter diverts progressively smaller fractions of streamflow
to a collection tank to sample suspended sediment flowing through a rectangular weir in
north-central Arizona. By also collecting bedload materials that settle in the catchment
basin upstream of the weir total sediment yields can be measured from a watershed
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PLATE 8.
Comparison of the Channel Evolution Model (from Simon and Hupp,
C Wildland
1986) and the corresponding four Rosgen stream types (from Rosgen, 2006.
Hydrology, with permission)
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PLATE 9.
The Rodeo-Chediski fire in Arizona, USA, burned more than 189 000 ha
of mostly ponderosa pine forest in 2002, resulting in serious soil and water impacts
(Photograph by the US Forest Service)
PLATE 10.
Urbanization creates more surface runoff and compounds localized
flooding – a 2011 scene of the Mississippi River at flood stage in St. Paul, Minnesota,
USA (Photograph by Mark Davidson)
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Postfire
Prefire
PLATE 11.
Streamflow discharge peak-flow stage shortly after cessation of the
Rodeo-Chediski Wildfire in a ponderosa pine watershed in Arizona; prefire peakdischarge stage noted
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PLATE 12.
A healthy riparian community is able to maintain an equilibrium between
the streamflow forces acting to produce change and the resistance of vegetative, geomorphic, and structural features to the change. Riparian area in Honduras (Photograph
by Mark Davidson)
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PLATE 13.
Wetlands are characterized by the permanent or frequent presence of
water and occur in a variety of inland and coastal land forms. Pictured is a black ash
wetland in northern Minnesota (Photograph by Mark Davidson)
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PLATE 14.
Undesirable effects on soils and water such as illustrated in this photo
can be avoided when BMPs are properly implemented. In this case, a perennial vegetated
buffer would provide some degree of protection for the water quality in this stream
(Photograph by Mary Presnail)
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PART 1
Watersheds,
Hydrologic
Processes, and
Pathways
PHOTO 1.
Measuring groundwater levels in a forested wetland with a pressure
transducer in a shallow well (Photograph by Chris Lenhart) (For a color version of this
photo, see the color plate section)
Hydrology and the Management of Watersheds, Fourth Edition. Kenneth N. Brooks, Peter F. Ffolliott and Joseph A. Magner.

C 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc.
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Part 1 Watersheds, Hydrologic Processes, and Pathways
Knowledge of the inherent characteristics of a watershed and hydrology provides the
foundation for understanding the role of integrated watershed management (IWM) in
achieving sustainable development and the use of land and water. An understanding of
the hydrologic cycle and energy relationships on earth is fundamental to the study of
hydrology and, therefore, necessary for making informed decisions in planning and implementing IWM practices. Embedded in this knowledge is an understanding of the nature
of precipitation falling on the watershed; the magnitudes of evaporation, interception, and
transpiration losses on a watershed; the infiltration and percolation of precipitation reaching
the ground surface; and the pathways of water flow into stream channels and recharging
groundwater aquifers. Methods of measuring or estimating streamflow discharges, including peak flows, minimum flows, volumes of flow, and the routing of streamflow from the
watershed of origin to downstream points are also basic to understanding hydrology. Concepts of groundwater hydrology and processes of groundwater and surface water exchange
are fundamental in understanding surface water–groundwater relationships in watersheds.
Part 1 of this book focuses on obtaining this information.
PHOTO 2.
Forested headwater watersheds are the source of most of the streamflow
in the United States as depicted in this scene in the Northern Cascades of Washington
(Photograph by Mark Davidson) (For a color version of this photo, see the color plate
section)
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July 17, 2012
5
Hydrologic processes are described in detail in Part 1. The topics presented in Chapter
1 of the book include a discussion on the inherent characteristics of a watershed; the importance of IWM; and fostering the sustainable use and development of natural resources
while coping with land and water scarcity. Chapter 2 focuses on the hydrologic cycle, the
water budget, the energy budget, and the energy processes that drive the hydrologic cycle.
Precipitation, including rainfall and snowfall – primary inputs to the water budget, are considered in Chapter 3. The processes of evaporation, interception, and transpiration losses
and their importance are discussed in Chapter 4. Infiltration processes and measurement
and the pathways of water flow within a watershed system including groundwater recharge
are the primary topics of Chapter 5. Methods of measuring and analyzing the streamflow
response of watersheds are presented in Chapter 6. Basic concepts of groundwater and
groundwater–surface water exchanges are presented in Chapter 7. The collective information in these chapters is basic to hydrology and provides the background necessary for a
more comprehensive appreciation of how climatic factors, watershed characteristics, and
land-use activities affect the hydrologic responses of watersheds.
PHOTO 3.
Students measuring streamflow with a current meter (Photograph by
Lucas Bistodeau) (For a color version of this photo, see the color plate section)
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CHAPTER 1
August 25, 2012
Introduction
OVERVIEW
Our perspective of watershed management is that water and land resources must be managed
in concert with one another. While hydrology and water quality are essential components
of watershed management and are the subjects of much of this book, we also recognize
the importance of ecosystem functions, land productivity, stream-channel morphology, and
the actions of people on the land as integral parts of watershed management. To emphasize
this holistic view, the concept of Integrated Watershed Management (IWM) is embedded in
discussions presented in the chapters of this book. IWM deals not only with the protection of
water resources but also with the capability and suitability of land and vegetative resources
to be managed for the production of goods and services in a sustainable manner. Few
watersheds in the world are managed solely for the production of water. Some municipal
and power company watersheds that drain into reservoirs are the exception. Since water
affects what we do on the landscape, watersheds serve as logical and practical units for
analysis, planning, and management of multiple resources and coping with water issues
regardless of the management emphasis.
A basic understanding of hydrology is fundamental to the planning and management
of natural resources on a watershed for sustainable use. Hydrology enters explicitly and
directly into the design of water resource projects including reservoirs, flood control structures, navigation, irrigation, and water quality control. Knowledge of hydrology also helps
us in balancing the demands for water supplies, avoiding flood damages, and protecting the
quality of streams, lakes, and other water bodies.
One of our concerns and an incentive for writing this book is that hydrology is
not always considered in the management of forests, woodlands, rangelands, agricultural
Hydrology and the Management of Watersheds, Fourth Edition. Kenneth N. Brooks, Peter F. Ffolliott and Joseph A. Magner.

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Part 1 Watersheds, Hydrologic Processes, and Pathways
croplands, or in the array of human development activities on rural landscapes even though
it should be! Ignoring development and land-use effects on soil and water resources is
shortsighted and can lead to unwanted effects on a site and in downstream areas. For example, altering forested uplands, riparian communities, and wetland ecosystems can affect the
flow and quality of water. Changes in vegetative cover that increase soil erosion can lead to
soil instability and long-term losses of plant productivity. The consequences of soil erosion
on upland watersheds can alter streamflow quantity and quality downstream. Changes in
streamflow and sediment transport can, in turn, alter stream-channel morphology and affect
the stability of rivers.
Hydrologic concepts and concerns about land use and water date back to some of the
earliest recorded history. The evolution of hydrology from Egyptian texts as early as 2500
bc, to the ancient Indian writings from Vedic times before 1000 bc, to decrees recognizing
the interrelationships between water and forests in Europe following the Dark Ages, to
contemporary publications into the twenty-first century all illustrate the growing awareness
of the importance of hydrology to the management of water and other natural resources. A
more detailed timeline of the history of hydrology and watershed management is presented
in Box 1.1.
Box 1.1
A Historical Look at Hydrology, Water, Watershed Management,
and People
Year
US populationa
Event
Reference
2125 BC
?
Baines (2011)
(www.bbc.co.uk)
1000 BC
?
∼900 BC
?
∼360 BC
?
1342
?
