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Environmental Hydrology
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ENN1RONMENTALHYDROLOGY
Water Science and Technology Library
VOLUME 15
Editor-in-Chief
V. P. Singh, Louisiana State University,
Baton Rouge, U.S.A.
Editorial Advisory Board
M. Anderson, Bristol, U.K.
L. Bengtsson. Lund, Sweden
A. G. Bobba, Burlington, Ontario, Canada
S. Chandra, Roorkee, U.P., India
M. Fiorentino, Potenza, Italy
W. H. Hager, Zurich, Switzerland
N. Harmancioglu, Izmir, Turkey
A. R. Rao, West Lafayette, Indiana, U.S.A.
M. M. Sherif, Giza, Egypt
Shan Xu Wang, Wuhan, Hubei, P.R. China
D. Stephenson, Johannesburg, South Africa
The titles published in this series are listed at the end of this volume.
ENVIRONMENT AL
HYDROLOGY
edited by
VUAYP. SINGH
Louisiana State University,
Baton Rouge, U.S.A .
.....
SPRINGER-SCIENCE+BUSINESS " MEDIA, B.V.
Library of Congress Cataloging-in-Publication Data
EnVlronmental hydrology I edIted by Vljay P. SIngh.
p. Cill.
Includes Index.
ISBN 978-90-481-4573-7 ISBN 978-94-017-1439-6 (eBook)
1. Hydrology. 2. Hydrology--Envlronmental aspects.
V. P. '(VI jay P.)
GB665.E58 1995
551.48--dc20
ISBN 978-90-481-4573-7
02-0897 -1 DO ts
Printed on acid-free paper
AU Rights Reserved
95-11259
CIP
© 1995 Springer Science+Business Media Dordrecht
Originally published by Kluwer Academic Publishers in 1995
No part of the material protected by this copyright notice may be reproduced or
utilized in any form or by any means, electronic or mechanical,
including photocopying, recording or by any information storage and
retrieval system, without written permission from the copyright owner.
DOI 10.1007/978-94-017-1439-6
To the People of the United States of America
Table of contents
Preface
1 What is environmental hydrology?
v.P. Singh
1.1 Environmental Continuum . . .
1.2 Integrated Environmental Management
1.3 Water Continuum ..... .
1.4 Integrated Water Management
1.5 Classification of Hydrology
1.5.1 Properties of Water .
1.5.2 Sources of Water ..
1.5.3 Scientific Content .
1.5.4 Solution Technique
1.5.5 Area of Emphasis .
1.5.6 Basin Size .....
1.5.7 Basin-Type orLand Use
1.6 Definition of Environmental Hydrology
1.7 Scope of Environmental Hydrology
1.8 Role of Environmental Hydrology . . .
2 Watershed acidification modelling
A.G. Bobba. D.S. Jeffries. w.G. Booty and v.P. Singh
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . .
2.1.1 Watershed Acidification Modelling ...... .
2.1.2 Interaction between Lake and Terrestrial Systems
2.1.3 Flow Pathways in Watersheds . . . . . . . . .
2.l.4 Bio-geochemical Processes: Chemical Species.
2.1.5 Basic Mechanism of Leaching - Mobile Anions
2.1.6 Effect of Acid Rain on Leaching .
2.1.7 Soil Buffering Systems.
2.2 Watershed Acidification Models
2.2.1 ILWAS Model ..
2.2.2 Birkenes Model . . . .
2.2.3 TMWAM Model ....
2.2.4 Enhanced Trickle-Down Model
2.2.5 RAINS Model . . . . . . . . .
2.2.6 MAGIC Model. . . . . . . . .
2.4 Stochastic Analysis to Predict Acid Shocks
2.4.1 Number of Exceedances
2.4.2 Time of Exceedance .. .
2.4.3 Time of Events ..... .
2.4.4 Duration of Exceedances .
2.4.5 Magnitude of Exceedances .
xiii
1
1
1
2
3
7
7
7
7
7
8
8
8
8
9
10
13
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15
16
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22
23
24
25
27
27
30
33
40
42
44
46
47
47
48
49
49
viii
3
2.5 Application of Models to the Turkey Lake
2.5.1 Statistical Analysis.
49
51
56
60
62
2.6
2.7
2.5.2 Acidification Events
Discussion .
Conclusions
Climatic change
M.A. Mimikou
69
3.1 Introduction . . . . . . . . . . . . 69
3.2 A Brief Overview of the Literature. 70
3.3 Climatic Variability and Change . . 72
3.3.1 Global Greenhouse Warming - Past, Present and Future Conditions 72
3.3.2 Methods for Evaluating Global and Regional Climatic Changes 75
3.3.3 Impacts of Climatic Changes. . . . . . . . . . . . . . . . . . . . 76
3.4 Hydrological Effects of Climatic Change ................. 77
3.4.1 Hydrological System Response to Climatic Changes. . . . . . . . 77
3.4.2 Methods for Evaluating Regional Hydrological Effects of Climatic
Changes .............................. 78
3.4.3 Sensitivity of Major Climatic and Hydrological Variables to Global
Warming .............................. 82
3.5 Climatic Change Impacts on Water Resource Planning and Management . 88
3.5.1 Basic Considerations. . . . . . . . . . . . . . . . . . . . . . .. 88
3.5.2 Sensitivity of Some Critical Water Management Issues to Climatic
Change. . . . . . . 90
3.6 Future Research Directions. . . . . . ................ " 97
4 Understanding river hydrology
B.L Finlayson and T.A. McMahon
107
4.1 Introduction . . . . . . . . . . . . . . .
4.2 Data .................. .
4.2.1 Types of Data and Their Sources.
4.2.2 Data Problems . . . . . . . . .
4.2.3 Synthetic Data . . . . . . . . . .
4.3 Characteristics of the River Hydrograph .
4.3.1 The Generation of the River Hydrograph
4.3.2 Hydrograph Separation
4.3.3 Flow Duration Curves
4.3.4 High Flows .. .
4.3.5 Low Flows ... .
4.4 Variability . . . . . . . .
4.4.1 Seasonal Regime.
4.4.2 Hydrograph Variability.
4.4.3 Interannual Variability
4.5 Catchments and Channels .
4.5.1 Fluvial Geometry .
4.5.2 Ungauged Streams.
4.6 Regulated Rivers ..... .
4.7 Waterways and Watersheds.
4.7.1 Influent and Effluent Streams
4.7.2 Environmental Aspects
4.8 Concluding Statement . . . . . . . .
107
108
108
108
109
110
110
112
113
115
118
118
120
120
121
123
123
124
127
129
129
130
131
5 Transport of reacting solutes in rivers and streams
RL Runkel and K.E. Bencala
5.1 Introduction ........ .
5.2 Physical Transport Processes.
5.3 Sources of Solutes . . . .
5.4 Geochemical Processes. . . .
5.4.1 Rates of Reaction ..
5.4.2 PrecipitationlDissolution.
5.4.3 SorptionlDesorption ...
5.4.4 pH, the Master Variable .