Canals move water from Nile
River to supply water for
irrigation and human
consumption
An understanding of the
hydrologic cycle was
indicated in Indian texts from
the Vedic times
Chinese scholars develop an
accurate understanding of the
hydrologic cycle
Plato writes about land
degradation, flooding, and
erosion
The first written record of a
“protection forest” being
established by a community
in Switzerland
Chandra (1990)
Kittredge (1948)
Kittredge (1948)
Kittredge (1948)
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Chapter 1 Introduction
Year
US populationa
Event
Reference
ca 1670
∼110,000
(immigrants)
Kittredge (1948)
1897
72 million
1903
81 million
1909
90 million
1912
95 million
1930s
123 million
1950s and
1960s
151–179 million
2010
308.7 million
Perrault accurately quantifies
the water balance of the Seine
River watershed in France
Organic Act . . . authorized the
establishment of National
Forests on public lands in the
west
Gifford Pinchot publishes A
Primer of Forestry in which he
states “A forest, large or
small, may render its service
in many ways . . . especially
against the dearth of water in
streams.”
Wagon Wheel Gap paired
watershed experiment
initiated in Colorado
Raphael Zon publishes Forests
and Water in Light of
Scientific Investigation as an
appendix to the Report of the
National Waterways
Commission
Coweeta Hydrologic Laboratory
founded near Asheville, North
Carolina
Watershed studies established
at H.J. Andrews, Oregon; Fool
Creek, Colorado; Beaver
Creek, Arizona; Fernow, West
Virginia; Hubbard Brook, New
Hampshire; Marcell,
Minnesota; and other
“barometer watersheds” on
National Forests
US population; more than 1200
locally led watershed
management districts,
associations, partnerships,
councils, and river basin
commissions emerged for
resolving watershed problems
and achieving the goals of
IWM
Glasser (2005)
Pinchot (1903)
Bates and Henry
(1928)
Zon (1927)
Ice and Stednick
(2004)
Glasser (2005)
Ice and Stednick
(2004)
US Census
Bureau
Source: US Census Bureau; 2010 population from website: http://2010.census.gov/2010census/data/
a
Linearly interpolated from decadal census data.
9
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WATERSHEDS
Watersheds are biophysical systems that define the land surface that drains water and waterborne sediments, nutrients, and chemical constituents to a point in a stream channel or a
river defined by topographic boundaries. Watersheds are the surface landscape systems that
transform precipitation into water flows to streams and rivers, most of which reach the
oceans. Watersheds are the systems used to study the hydrologic cycle (see Chapter 2),
and they help us understand how human activities influence components of the hydrologic
cycle.
Watersheds and Stream Orders
Watersheds and stream channels can be described according to their position in the landscape. It is useful to refer to an established nomenclature of stream orders (Horton, 1945;
Strahler, 1964) in discussing watersheds and the water in streams that emanates from them.
The commonly used method of stream orders classifies all unbranching stream channels as
first-order streams (Fig. 1.1). A second-order stream is one with two or more first-order
stream channels; a third-order stream is one with two or more second-order stream channels, and so forth. Any single lower stream juncture above a larger order stream does not
change the order of the larger order stream. Thus, a third-order stream that has a juncture
with a second-order stream remains a third-order stream below the juncture.
The watershed that feeds the stream system takes on the same order as the stream.
That is, the watershed of a second-order stream is a second-order watershed and so on.
While there is little evidence that streamflow and watershed characteristics are related to
stream order, the use of this terminology helps one place a stream channel or a watershed
in the context of the overall drainage network of a river basin. The physical and biological
characteristics of watersheds and the climate in which they exist determine the magnitude
FIGURE 1.1.
Stream order system by Horton (1945) as modified by Strahler (1964)
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and pathways of water flow. Furthermore, the hierarchy of watersheds within a river basin
generally influences the magnitude of water flow.
A Geomorphologic Perspective
As the upper-most watersheds in a river basin, first-order watersheds, also called headwater
watersheds, are the most upstream watersheds that transform rainfall and snowmelt runoff
into streamflow. Headwater streams comprise 70–80% of total watershed areas (Sidle et al.,
2000) and contribute most of the water reaching the downstream areas in river basins
(MacDonald and Coe, 2007). Headwater watersheds are often forested or once were prior
to the expansion of agriculture, urban areas, and other human development activities. These
headwaters are particularly important in water resource management. First-order streams
in mountainous regions occur in steep terrain and flow swiftly through V-shaped valleys.
High rainfall intensities can erode surface soils and generate large magnitude streamflow
events with high velocities that can transport large volumes of sediment downstream.
Over geologic time, mountains erode and sediment becomes deposited downstream (Fig.
1.2). As water and sediment from headwater streams merge with higher order streams,
sediment is deposited over vast floodplains as rivers reach sea level. A transitional zone
EROSION ZONE:
Tributaries
TRANSITION ZONE:
Main Stem
DEPOSITION ZONE:
Mouth
ELEVATION
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DISTANCE UPSTREAM FROM MOUTH
New Floodplain
Old Floodplain
FIGURE 1.2.
Rivers generally flow from an upper, high-gradient erosion zone
through a transition zone to a low-gradient deposition zone (Schumm, 1977, as modified
by Verry, 2007)
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Part 1 Watersheds, Hydrologic Processes, and Pathways
exists between the steep headwater steams and the lower zone of deposition at the mouth of
major rivers and is typically characterized by broad valleys, gentle slopes, and meandering
streams.
The “work” of water on soils, hillslopes, and within rivers forms landscapes with topography and soils that are better suited for some types of land use than others. Agricultural
centers have developed in the transitional and depositional areas of a river basin while the
steeper uplands are likely to prohibit intensive agricultural cultivation, resulting in landscapes with forests, woodlands, and rangelands suitable for forestry and livestock-grazing
enterprises.
Watershed Assessments
The hydrologic response of watersheds to climatic variability and land-use changes will
be discussed in this book requiring that methods of delineating watershed boundaries,
determining watershed areas, and assessing a myriad of watershed metrics be understood.
Geographic Information Systems (GIS), maps, and other tools, such as Google Earth,
provide the means to quantify and describe watersheds, their vegetative cover, geology,
and soils, and help delineate people’s activities occurring across the landscape. Physical
descriptions, such as watershed area, slope, stream-channel lengths, and drainage density
(the sum of all channel lengths in a watershed divided by the watershed area), are important
descriptors of watersheds.
Considerable information is available for assessing watersheds in the United States.
Hydrologic Unit Codes (HUCs) are used by the U.S. Geological Survey to classify four
levels of hydrologic units beginning with 21 major geographic regions that contain either
major river basins or a series of river basins in a particular region. The major regions are
subdivided into 221 subregions with 378 accounting units and ending with 2264 watershed
units (Seaber et al., 1987). HUCs are used for mapping and describing areas on the landscape and in some cases coincide broadly with ecoregions (Omernik, 2003). HUCs can be
watersheds or other land units that have similar characteristics of climate, vegetation, geology, soils, land use, and topography. As such, HUCs would be expected to have common
hydrologic properties that differ from those HUCs with a different set of characteristics.
Methods of assessing watershed characteristics that are needed for certain applications
are presented in the context of those applications in this book. A discussion of tools and
emerging technologies for making hydrologic and watershed assessments is presented in
Chapter 16.
INTEGRATED WATERSHED MANAGEMENT
IWM involves an array of vegetative (nonstructural) and engineering (structural) practices
(Gregersen et al., 2007). Soil conservation practices, constructing dams, and establishing
protected reserves can be tools employed in IWM as can be land-use planning that entails developing regulations to guide timber-harvesting operations, road-building activities,
urban development, and so forth. The unifying focus in all cases is placed on how these
varying activities affect the relationships among land, water, and other natural resources
on a watershed. The common denominator or the integrating factor is water. This focus on
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water and its interrelationship with land and other natural resources and their use is what
distinguishes IWM from other natural resource management strategies.
On the one hand, IWM is an integrative way of thinking about people’s activities
on a watershed that have effects on, or are affected by, water. On the other hand, IWM
includes tools or techniques such as the physical, regulatory, or economic means for responding to problems or potential problems involving the relationship between water and
land uses. What sometimes confuses people is the fact that these tools and techniques are
employed not only by those designated as “watershed managers” but also by foresters,
farmers, soil conservation officers, engineers, and so forth. In reality, all are watershed
managers.
This fact is both the dilemma and the strength of watershed management. In practice,
activities using natural resources are decided on and undertaken by individuals, local
governments, and various groups that control land in a political framework that has little
relationship to, and often ignores, the boundaries of a watershed. For example, forested
headwater areas of a river basin could be under the control of a governmental forestry
agency but the middle elevations and lowlands could be a composite of private, municipal,
and individual ownerships.
Activities are often undertaken independently with little regard to how they affect other
areas. Despite this real world of disaggregated and independent political and economic
actions, it remains a fact that water and its constituents flow from higher to lower elevations
according to watershed boundaries, regardless of the political boundaries. What one person
or group does upstream can affect the welfare of those downstream. Somehow, the physical
facts of watersheds and the political realities have to be brought together to achieve IWM.
Within this broad focus there is concern with both how to prevent deterioration of an existing
sustainable and productive relationship between the use of land, water, and other natural
resources and how to restore or create such a relationship where it has been damaged or
destroyed in the past.
Embodied in IWM are
r preventive strategies aimed at preserving existing sustainable land-use practices;
and
r restorative or rehabilitation strategies designed to overcome identified problems or
improve conditions to a desirable level where desirable is defined in ecological,
environmental, and political terms.
Both strategies respond to the same types of problems. However, in one case the
objective is to prevent a problem from occurring while in the other case the objective
is to improve conditions once the problem has occurred. In reality, we are dealing with
a continuum from regulatory support and reinforcement of existing sustainable land-use
practices (preventive strategy) to such actions as emergency relief, building of temporary
water-control structures, restoring wetlands, or changing land use on fragile and eroded
lands (rehabilitation strategy). These routine preventive strategies and actions are as important as the more dramatic and visible restorative actions. Losses avoided through preventive
strategies are as important to people as gains from rehabilitating a degraded watershed. In
economic terms, the cost of preventing losses of productivity in the first place can be much
lower than the cost of achieving the same benefit through more dramatic actions to restore
productivity on degraded lands.
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SUSTAINABLE USE AND DEVELOPMENT OF
NATURAL RESOURCES
There are many examples of the need to manage watersheds better to meet current and
future demands for water and other natural resources in a more sustainable manner. It must
be recognized, however, that practices relating to resource use and management do not
depend solely on the physical and biological characteristics of watersheds. Institutional,
economic, and social factors such as the cultural background of rural populations and the
nature of governments need to be fully integrated into solutions that meet environmental,
economic, and social objectives (Gregersen et al., 2007). How these factors are interrelated
can best be illustrated by looking at specific issues.
Land and Water Scarcity
Arable land and water resources are becoming scarcer with the earth’s expanding population
of people. These scarcities and the human responses to these scarcities pose challenges to
sustainable development and can have serious environmental consequences. Changing
climatic conditions and weather patterns add uncertainty to future land and water resource
management. It is unclear where and how much the supplies of freshwater will change with
changes in climate and weather. What is clear, however, is that the increasing water demands
caused by increasing populations of people and expanding economic developments will
pose far greater problems by the year 2025 than currently (Vorosmarty et al., 2000).
Knowledge, information, and technologies will no doubt expand our capability to
deal with shortages of resources. To harness these capabilities, however, will require an
integrated and interdisciplinary approach to planning and managing natural resources. The
lack of such an approach has been problematic as emphasized by Falkenmark (1997) who
stated, “In environmental politics, land and water issues are still seen as belonging to
different worlds, taken care of by different professions with distinctly different education
and professional cultures.” Land, water, and other natural resources have traditionally been
managed in isolation of one another as a consequence. However, we recognize that landuse affects the quantity and quality of water flow in a watershed. Water development,
conversely, affects land use. Understanding these relationships and linkages is essential to
achieving sustainable use and development of natural resources.
Land Scarcity
Land scarcity has been exacerbated in many developing countries by the rural poor who clear
forests to grow agricultural crops, cultivate steep uplands, and overgraze fragile rangelands
to meet their food and natural resource needs. Watershed degradation frequently results,
further reducing land productivity that in turn causes more extensive and intensive land use.
In other instances, irrigation practices intended to expand agricultural productivity have
been inappropriate and have caused land productivity to diminish through salinization.
As watershed conditions deteriorate, the people living in both uplands and downstream
areas become impacted. This cycle of deteriorating land productivity is a clear indicator of
nonsustainable land use.
Of the 8.7 billion ha of agricultural land, forests, woodlands, and rangelands worldwide,
almost 25% has been degraded since the mid-1900s with 3.5% being severely degraded
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(Scherr and Yadav, 1996). Problems of desertification have received global attention as the
productivity of some of the poorest countries in the world continues to decline. Ironically,
we have the know-how to manage resources in a sustainable manner. What is most often
lacking are the policies and institutions to promote and sustain sound resource use.
Water Scarcity
Water scarcity has attracted attention globally and is considered the major environmental
issue facing the twenty-first century. The United Nations commemorated a World Day for
Water in March 2001 where speakers concluded that demands for freshwater exceeded
supplies by 15–20% and that two-thirds of the world’s population will experience severe
water shortages in the next 25 years. Although the 9000–14,000 km3 of freshwater on
earth should be sufficient to support expanding human populations for the foreseeable
future, unequal distribution results in water scarcity (Rosegrant, 1997). For example, per
capita freshwater supplies in Canada are approximately 120,000 m3 compared to Jordan’s
300 m3 . In China, per capita freshwater supplies are 2700 m3 with 600 of its major
cities suffering from water shortages. Although China encompasses the Yangtze River, the
third largest river in the world, water scarcity is pronounced in areas north of the river
which represent 63.5% of the country’s land area but only 19% of its water resources.
To enhance water supply in northern China, where more than one-third of the county’s
population lives, the largest water diversion project is underway to divert 44.8 billion m3
of water annually from the Yangtze, Huaihe, and Haine rivers in the south to the drier
north (www.water.technology.net/projects/south_north/). Other diversions are planned for
western regions of the country.
Such programs have dramatic effects on land and water, placing greater importance on
improved watershed planning and management to protect the life of major reservoir and
diversion investments. The fact is that water has become a global environmental priority,
exceeding climate change according to a GlobeScan/Circle of Blue Report (Box 1.2).
Examples of the role of IWM in coping with water scarcity and other natural resource
issues are outlined in Table 1.1.
UNESCO (www.unesco.org, 2011) estimates that global water withdrawals have
tripled over the past 50 years. The International Water Management Institute (IWMI)
paper of 2007 (www.iwmi.cgiar.org) reported that much of the increased use of water is
attributed to irrigation of agricultural lands. Competition for water between agricultural and
other interests (municipal and environmental) will undoubtedly increase in coming decades
with the world’s population projected to grow to 9 billion people by 2050.
Coping with Hydrometeorological Extremes
Floods, landslides, and debris torrents that result from excessive precipitation events and
the droughts that result from deficient precipitation represent both dimensions of hydrometeorological extremes. The disasters and famine that can result present some of the greatest
challenges to land and water resource managers. Even though we call them extreme events,
it should be recognized that floods and droughts occur naturally and are not necessarily rare.
However, forecasting when and where they will occur and at what magnitude is uncertain.
Whether these hydrometeorological extremes end up as disasters depends on their impacts
on people and natural ecosystems. What can be done to cope better with such extreme
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TABLE 1.1.
The role of watershed management in developing solutions to natural
resource problems
Problem
Possible alternative
solutions
Associated watershed management
objectives
Deficient water
supplies
Reservoir storage and water
transport
Minimize sediment delivery to
reservoir site; maintain watershed
vegetative cover
Develop localized collection and
storage facilities
Convert from deep-rooted to
shallow-rooted species or from
conifers to deciduous trees
Maintain vegetative cover to
minimize erosion
Not applicable
Management of recharge areas
Water harvesting
Vegetative manipulation;
evapotranspiration
reduction
Cloud seeding
Flooding
Desalinization of ocean water
Pumping of deep
groundwater and irrigation
Reservoir storage
Construct levees,
channelization, etc.