5.5 Coupling of Hydrologic and Geochemical Processes .
5.6 Summary ..................... .
6 Water and contaminant transport in the vadose zone
P.l. Wierenga and ML Brusseau
6.1 Introduction . . . . .
6.2 Water Storage ..... . .
6.2.1 Water Content . . .
6.2.2 Energy Relationships
6.3 Water Movement. . . . . . .
6.3.1 Steady Flow .....
6.3.2 Hydraulic Conductivity
6.3.3 Transient Flow
6.3.4 Infiltration...
6.3.5 Redistribution.
6.4 Contaminant Transport .
6.4.1 Equations For Nonreactive Solutes.
6.4.2 Equations for Reactive Solutes. . .
6.4.3 Transport in Uniform Soil: Steady flow
6.4.4 Transport in Uniform Soil: Transient flow
6.5 Nonideal Contaminant Transport.
6.5.1 Heterogeneity ......... .
6.5.2 Preferential Flow ........ .
6.5.3 Nonlinear, Rate-Limited Sorption
6.5.4 Facilitated Transport ...... .
6.5.5 Vapor and Immiscible-Liquid Phase Transport.
6.5.6 Coupled Physical, Chemical, and Biological Processes
6.6 Conclusion.........................
ix
137
137
139
147
152
153
155
]57
159
160
163
165
165
]66
]66
166
170
170
170
17]
172
175
175
175
177
178
]80
180
180
182
183
186
186
187
188
7 Transport of moisture and solutes in the unsaturated zone by 193
preferential flow
F Stagnitti. 1. -Y. Parlange. IS. Steenhuis. 1. Boll. B. Pivetz and D.A. Barry
7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . 194
7.2 Preferential Moisture and Solute Transport. . . . . . . 195
7.3 Sampling for Preferential Flow in the Unsaturated Zone 199
7.4 Characterisation of Soil Structure . 200
7.5 Modelling Preferential Transport. . . . . . . . . . . . . 203
7.6 Transport of Pesticides . . . . . . . . . . . . . . . . . . 210
7.7 Influence of Macropores on Biological Degradation of Pesticides . 213
7.7.1 Biodegradation of p-Nitrophenol (PNP) . . . . . . . . . . 213
7.7.2 Biodegradation of 2,4-D (2,4-dichlorophenoxyacetic acid) . . 216
7.8 Summary and Conclusions. . . . . . . . . . . . . . . . . . . . . . 218
x
8 Groundwater contamination modelling 225
A.G. Bobbaand v.P. Singh
8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . 225
8.2 Classification of Groundwater Contamination Models . 227
8.2.] Models ...... . 227
8.2.2 Model Development . . . . . . . . . . . . . . 227
8.3 Groundwater Modelling . . . . . . . . . . . . . . . . 228
8.3.1 Principles and Concepts used in Groundwater Modelling . 229
8.3.2 Darcy's Law . . . . . . . . 229
8.3.3 Hubbert's Force Potential . 231
8.3.4 Conservation of Mass . 232
8.4 Flow Models . . . . . . . . . . . . 232
8.4.1 Basic Assumptions. . . . . 232
8.4.2 Two Dimensional Horizontal Flow. . 233
8.4.3 Definition of boundary and initial conditions. . 233
8.4.4 Initial conditions ........ . 235
8.5 Contaminant Transport Models. . . . . . . . . . . . . 236
8.5.1 Advective Dispersion Phenomena . . . . . . . 236
8.5.2 Basic Assumptions. . . . . . . . . . . . . . . 236
8.5.3 Advective Dispersion Equation in Cartesian Coordinates . 238
8.5.4 Boundary and Initial conditions . 238
8.6 Hydrodynamic Dispersion . . . . . . . . . . . . . . . . . . . 239
8.6.1 Effects of Dispersion. . . . . . . . . . . . . . . . . . 24]
8.6.2 Quantification of Dispersion . . . . . . . . . . . . . . 242
8.6.3 Determination of Coefficient of Molecular Diffusion . 243
8.7 Chemical and Biological Activity . 244
8.7.1 Chemical Processes . 244
8.7.2 Linear Adsorption . . 246
8.7.3 Freundlich Isotherm . 246
8.7.4 Langmuir Isotherm . . 246
8.7.5 Biological Processes .247
8.8 Development of Contaminant Transport Models . 247
8.8.1 Analytical and Numerical Models . . 248
8.8.2 Types of models . . . . . . . . . . . 248
8.8.3 Analytical Models . . . . . . . . . . 250
8.8.4 Analytical Element Method Models . 25]
8.8.5 Finite Difference Models. . . . . . 252
8.8.6 Finite Element Models . . . . . . . 260
8.8.7 Application of Galerkin's method . 264
8.8.8 Application of Green's theorem . 265
8.8.9 Boundary Element Models. . 271
8.8.10 Velocity Based Modelling . 272
8.9 Accuracy of Numerical Models .. . 272
8.9.1 Numerical Dispersion . . . . 273
8.10 Available Numerical Models. . . . . 273
8.10.1 General Guidance for Groundwater Contamination Model Use . 274
8.10.2 Data Requirements. . 276
8.10.3 Physical Framework . . . . . . . . . . . . . . 276
8.11 Stresses on System . . . . . . . . . . . . . . . . . . . 277
8.11.1 Groundwater Flow . . . . . . . . . . . . . . . 277
8.11.2 Contaminant Transport (in addition to above) . 277
8.12 Physical Interpretation . . . . . . . . . . . . . . . . . 277
Xl
8.12.1 Effect of Velocity Ratios on Contaminant Transport Model . 278