Floodplain management
Energy shortages
Revegetate disturbed and
denuded areas
Utilize wood for fuel
Develop hydroelectric power
project
Food shortages
Develop agroforestry
Increase cultivation
Increase livestock production
Import food from outside
watershed
Erosion/sedimentation
from denuded
landscapes
Erosion control structures
Contour terracing
Revegetate
Minimize sediment delivery to
reservoir site; maintain watershed
vegetative cover
Minimize sediment delivery to
downstream channels
Zoning of lands to minimize human
activities in flood-prone areas;
minimize sedimentation of
channels
Plant and manage appropriate
vegetative cover
Plant perpetual fast-growing tree
species; maintain productivity of
sites; minimize erosion
Minimize sediment delivery to
reservoirs and river channels;
sustain water yield
Maintain site productivity; minimize
erosion; promote species
compatible with soils and climate
of area
Restructure hillslopes and other
areas susceptible to erosion; utilize
contour plowing, terraces, etc.
Develop herding–grazing systems for
sustained yield and productivity
Develop forest resources for pulp,
wood, wildlife products, etc., to
provide economic base
Maintain life of structures by
revegetation and management
Revegetate, mulch, stabilize slopes
and institute land-use guidelines
Establish, protect, and manage
vegetative cover until site recovers
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17
(Continued)
Possible alternative
solutions
Associated watershed management
objectives
Poor-quality drinking
water
Develop alternative supplies
from wells and springs
Treat water supplies
Polluted streams/
reduced fishery
production
Control pollutants entering
streams
Protect groundwater from
contamination
Filter through wetlands or upland
forests
Develop buffer strips along stream
channels; maintain vegetative
cover on watersheds; develop
guidelines for riparian zones
Use forests and wetlands as
secondary treatment systems for
wastewater
Problem
Treat wastewater
events is a critically important question that planners and managers as well as students of
hydrology and watershed management should be prepared to address.
Floods, landslides, and debris torrents result in billions of dollars being spent globally
each year for flood prevention, flood forecasting, and stabilization of hillslopes and stream
channels. However, the cost of lives and property damage due to floods, landslides, and
debris flows is staggering. The impacts of these naturally occurring phenomena can be
exacerbated by human encroachment on floodplains and other hazardous areas, which is
often the result of land scarcity.
Responses to disasters caused by too much water from flooding or too little water
that results from droughts are most often short term and limited to helping those who
have been directly impacted rather than dealing with the causes. Interest in coping with
extreme events flourishes immediately after they have taken their toll on human lives and
property but becomes lower priority with reduced funding as memories fade with time.
The term crisis management captures this approach. It is not unreasonable to ask the
question: what (if anything) can be done to prevent such disasters? There is no single
approach that applies to all situations. To paraphrase Davies (1997), there are essentially
three options available to reduce or prevent disasters caused by excessive amounts of water.
These options are modifying the natural system, modifying the human system of behavior,
and some combination of these two.
It is important to recognize that no matter how advanced our technologies might
become, the degree to which such events result in disasters is largely because of the
behavior of people on watersheds. Nevertheless, while solutions are not necessarily easily
found, they must be based on sound hydrological principles.
Attempts to control events by modifying hydrologic and other natural systems most
often entail the use of engineering measures including reservoirs, levees, and channelization
aimed at controlling floods (see Table 1.1). There are increasing attempts to apply vegetative
management and bioengineering measures along with structures to exert some control
over extreme events. However, it is unrealistic to believe that we can mitigate the most
extreme of hydrometeorological events. The development of such measures requires an
understanding of natural hydrologic systems and processes, how these systems function,
and how mitigation impacts other aspects of the environment.
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Box 1.2
Water – The Top Global Environmental Concern?
http://www.circleofblue.org/waternews/2009/world/waterviews-water-topsclimate-change-as-global-priority/. Accessed April 7, 2010
A poll of 1000 people in each of 15 countries, including Canada, China,
India, Mexico, Russia, United Kingdom, and the United States, suggested that
fresh clean water has a greater impact on people’s quality of life, exceeding
concerns of air pollution, species extinction, loss of habitat, depletion of
natural resources, and climate change. Water concerns range across the
spectrum of water resources with top concerns expressed by countries as
follows:
r India – lack of safe drinking water; water pollution; access to fresh
clean water is highest priority; the Yamuna River is one of the most
polluted waterways in the world.
r Russia – industrial water pollution of major rivers is of foremost
concern.
r China – industrial pollution of the Yellow River and water scarcity
plague many areas of China.
r United States – dependence on the Colorado River for irrigation in
Imperial Valley and Coachella Valley of southern California has made
water extremely scarce and valuable; the demise of the Salton Sea is
attributed to high consumptive use and evaporation of water plus
pollution from agricultural chemicals and trace metals.
r Mexico – Mexico City’s supply is insufficient to meet the demands of
its human population of more than 25 million; this has led to the
depletion of underground aquifers and impaired waters due to
human sewage.
We can view the options presented in Table 1.1 as those that attempt to move or control
water and those that attempt to move or manage people on a watershed. Furthermore,
some of the measures listed tend to be the responsibility of water resource organizations
with an engineering bias such as governmental agencies responsible for flood control or
irrigation projects, or private utilities companies responsible for hydropower. Organizations
responsible for land management often have a land production–ecological viewpoint. Too
often the separation of responsibilities and the lack of coordination lead to piecemeal
programs that result in unwanted effects and, furthermore, do not have the long-term
perspective needed to develop sustainable solutions.
While it is true that land use might not affect the magnitude of the most extreme
hydrometeorological events, people’s activities on a watershed and the cumulative effects
of these activities determine the extent to which the events impact human life and welfare.
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The reliance on large-scale engineering structures to store or divert water with the intention
of meeting water management objectives has been met with objections from environmental
groups for some time. However, there is concern by some professionals that an over-reliance
on engineering measures such as dams, levees, or stream channelization has
r led to unintended and unwanted effects;
r reduced the hydrologic function and environmental values associated with natural
rivers, floodplains, and estuaries; and
r imparted a false sense of security to those living in downstream areas.
Agricultural expansion and urban development have further reduced natural riparian communities, wetland ecosystems, and floodplains with often serious consequences
(Box 1.3).
Box 1.3
Effects of Altering Natural Ecosystems on Flooding: Examples in
the Upper Midwest, United States
The aftermath of record flooding in the upper Mississippi River in 1993 and
2001, and the 1997, 2001, 2007, and 2010 floods in the Red River of the North
along the Minnesota–North Dakota boundary, led to questions concerning
the extent to which the floods were the result of human modifications of
watersheds. Since the beginning of the twentieth century, efforts designed
largely to expand farmland have resulted in extensive drainage of wetlands
and the use of tile drains on croplands. Flood control has involved levee construction and channelized rivers, with a resulting loss of floodplain storage
and riparian forests.
Following the 1993 Mississippi River flood that caused $10 billion in damage (Tobin and Montz, 1994), Leopold (1994) found that in several locations
the levees caused river stages to be higher than they would have been without levees being in place. As long as levees did not fail or were breached,
the increased height of flood stages was retained within the channel system.
However, many levees were breached, causing greater devastation than
would have otherwise occurred without levees. The cumulative loss of wetland and floodplain storage along channels and in upper watersheds could
have exacerbated flooding in many locations. The question of the extent to
which land use and channel modifications have affected the magnitude of
floods throughout the Mississippi River and the Red River of the North is uncertain. A better understanding of cause-and-effect relationships is needed;
this book provides the background to help understand and address these
issues.
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WATERSHEDS, ECOSYSTEM MANAGEMENT, AND
CUMULATIVE EFFECTS
Taking a watershed perspective has utility in addressing the many natural resource issues
that span decades and involve varying climatic and land-use changes. The merit in taking a
watershed perspective is clear when dealing with issues of water supply and events such as
floods and droughts (see Table 1.1). Not so obvious are the effects that become compounded
spatially and temporally and that at first glance might not appear to be related to hydrologic
processes.
During the past century in the United States, vegetation and land-use changes on the
landscape have dramatically altered the hydrologic characteristics of watersheds. Since
the earliest European settlers, extensive areas of native forests and grasslands have been
converted to agricultural cropland. Urban areas and road systems have expanded, riparian
corridors altered, wetlands drained, and natural river systems modified. These landscape
changes have resulted in hydrologic changes through modifications of watersheds, their
stream systems, and surface–groundwater linkages. Changes in water flows and water
quality can affect people and ecosystems in both upstream and downstream areas.
The challenge to people living on watersheds is that of mitigating the effects of
all of the changes that adversely impact human welfare and the functioning of natural
ecosystems. Increased attention is being paid to maintaining or restoring natural streamchannel systems, riparian communities, wetland ecosystems, and floodplains as needed to
maintain the watersheds in a good hydrologic condition. For example, Hey (2001) called
for a major program to maximize the natural storage of wetlands and floodplains and
minimize conveyance in the upper Mississippi River Basin. Such a program would in effect
reverse some of the effects of the past 200 years of levee construction and other engineering
practices in the basin.
The role of watersheds as units for ecosystem management and analysis has gained
international recognition in the past decades. Ecosystem management, a focus of natural
resource management in the 1990s, is based on maintaining the integrity of ecosystems
while sustaining benefits to human populations. The usefulness of a watershed approach
in ecosystem management became apparent in the United States with the attempts to reconcile conflicts over the use and management of land and water in the Pacific Northwest
(Montgomery et al., 1995). Environmental, economic, and political conflicts involving the
use of old-growth forests for both spotted owl habitats and the production of timber were
heated in this region. At the same time, concern over the effects of timber harvesting and
hydropower dams on native salmon emerged. A watershed analysis approach facilitated the
examination of the interrelationships between land and water use and many of the environmental effects. This approach was taken in the Pacific Northwest because watersheds define
“basic, ecologically, and geomorphologically relevant management units” and “watershed
analysis provides a practical analytical framework for spatially-explicit, process oriented
scientific assessment that provides information relevant to guiding management decisions”
(Montgomery et al., 1995, p. 371).
Cumulative watershed effects are the combined environmental effects of activities in a
watershed that can adversely impact beneficial uses of the land (Reid, 1993; Sidle, 2000).
The viewpoint taken here is that there are interactions among different land-use activities
and there can be incremental effects that can lead to more serious overall impacts when
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added to past effects. Individually, these environmental effects might not appear to be
relevant but collectively they can become significant over time and space. For example,
the conversion from forest to agricultural cropland on one part of a watershed can cause
an increase in water and sediment flow. Road construction and drainage can have similar
effects elsewhere in a watershed as can the drainage of a wetland at another location.
These activities are likely to occur over a long period and, incrementally, have little
obvious effect. At some point in time, however, the increased streamflow discharge in combination with additional sediment loads can lead to more frequent flooding. River channels
can adjust to these changes in the flow of water and sediment to cause additional impacts
downstream. These same changes can alter aquatic habitats. A watershed perspective forces
one to look at multiple and cumulative effects in a unit and attempt to identify key linkages
between terrestrial and aquatic ecosystems.
RECONCILING WATERSHED AND
POLITICAL BOUNDARIES
Historically, land, water, and other natural resources have been managed not only by people
with different technical backgrounds but also by organizations that have very focused and
different missions that impact one another. There are often overlapping responsibilities.
In contrast, it is rare to find organizations that have the explicit mission of watershed
management. There are few organizations with responsibilities that coincide with watershed
boundaries as a result. However, there is a need to cope with issues that arise between
upstream and downstream entities. In response to these needs, watershed management
organizations and institutions have emerged with responsibilities of planning and managing
resources to resolve upstream–downstream conflicts that arise (see NRC, 2000).
Locally led watershed management organizations and initiatives have been established
under various names, including watershed associations, partnerships, councils, and other
“co-management” schemes administered jointly by governments and local communities.
They all emphasize the growing awareness of the important hydrologic linkages between
uplands and downstream areas. Furthermore, there is a growing recognition that institutional
arrangements and supportive policies are needed so that people can develop sustainable solutions to land and water problems and avoid land and water degradation. By the late 1990s,
more than 1500 locally led initiatives were established in the United States in response to
water resource problems that were not being addressed (Lant, 1999). These initiatives
were largely in response to the issues discussed above, including the effects of urbanization, intensification of agriculture, forest management activities, stream channelization, and
wetland drainage on flooding, water yield, and water quality.
A river basin management strategy has emerged in North Carolina, United States
(www.ncwater.org/basins, accessed May 24, 2011), as a result of a decade of water supply
demands exceeding water supplies in many locations of the state. Water shortages during a
4-year drought intensified in 2002, suggesting that actions would be needed to avoid adverse
economic impacts caused by insufficient water supplies. The river basin strategy took the
approach that river basins provide the fundamental units for local and state governmental
organizations to monitor water availability and use and to facilitate planning to develop
50-year water supply plans to solve water supply problems. A similar strategy, but for
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Box 1.4
Minnesota’s Major Watershed Restoration and Protection Strategy
In November of 2008 after the US stock market crashed and votes throughout
the United States sent Barack Obama to Washington DC, Minnesotans voted
to raise their sales tax by 3/8’s of a percent. A new clean water and land
legacy fund was created to clean up the state’s impaired waters and preserve
valued hunting land and wetlands over a 25-year period. The state developed
a strategy for not only restoring impaired waters, but also identifying land at
risk of developmental changes that could adversely impact water resources
in the future. The strategy focused on several key areas:
(1) Assess the overall condition and health of the many lakes, streams,
rivers, wetlands by major watersheds using biological metrics in the
form of an Index of Biological Integrity (IBI). A major watershed is
defined as an eight-digit HUC according to the U.S. Geological Survey;
Minnesota has 81.
(2) Develop an assessment document, including additional data such as
Lake Secchi, Chlorophyll A, and total Phosphorus that would guide
listings for the Section 303(d) list of the Clean Water Act.
(3) Conduct a Stressor Identification analysis to validate true water quality
impairment. This step involves more than examining toxic influences
upon fish; hydrologic, geologic, geomorphic, and connectivity data
must be gathered to define natural background and superimposed
anthropogenic activity.
(4) Construct computer models for each major watershed to help manage
current land-use conditions and predict future water quality scenarios
based on climate and land-use change.
(5) Perform statistical analysis and synthesis of all data and model results
to identify Priority Management Zones for restoration or protection
action.
(6) Lastly, with Clean Water Legacy funds implement Best Management
Practices where they are most needed on the landscape to form
watershed-based treatment trains from upland through riparian and
into channels, lakes, and wetlands.
This approach is unique in the United States; no other state has this
level of local funding focused on an IWM approach (www.pca.state.mn.us/
index.php/view-document.html?gid = 14224).
different purposes, has been developed in Minnesota where major watersheds are used as
the units of management to meet water quality objectives (Box 1.4).
Institutional responses to international water issues are also emerging. For example,
transboundary issues in the Nile River Basin have focused attention on watersheds as a
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Chapter 1 Introduction
23
logical framework for analysis and understanding of the effects of human actions in one
part of the basin on those living downstream. Here, concerns about water, human poverty,
biodiversity, and wildlife are paramount. The disparity between watershed and political
boundaries prompted the Nile Basin Initiative and an extensive study into transboundary
issues of the Nile River Basin (Box 1.5). The inequities in rainfall and water supply among
the ten riparian countries of the Nile River Basin have threatened sustainable development
in the region and have historically resulted in political conflict (Baecher et al., 2000).
While a necessary and daunting task, developing solutions to water supply problems is
Box 1.5
The Nile River Basin: A Case for Watershed Management (Baecher
et al., 2000)
The Nile River is the longest river of the world, traversing 6700 km from the
rift valleys of East Africa, connecting with the Blue Nile from the Ethiopian
highlands, and emptying into the Mediterranean Sea through the broad delta
of Egypt. Annual rainfall amounts vary to the extreme within the basin. The
Ethiopian highland watersheds experience more than 2000 mm of annual
rainfall and contribute 60–80% of the water flow in the Nile River, yet represent less than 10% of the land area in the Nile Basin. Vast areas of the
northern Basin, on the other hand, average less than 50 mm of annual rainfall. Consequently, the watersheds of Egypt and Sudan, which constitute
more than 75% of the basin, contribute negligible flow to the Nile River.