8.12.2 Effect of Adsorption on Contaminant Concentration . . 279
8.12.3 Effect of Velocity Ratios on Concentration. . . . . 283
8.12.4 Effect of Dispersivity on Contaminant Transport. . . . 284
8.12.5 Contaminant Recovery Phase . . . . . . . . . . . . . 287
8.13 Application of Groundwater flow and Contaminant Transport Models . 288
8.13.1 Prot Granby Radioactive Disposal Site, Ontario, Canada. .. . 288
8.13.2 Application of SUTRA model to Lambton county, Ontario, Canada 295
8.13.3 Numerical Results . . . . . . . . . . . . . . . . . . . . . . . . . 306
8.14 Limitations and Source of Error in Groundwater Contamination Models. . 310
8.14.1 Model Misuse . . . . 313
8.15 Improvements in Modelling . . . . 314
8.15.1 Output Formats ..... .314
8.15.2 Data Collection Methods. . 314
8.15.3 Further Development of Flow and Transport Models . 314
8.16 Further Research in Chemical Kinetics. . . . . . . 315
8.17 Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . 315
9 Modeling subsurface transport of microorganisms 321
Y. Tan and w.J. Bond
9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 321
9.1.1 Development of Mechanistic Models of Transport . . . . 322
9.1.2 Approaches to the Pore-Scale Distribution of Bacteria . . 323
9.2 Processes Affecting Transport and Fate of Microorganisms . 324
9.2.1 Movement of Microorganisms . . . . . . 324
9.2.2 Growth and Decay of Microorganisms . . . . . . . . 327
9.2.3 Attachment and Detachment Processes ...... . 331
9.2.4 Mathematical Description of Attachment and Detachment. . 334
9.3 Transport of Microorganisms in Porous Media. . . . . . . . 337
9.3.1 General Transport Equation for Microorganisms. . . 337
9.3.2 Special Cases of the Microbial Transport Equation . 339
9.3.3 Simultaneous Transport of Substrates with Bacteria . 340
9.4 Comparison of Models with Experiments . 342
9.4.1 Transport of Bacteria. . . . . . . . 342
9.4.2 Transport of Viruses . . . . . . . . 346
9.4.3 Summary of Experimental Studies . 347
9.5 Summary and Conclusions . . . . . . . . . 347
10 Assessment and control of loading uncertainty for managing
eutrophication and toxic chemical fate in lakes
T. C. Young and D.M. Dolan
10.1 Introduction . . . . . . . . . . . . .
10.2 Loadings and Sources of Uncertainty
10.2.1 Tradeoffs. . . . . . . .
10.3 Assessing Loading Uncertainty
10.3.1 Demonstration .....
10.4 Managing Loading Uncertainty
10.4.1 Tributary Mouth Loadings
10.4.2 Point Sources ..... .
10.4.3 Non-Point Sources ... .
10.4.4 Data Handling Improvements
10.5 Conclusions . . . . . . . . . . . . .
357
.357
.359
.360
.360
.362
.363
.365
.369
.370
.373
.373
xii
11 Modeling watershed water quality 377
A.S. Donigian, J. C. Imhoff and R.B. Ambrose, Jr.
11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . 377
11.2 Evolution of Watershed-Scale Water Quality Modeling . 378
11.3 Why Model? . . . . . . . . . . . . . . . . . . . . . . . 379
11.3.1 Role of Modeling in Water Quality Analyses . . 380
11.3.2 Environmental Analyses That Benefit from Modeling . 380
11.4 Introduction to Watershed Models and Modeling. . . . . . 381
11.4.1 Modeling Fundamentals . . . . . . . . . . . . . . . . 381
11.4.2 Relevant Model Features. . . . . . . . . . . . . . . . 383
] ] .4.3 Watershed Water Quality Modeling Components .. . 387
11.5 Methods and Models for Nonpoint Source Water Quality Modeling . 387
11.5.1 Urban Runoff Modeling Methods . . . . 387
11.5.2 Urban Runoff Models . . . . . . . . . . 392
1] .5.3 Non-Urban Runoff Modeling Methods. . 395
] ] .5.4 Non-Urban Runoff Models . . . . . . . . 399
11.5.5 Guidance on Nonpoint Source Mode] Selection and Use. . 405
] ].6 Methods and Models for Surface Receiving Water Quality Modeling . 407
11.6.1 Methods . . . . . . . . . . . . . . . . . . . . . . . . . 407
11.6.2 Guidance on Receiving Water Model Selection and Use . . 415
11. 7 The Future of Watershed Water Quality Modeling . 416
11.7.1 Algorithm/Computation Enhancements . 417
11.7.2 User Interaction . . . . . . . . . . . . . . 419
1 ].8 Closure. . . . . . . . . . . . . . . . . . . . . . . 421
12 Eco-hydrological consequences of environmental degradation: 427
Hydrology, ecology and environmental impacts
P.H. Templet and J. Sorensen
12.1 Introduction . . . . . . . . . . . . . . . . . . 427
12.2 Environmental Change . . . . . . . , . . . . . 429
] 2.2.1 A Partial List of the Agents of Change . . 429
] 2.3 Comparative Hydrology and Landforms . 429
12.4 Weighting Impacts. . . . . . . . . . . . . . . . 431
12.5 Overview of The Planning Process. . . . . . . . 437