Efforts to relocate waters of the Nile Basin for economic development have
led to environmental concerns and potential political conflict. Lake Nassar,
created by the large Aswan Dam of the lower Nile River, is essential to
Egypt’s economy but is vulnerable to water losses upstream. The controversial Jonglei Canal was proposed to enhance downstream water supplies by
diverting water from the vast Sudd wetland to the lower and drier north – at
the expense of the loss of valuable habitat. In contrast, Ethiopia as one of the
poorest countries in the basin would benefit economically by expanding its
irrigation and hydropower production through construction of dams in the
uplands. However, such projects would meet with objections from downstream riparian countries. Furthermore, such projects, whether in uplands or
lowlands, are threatened by intensive land use, watershed degradation, and
deforestation that impair the quantity and quality of streamflow and aquatic
habitat. Forest cover in the Ethiopian Highlands decreased from more than
15% in the 1950s to less than 4% (43.4 million ha) by 2000. FAO (2007) indicated that annual deforestation was occurring at a rate of 1410 km2 /year.
Overgrazing and cultivation of hillslopes lead to soil erosion, loss of productivity of the land, and increased sediment delivery downstream. All these
issues focus on the need for watershed management that extends beyond
country boundaries.
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not necessarily sufficient. The growth and distribution of human populations, widespread
poverty, and inadequate policy responses to land and water resource issues compound
problems in the basin. A major theme in the Nile Basin Initiative is that land and water
must be managed in harmony so that environmental conditions and human welfare benefit.
IWM is one of the key themes in this effort.
A proactive approach to IWM involves establishing and sustaining preventive practices. This approach requires instituting guidelines of land use and implementing land-use
practices on a day-to-day basis that result in long-term, sustainable resource development
and productivity without causing soil and water problems. Land and natural resource management agencies in the United States and many other countries have established policies
that embody sound watershed management principles. These preventive measures might
not make for exciting reading but in total they represent the ultimate goal of IWM. Achieving this goal requires that managers recognize the implications of their actions and that
they work effectively within the social and political setting in which they find themselves.
The dilemma in watershed management is that land-use changes needed to promote
the survival of society in the long term can be at cross-purposes with what is essential to
the survival of the individual in the short term. Requirements for food, high-quality water,
and the commodities and amenities provided by natural resources today should not be
met at the expense of future generations. Therefore, any discussion of sustainable natural
resource development should consider watershed boundaries, the linkages between uplands
and downstream areas, and the effects of land-use practices on long-term productivity. Land
use that is at cross-purposes with environmental capabilities cannot be sustained. However,
sustainable productivity and environmental protection can be achieved with the integrated,
holistic approach that explicitly considers hydrology and the management of watersheds.
SUMMARY AND LEARNING POINTS
Watersheds are biophysical systems that describe how units of land are connected by water
flow. They also represent systems that are suitable for developing sustainable use and
management of multiple resources under the tenet that land and water should be managed
in concert with one another – the basis for IWM. This introductory chapter lays out the
reasons for taking an IWM approach – which the reader should be able to understand – as
well as the role of IWM in coping with land and water scarcity and hydrometeorological
extremes.
REFERENCES
Baecher, G.B., Anderson, R., Britton, B. et al. (2000) The Nile basin, environmental transboundary
opportunities and constraints analysis. Washington, DC: U.S. Agency for International
Development, International Resources Group, Limited.
Bates, C.G. & Henry, A.J. (1928) Forest and streamflow experiment at Wagon Wheel Gap,
Colorado. U.S. Weather Bureau Monthly Weather Review, Supplement 30.
Chandra, S. (1990) Hydrology in ancient India. Roorkee, India: National Institute of Hydrology.
Davies, T.R.H. (1997) Using hydroscience and hydrotechnical engineering to reduce debris flow
hazards. In Debris-flow hazards mitigation: Mechanics, prediction, and assessment –
Proceedings of the First International Conference, ed. C. Chen, 787–810. New York:
American Society of Civil Engineers.
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25
Falkenmark, M. (1997) Society’s interaction with the water cycle: A conceptual framework for a
more holistic approach. Hydrol. Sci. J. 42:451–466.
Food and Agricultural Organization of the United Nations (FAO) (2007) State of the world’s forest
report. Rome: FAO.
Glasser, S.P. (2005) History of watershed management in the US Forest Service: 1897–2005. J.
Forest. July/Aug:255–258.
Gregersen, H.M., Ffolliott, P.F. & Brooks, K.N. (2007) Integrated watershed management:
Connecting people to their land and water. Wallingford, Oxfordshire: CABI Press.
Hey, D.L. (2001) Modern drainage design: The pros, the cons, and the future. Paper presented at the
annual meeting of the American Institute of Hydrology – Hydrologic Science: Challenges
for the 21st Century, Bloomington, MN.
Horton, R.E. (1945) Erosional development of streams and their drainage basins: Hydrophysical
approach to quantitative morphology. Geol. Soc. Am. Bull. 56:275–370.
Ice, G.G. & Stednick, J.D. (eds.) (2004) A century of forest and wildland watershed lessons.
Bethesda, MA: Society of American Foresters.
Kittredge, J. (1948) Forest influences. New York: McGraw-Hill.
Lant, C.L. (1999) Introduction, human dimensions of watershed management. J. Am. Water Resour.
Assoc. 35:483–486.
Leopold, L.B. (1994) Flood hydrology and the flood plain. The Universities Council on Water
Resources. Water Resour. Update 94:11–14.
MacDonald, L.H. & Coe, D. (2007) Influence of headwater streams on downstream reaches in
forested areas. For. Sci. 53:148–168.
Montgomery, D.R., Grant, G.E. & Sullivan, K. (1995) Watershed analysis as a framework for
implementing ecosystem management. Water Resour. Bull. 3:369–386.
National Research Council (NRC) (2000) Watershed management for potable water supply,
assessing the New York City strategy. Washington, DC: National Academy Press.
Omernik, J.M. (2003) The misuse of hydrologic unit maps for extrapolation, reporting, and
ecosystem management. J. Am. Water Resour. Assoc. 39(3):563–573.
Pinchot, G. (1903) Primer of forestry. Washington, DC: Bureau of Forestry.
Reid, L.M. (1993) Research and cumulative watershed effects. USDA For. Serv. Gen. Tech. Rep.
PSW-GTR-141.
Rosegrant, M.W. (1997) Water resources in the twenty-first century: Challenges and implications
for action. Food, Agriculture and the Environment Discussion Paper 20. Washington, DC:
International Food Policy Research Institute.
Scherr, S.J. & Yadav, S. (1996) Land degradation in the developing world: Implications for food,
agriculture, and the environment in 2020. Food, Agriculture, and the Environment
Discussion Paper 14. Washington, DC: International Food Policy Institute.
Schumm, S. A. (1977). The fluvial system. New York: Wiley & Sons.
Seaber, P.R., Kapinos, F.P. & Knapp, G. L. (1987) Hydrologic unit maps. U.S. Geological Survey
Paper 2294.
Sidle, R.C. (2000) Watershed challenges for the 21st century: A global perspective for mountainous
terrain. In Land stewardship in the 21st century: The contributions of watershed
management, tech. coords. P.F. Ffolliott, M.B. Baker, Jr., C.B. Edminster, M.C. Dillon &
K.L. Mora, 45–56. USDA For. Serv. Proc. RMRS-P-13. Fort Collins, CO: Rocky
Mountain Research Station.
Sidle, R.C., Tsuboyama, Y., Noguchi, S., et al. (2000) Streamflow generation in steep headwaters: A
linked hydro-geomorphic paradigm. Hydrol. Process 14:369–385.
Strahler, A.N. (1964) Quantitative geomorphology of drainage basins and channel networks. In
Handbook of applied hydrology, ed. V.T. Chow, Section 4.2. New York: McGraw-Hill.
Tobin, G.A. & Montz, B.E. (1994) The great Midwestern floods of 1993. New York: Saunders
College Publication.
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United Nations World Water Assessment Program (UNWWAP) (2006) United Nations world water
development report: Water, a shared responsibility. Paris: UNWWAP.
Verry, E. S. (2007). Integrating forest management and forest hydrology: Landscape to site level.
Presentation at annual meeting of the Wisconsin Society of American Foresters,
Chippewa Chapter, October 16–17, Telemark Resort, WI.
Vorosmarty, C.J., Green, P., Salisbury, J., et al. (2000) Global water resources: Vulnerability from
climate change and population growth. Science 289:284–288.
Zon, R. (1927) Forests and water in the light of scientific investigation. Washington, DC: United
States Department of Agriculture – Forest Service, GPO.
WEBLIOGRAPHY
www.bbc.co.uk/history/ancient/egyptians/nile 01.shtml – Baines, J. (2011) The story of the Nile.
Accessed September 9, 2011.
www.circleofblue.org/waternews/2009/world/waterviews water tops climate change as global
priority. Accessed April 7, 2010.
www.iwmi.cgiar.org/publications/other/pdf/water scarcity flyer.pdf – International Water
Management Institute (IWMI) (2007). Water for food, water for life. EARTHSCAN.
www.iwmi.cgiar.org/About IWMI/Strategic Documents/Annual Reports/index.aspx – Publications
and reports by the International Water Management Institute (IWMI), Sri Lanka.
www.ncwater.org/basins – North Carolina basin management strategy. Accessed May 24, 2011.
www.pca.state.mn.us/index.php/view-document.html?gid = 14224 24 – Minnesota major watershed
restoration and protection strategy. Accessed February 4, 2012.
www.unesco.org/water/news/newsletter/250.shtml#know – UNESCO Water e-Newsletter No. 250:
Trends in water use. Accessed May 23, 2011.
www.watertechnology.net/projects/south north/ – South-to-North Diversion Project, China.
Accessed February 4, 2012.
www.water.usgs.gov/GIS/huc.html – Hydrologic Unit Maps of the United States.
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CHAPTER 2
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Hydrologic
Cycle and the
Water Budget
INTRODUCTION
Hydrology is the science and study of water. In this book, we focus on the occurrence of
water on earth and the factors that influence the amount, distribution, circulation, timing,
and quality of water from a watershed perspective. Water flows in accordance with physical
laws and in many ways alters the landscape features of watersheds. In conjunction with
climate, water affects the development of soils and the type of vegetative cover that grows
on a watershed. The quantity, quality, and timing of water that flows from watersheds are
dependent on the interactions of climate and the activities of people on the watershed. An
understanding of the causes, processes, and mechanisms of water flow on earth and through
its watersheds provides the underpinning for the study of watershed hydrology.
This chapter begins with a review of the properties of water and examines the basic principles and processes involved in the flow of water from a watershed and the corresponding
flows of energy that drive the hydrologic cycle.
PROPERTIES OF WATER
Water is commonplace but also unique. As the most abundant compound on the earth’s
surface, water is the only common substance that exists naturally in all three states – liquid,
solid, and gas. The chemical formula for water, H2 O, is two hydrogen atoms covalently
bonded to one oxygen atom. The water molecule is polar; that is, it has a net positive charge
on the hydrogen atoms and a net negative charge on the oxygen atom because hydrogen
attracts electrons less strongly than oxygen. This property of polarity results in the physical
attraction of water molecules to one another (hydrogen bonding) that explains many of the
physical and chemical properties of water that are integral to life on earth.
Hydrology and the Management of Watersheds, Fourth Edition. Kenneth N. Brooks, Peter F. Ffolliott and Joseph A. Magner.

C 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc.
27
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FIGURE 2.1.
Water droplet on hydrophobic soil illustrates the cohesive property of
water molecules
Importance of Polarity
Water polarity results in the important properties of cohesion, adhesion, and capillarity.
Hydrogen bonds help keep water molecules together (cohesion) while polar bonding to other
materials (adhesion) explains the attraction of water molecules to soil particles and plant
cells. The cohesion between water molecules explains the high surface tension of water,
which is the reason why a water droplet maintains its form as a bubble on a nonabsorbent
or water-repellent surface (Fig. 2.1). The properties of surface tension and adhesion explain
why water can move upward into a small capillary tube against the force of gravity. This
capillary action, or capillarity, also explains why water can move against the force of gravity
through soils and plants. Water moves into and up a plant as a result of both the cohesion of
water molecules that keep the water column intact and the adhesion properties of water that
attach water to plant cell walls in the xylem. The cohesive properties and surface tension of
water molecules explain why water droplets form in the atmosphere and grow in size until
they fall to the earth’s surface under the influence of gravity.
The polarity of water molecules results in water being a good solvent, sometimes
referred to as a universal solvent. While not all substances such as oils and waxes mix with
water, many substances such as other polar or ionic molecules (salts, acids, and alcohols)
mix well with water. As a result, water plays an important biological role in transforming
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TABLE 2.1.
29
The unique water molecule properties of water at normal terrestrial
temperatures (Maidment 1993 and others)
Density
Heat capacity (Cp )
Heat of
vaporization (L)
Temperature
(â—¦ C)
(kg/m3 )
(g/cm3 )
(J(g/K))
(cal/(g/â—¦ C))
(J/g)
(cal/g)
0 (ice)
0 (liquid)
4
10
20
30
40
916.70
999.84
999.97a
999.70
998.21
995.65
992.20
0.92
1.00
1.00
1.00
1.00
1.00
0.99
2.05
4.22
4.19
4.18
4.18
4.18
0.5
1.0
1.0
1.0
2835
2500
2491
2477
2453
2429
2406
677.3
597.3
595.2
591.7
586.0
580.4
574.9
a
Maximum density of water and an important characteristic that explains why the water in lakes becomes stratified
in summer and turns over as water cools in the fall/early winter.
and moving many mineral salts and nutrients in solution through soils to plants and from
groundwater. Any nutrients and mineral salts that are in solution in soil water are easily
moved by excess water draining a soil or flowing off the soil surface and subsequently
entering groundwater or surface water bodies. Because of the physical forces of water
flowing across the soil surface, both nonsoluble and soluble materials can be transported
into stream channels and other water bodies.
State of Water
The form or state of water that occurs at any location on earth, whether gas, liquid, or
solid, is a function of the properties of water and energy. Most water on earth occurs in
liquid form. Liquid water has the highest specific heat capacity of all known substances
(Table 2.1). Changes in state from liquid to water vapor or to ice and vice versa involve
exchanges of energy. Large amounts of energy are required to convert liquid water to its
gaseous state, called the latent heat of vaporization. Therefore, the transformation of liquid
water to water vapor, called evaporation, consumes large amounts of the total solar energy
absorbed by the earth’s surface.
The energy required to change ice at 0â—¦ C to vapor requires the sum of the heat of
fusion (the energy required to melt ice is 334 J/g or 80 cal/g at 0â—¦ C) plus the latent heat of
vaporization for liquid water at 0â—¦ C. Given the abundance of water on earth, these properties
of specific heat and latent heat of vaporization explain why water acts as a thermal regulator
that moderates the earth’s climate.
In contrast to evaporation, the transformation of water vapor in the atmosphere to
liquid water, a process called condensation, releases the latent heat of vaporization. When
condensation occurs, energy becomes available to heat the air, melt snow, or heat the surface
on which condensation occurs. The changes in state of water in the atmosphere affect air
temperature, precipitation (see Chapter 3), and weather conditions in general.
Water vapor mixes completely in air and exerts a vapor partial pressure, called vapor
pressure. The saturation vapor pressure (es ) is determined by air temperature alone and
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is the partial pressure of water vapor (mb) in a saturated atmosphere as described by Lee
(1978):

5347.5
(2.1)
1n es = 21.382 −
T
where T is the absolute temperature in K, which is equal to â—¦ C + 273.
At any given temperature, a parcel of air in an unsaturated atmosphere will have a
partial pressure that is less than the saturation vapor pressure: the drier the air, the greater
the difference between the vapor pressure of the air and its saturation vapor pressure. The
magnitude of this vapor pressure difference affects the rate and amount of evaporation
(discussed in Chapter 4).