12.5.1 Water Conservation . . . . . . . . . . . 439
12.6 Detailed Planning and Impact Assessment Process. . 439
12.6.] Description of the Proposed Action . . . . . 440
] 2.6.2 Screening ................. . 44]
12.6.3 Scoping . . . . . . . . . . . . . . . . . . . 446
12.6.4 Identification and Selection of Alternatives . 448
12.6.5 Presentation . . . . . . . . . . . 455
] 2.6.6 Mitigation of Adverse Impacts . . 456
] 2.6.7 Review and Comment . . . . . . 457
]2.6.8 The Decision. . . . . . . . . . . 458
]2.6.9 Auditing or Monitoring Impacts . 460
12.7 Conclusion. . . . . . . . . . . . . . . . 460
Glossary of terms 463
Contributors 475
Subject Index 477
Preface
In environmental planning, management and restoration, hydrology plays a fundamental role. Collection of hydrologic concepts that apply to solution of environmental problems constitutes the subject matter of environmental hydrology.
A book exclusively devoted to discussion of these concepts and their application
to environmental issues is lacking. Because of the increasing significance of environmental management, an attempt has been made to cover some aspects of
environmental hydrology in this book. It is hoped that publication ofthis book will
encourage others to write comprehensive texts on this subject of enormous interest
and growing significance.
The subject matter of this book is divided into twelve chapters. Introducing
environmental hydrology in the first chapter, watershed acidification modeling is
discussed in Chapter 2. The precipitation of acidic materials from the atmosphere to
terrestrial and aquatic systems has received international attention, and is, therefore,
an appropriate start for the subject of environmental hydrology. If the climate as
we know it today changes in a significant way, its implications are so gigantic
that it is even difficult to fathom its full impact on the human race. Because of
considerable expenditure of resources and debate now being devoted to climate
change, its discussion is given in Chapter 3. River hydrology constitutes the core
of surface-water hydrology and much of chemicals introduced into our land and
water resources find their way into surface waters through rivers. Chapter 4 is
devoted to discussion of river hydrology.
Chemicals introduced into the environment migrate through rivers and streams,
unsaturated zone, and/or groundwater formations. The next four chapters deal with
the fate and migration of these chemicals through the water continuum. Specifically,
Chapter 5 discusses the fate and migration of reacting solutes in rivers and streams,
physical mechanisms influencing solute transport, and the spatial and temporal
distribution of the sources contributing the solute mass. Chapters 6 and 7 focus on
solute transport in subsurface unsaturated zone. Basic principles of water storage,
and of flow and contaminant movement through unsaturated soil are emphasized
in Chapter 6. Transport of water and solutes through the vadose zone under field
conditions is presented in Chapter 7, with emphasis on the nature and impact
of preferential flow. Chapter 8 discusses the role of groundwater contamination
models in planning, management, and regulation of groundwater systems, with
a focus on generic and site specific contamination. A case history from Canada
is presented to illustrate the current modeling technology. Besides chemicals,
microbial transport is receiving increasing attention these days, partly because of
the role bacteria and viruses play in environmental pollution as well as in waste
remediation. Subsurface transport of microorganisms is presented in Chapter 9,
summ:lfiz';iig the progress made to date in this area of growing significance.
X III
xiv
The last 3 chapters deal with water quality and its modeling. Of particular
concern are loadings of nutrients and toxic chemicals. Chapter 10 reviews major
sources of uncertainty associated with tributary loads, other non point sources,
and point sources, with particular emphasis on fluvial loadings. It also discusses
alternative monitoring and sampling designs. Water quality modeling is presented
in Chapter 11. It summarizes the historical development of watershed-scale waterquality modeling, presents the best of the current generation of modeling tools,
and explains potential areas for future model development and research. The
last Chapter 12 dwells upon eco-hydrological consequences of environmental
degradation. It provides a guide to those interested in impact assessment or more
detailed analyses.