Atmospheric pressure (not to be confused with vapor pressure) decreases with height,
causing a rising parcel of air to expand. As a result of this expansion, the temperature of
the air will decrease while that of a descending parcel of air will become compressed and
warmed. These relationships can be explained by the natural or ideal gas law:
P V = n RT
(2.2)
where P is the pressure in pascals; V is the volume in m3 ; n is the number of moles of gas;
R is the universal or ideal gas constant and has the value 8.314 J/K/mol; and T is the air
temperature in K.
The process is called adiabatic if no heat is gained or lost in the parcel of rising or
falling air mass by mixing with surrounding air. The rate at which unsaturated air cools
by adiabatic lifting is approximately 1â—¦ C/(100 m), called the dry adiabatic lapse rate.
Movements of large air masses can change the lapse rate and conditions can even reverse
the lapse rate when large cold air masses underlie less-dense warm air, a condition called
an inversion in which temperature increases with height.
The properties of water and the interactions of water and energy provide the foundation
for understanding the circulation of water and energy across the globe.
THE HYDROLOGIC CYCLE
The circulation of water on earth, called the hydrologic cycle, involves the processes and
pathways by which water evaporates from the earth’s surface to the atmosphere and returns
to the surface as precipitation or condensation (Fig. 2.2). With the earth’s surface being
about 70% water, most of the atmospheric water originates from the oceans and other water
bodies. With few exceptions, much of the precipitation falling on land surfaces does not
reach the oceans as streamflow or groundwater flow but rather evaporates back into the
atmosphere. This partitioning of water between evaporation, storage, and watershed liquid
water flows involves numerous hydrologic processes.
Hydrologic Processes
Water evaporates from many surfaces located throughout a watershed. Precipitation that
is caught by plant surfaces and evaporates back to the atmosphere is called interception.
This part of precipitation does not reach the soil surface. Evaporation also occurs from
water bodies located in a watershed and from soil surfaces. Water that is extracted from the
soil by plant roots and that evaporates from within plant leaves is transpiration. The total
amount of water that evaporates from a watershed is evapotranspiration, which is the sum
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Chapter 2 Hydrologic Cycle and the Water Budget
FIGURE 2.2.
plate section)
August 28, 2012
31
The hydrologic cycle (For a color version of this figure, see the color
of interception, transpiration, and evaporation from soils and water bodies. This evaporated
water is temporarily lost from the watershed to the atmosphere but eventually returns to the
earth’s surface as precipitation at some other location and the cycle continues.
Precipitation falling on a watershed that is not returned to the atmosphere via evapotranspiration can either flow over the soil surface, reaching stream channels as overland
flow, or surface runoff, or it infiltrates into the soil. The fate of infiltrated water depends on
r the moisture status of the soil;
r the water-holding capacity of the soil; and
r the network and size of pores within the soil matrix.
Infiltrated water that is in excess of the soil water holding capacity can flow downward
under the influence of gravity until reaching groundwater. If the downward drainage of
water, called percolation, reaches a strata of soil or rock with limited permeability, water
can be diverted laterally through the soil and discharge into a stream channel or other
surface water body as subsurface flow, often called interflow. The fate of water that reaches
groundwater depends on subterranean characteristics of earth materials and geologic strata
that influence the pathways by which groundwater can flow. Some groundwater intersects
river channels or other water bodies, thereby returning to surface waters. Water that seeps
into deep groundwater aquifers can be stored for centuries before returning to surface
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Groundwater < 0.5 mi deep (4,150,000 km³) Groundwater > 0.5 mi
deep (4,150,000 km³)
Soil moisture
(67,000 km³)
Water vapor
(13,000 km³)
Lakes,
rivers
and streams
(126,250 km³)
Frozen
(29,000,000 km³)
FIGURE 2.3.
Distribution of freshwater on earth from van der Leeden et al. (1990)
waters. Therefore, the journey of water falling as precipitation on watersheds can follow a
myriad of pathways to the ocean only to be evaporated and recycled again.
A fundamental concept of the hydrologic cycle is that water is neither lost nor gained
from the earth over time. However, the quantities of water in the atmosphere, soils, groundwater, surface water, glaciers, and other components are constantly changing because of
the dynamic nature of the hydrologic cycle. A few facts to remember about the hydrologic
cycle are as follows:
r Solar energy provides the energy that drives and sustains the cycling of water on
earth.
r There is no beginning or end to the cycle.
r The supply of water on earth is constant, but the allocation of water in storage or in
circulation can vary with time.
The portion of water that is in various types of storage can be approximated over
periods of time. If we consider the total water resource on the earth, only about 2.7% is
freshwater of which about 77% exists in polar ice caps and glaciers (Fig. 2.3). About 11% of
freshwater is stored in deep groundwater aquifers leaving about 12% for active circulation.
Of this 12%, only 0.56% exists in the atmosphere and in the biosphere. The biosphere is
from the top of trees to the deepest roots. The atmosphere redistributes evaporated water
by precipitation and condensation. Components of the biosphere partition this water into
runoff, soil and groundwater storage, groundwater seepage, or evapotranspiration back to
the atmosphere.
The hydrologic processes of the biosphere and their interactions with vegetation and
soils are of interest in understanding the hydrology and management of watersheds. This
aspect of hydrology has been long emphasized in forest hydrology and watershed management courses taught in many natural resource programs; the emergence of ecohydrology in
many academic programs takes this same perspective. Precipitation and the flow of water
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33
into, through, and out of a watershed all can be affected by land use and management
activities. Likewise, human activities can alter the magnitude of various watershed storage
components including soil water, snowpacks, rivers, lakes, and reservoirs. With the water budget approach, we can examine existing watershed systems, quantify the effects of
management impacts on the hydrologic cycle, and in some cases predict or estimate the
hydrologic consequences of proposed activities.
Water Budget
The hydrologic cycle is complex and dynamic but can be simplified if we categorize
components into input, output, and storage (Fig. 2.4). Over any period of time, all inputs of water to a watershed must balance with outputs from the watershed and changes
in storage of water within the watershed. This hydrologic balance, or water budget,
follows the principle of the conservation of mass law expressed by the equation of
continuity:
I − O = ⌬S
(2.3)
where I is the inflow, O is the outflow, and ⌬ S is the change in storage.
If inflows exceed outflows over a period of time, storage must increase, or if outflows
exceed inflows then, storage must decrease – it is the conservation of mass law. The water
budget can be expanded to identify the key inputs and outputs for a watershed over a specific
period of time as follows:
P + GWi − Q − E T − GWo = ⌬ S
(2.4)
where P is the precipitation (mm); GWi is the groundwater flow into the watershed (mm);
Q is the streamflow from the watershed (mm); ET is the evapotranspiration (mm); GWo is
the groundwater flow out of the watershed; ⌬ S is the change in the amount of storage in
the watershed, S2 − S1 (mm), where S2 is the storage at the end of a period and S1 is the
storage at the beginning of a period.
Note that in the water budget of a watershed (Equation 2.4), groundwater flows only
need to be taken into account if they contribute to, or diminish, surface water on the watershed. Therefore, deep groundwater exchanges that have no contact with surface water are
not considered. In some situations, there can be surface water seepage to deep groundwater
aquifers, which is a net loss of water from the watershed, expressed as GWo .
Soil Moisture and the Water Budget
The status of soil moisture in first-order watersheds largely dictates what portion of rainfall
or snowmelt contributes to streamflow or to groundwater recharge. Therefore, the status of
soil water content at any point in time, referred to as antecedent soil moisture, is an important
hydrologic condition of a watershed that determines how the watershed responds. The
change in storage for most first-order watersheds is determined largely by changes in soil
moisture that is in response to precipitation, ET, runoff or streamflow (Q), and groundwater
leakage (if any) over a period of time.
The amount of water that a soil can store is largely determined by soil texture (Fig. 2.5),
although organic material in topsoil can increase the water-holding characteristics associated with the soil texture. Field capacity (FC) refers to the maximum amount of water that
a given soil can retain against the force of gravity. If a soil were saturated and allowed
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Part 1 Watersheds, Hydrologic Processes, and Pathways
FIGURE 2.4.
The hydrologic cycle consists of a system of water storage compartments and the solid, liquid, or gaseous flows of water within and between the storage
points (from Anderson et al., 1976)
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Chapter 2 Hydrologic Cycle and the Water Budget
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