The editor expresses his deep appreciation to the various contributors who
generously contributed to the book despite their busy schedule.
v.P. SINGH
Baton Rouge, Louisiana
CHAPTER 1
What is environmental hydrology?
V.P. Singh
1.1 Environmental Continuum
Air, soil, and water constitute the environmental continuum, and are vital components for sustaining life on earth. These components are interactive and interactions
amongst them are complex. Stated another way, a management change imposed on
one component of the environmental continuum has effects that propagate to other
components, and some of these effects are unknown and cannot be quantified.
The interactive nature requires that the environment is managed and protected as
a cohesive whole (or a system). This can be accomplished through an integrated
approach to environmental management that clearly has to be interdisciplinary.
Such an approach may entail management of individual components with proper
accounting of their interactions and dependencies. Thus, the management of air,
soil, and water must be undertaken in an integrated manner.
1.2 Integrated Environmental Management
Worldwide, environmental systems are rarely managed as a cohesive whole. That
is partly because air, soil, and water resources have been and continue to be
managed by independent organizations having little interaction amongst them. For
example, at the federal level in the United States there is the Environmental Protection Agency (EPA) charged with the responsibility of protecting the environment,
establishing guidelines for environmental protection, and overseeing environmental restoration works. Other federal departments such as the U.S. Department of
Agriculture, U.S. Army Corps of Engineers, U.S. Bureau of Reclamation, U.S.
Geological Survey, etc., are also concerned with safeguarding the environmental
quality of land and water resources within their jurisdiction. Then there are the
departments of natural resources or environmental quality at the state level. These
departments have the responsibility of protecting the environment at the state and
local level. In a similar vein, there is a plethora of organizations in the United
V. P. Singh (ed.). Environmental Hydrology. 1-12.
© 1995 Kluwer Academic Publishers.
2 v.P. Singh
States that have authorities to manage nation's water resources at the local, state,
regional, and national level. The U.S. Bureau of Reclamation and the U.S. Army
Corps of Engineers are two of the federal agencies with broad authority for water resources management. Then there are the departments of water resources or
natural resources at the provincial (state) level having authority to manage water
resources in their individual states. At the local (county or parish) level, there are
the water boards, commissions, and the like that manage local water resources. The
nation's land resources are managed in an analogous manner. The management of
land and water resources at all these levels must be accomplished in conformity
with environmental protection guidelines.
1.3 Water Continuum
Water exists in different forms - as water vapor in the atmosphere, as liquid water in oceans, streams, lakes, etc. (or hydrosphere) and lithosphere, and as snow
and ice on the land surface. The flow of water observed in rivers evolves as a
continuum comprising surface flow (overland and channel flow), interftow, and
baseflow. These components occur concurrently but their relative magnitudes vary
with time. For example, for a burst of rainfall, surface runoff predominates during
the rising part of the streamflow hydrograph, interftow during the early part of its
recession, and base flow during the delayed part of its recession. The mechanisms
governing these components are different but are influenced by dynamic interactions prevailing between them. The factors controlling flow generation can be
summarized as climate, geology, topography, soil characteristics, vegetation, and
land use. The relative significance of each factor varies in time and space.
The occurrence of surface runoff over the entire watershed is based on the
assumption that rainfall intensity exceeds the infiltration capacity over the whole
watershed. However, in many geographic regions surface runoff is rarely observed.
Many watersheds yield well-defined hydrographs from storms whose rainfall intensity is less than the infiltration capacity of the soil-vegetation-Iand use (SVL)
complex. This leads to the existence of the various sources of water generating
a storm hydrograph: (1) Overland flow may occur over the entire watershed or a
portion thereof. (2) Overland flow may occur on an area saturated by downslope
groundwater flow as well as infiltrating water. (3) Subsurface flow may occur due
to rapid infiltration and transmission through unsaturated and saturated zones. Saturated areas grow during a storm and shrink during interstorm periods. Therefore,
the initial size of the contributing area depends upon the time history of rainfall
during the prestorm period and the growth depends on the time history of rainfall
during the storm.
Solute transport in surface and subsurface waters is affected by a complicated
mix of physical, chemical, and microbiological processes. Because water is the
principal carrier of solutes and sediments, a complete understanding of water
continuum is critical to determining the fate and migration of these contaminants.
For example, huge quantities of chemical fertilizers and herbicides are used in
agriculture every year in the United States, Canada, and Europe. The release and
migration of nutrients, pesticides, and other chemicals in runoff from agricultural