EOS Transactions, 8th June, 1999
Isotope Tracers in Catchment Hydrology
C. Kendall and J. J. McDonnell (Eds.).
Elsevier Sci., New York, xxix + 839 pages. ISBN 0444-50155-X, 1998,
The drainage basin has been a fundamental unit of study in hydrology
and geomorphology throughout the modern era of these Earth sciences.
The basin, or catchment, is a convenient unit because it typically
is well defined topographically, can be studied as a series of nested
units of increasing size, and is an open system for which inputs
and out- puts of mass and energy can be defined and measured. Small
catchments have been the "outdoor laboratories for hydrologists
interested in rainfall-runoff relationships, for geochemists interested
in the export of weathering products from the landscape, and for
ecosystem ecologists interested in biogeochemical cycling.
Research studies on small catchments have evolved from a strong
focus on strictly input- output relationships (a 'black-box' approach)
to a focus on understanding flow paths, residence times, and biological
and chemical reactions within the catchment. Tracers are essential
tools for such work. In catchment studies, tracers that move with
the water but do not interact with biota, soils, and rocks are sought
so that water itself can be followed. Other tracers are used to
follow selected biogeochemical reactions. A multitude of stable
and radioactive isotopes has been used as tracers in catchment studies.
In Isotope Tracers in Catchment Hydrology, editors Carol
Kendall and Jeff McDonnell have assembled 22 chapters that cover
a wide range of topics related to the use of isotope tracers in
catchment studies. (In fact there is good coverage of some topics
not typically considered to be in the realm of catchment hydrology,
such as lake evaporation and groundwater recharge.) A chapter reviewing
the basics of isotope geochemistry is extremely valuable for someone
from the physical hydrology side of the house – like me! A suite
of chapters on processes (such as isotopic fractionation in snow),
on hydrological case studies (such as and catchments), on geochemical
case studies (such as nitrogen cycling), and on synthesis (such
as modeling hydrogeochemical responses) form the core of the book.
As I launched into this book, I admit I had some concern that I
would find authors overly euphoric about what could be done with
tracers. Users of particular tools often are so taken with the techniques
that they become effective, if not always impartial, salespersons.
My worry was not assuaged when I read ". . why do environmental
hydrogeologists continue to under utilize isotopes?" (One answer
given - “fear of the unknown" - was enough to make me cringe.) Any
fears that I had were dispelled on reading the book, however. The
material presented is thorough both in showing the power of using
isotope tracers and in exposing the difficulties.
When tracers are used to infer flow paths and residence times in
a catchment, a model of the hydrological system is required. That
is, the data are used in an inverse solution to infer parameters
of the underlying conceptual model, which may be a simple two-component
mixing model or may be much more complex. Problems arise because
the proper model may not be known, because the in- verse solution
may not be unique, and be- cause data may not be adequate to constrain
The most striking result of isotope studies of catchment hydrology
over the past several decades is the large and often dominant role
of subsurface water in generating stream ru n- of f in humid headwater
catchments. This is a result inferred by assuming a two-component
(sometimes three-component) mixing model.
Going beyond this now well established result has proven to be
much more problematic. Inverting more complex models requires careful
and detailed hydrometric measurements as well as measurements of
isotope tracers. The problems of spatial heterogeneity that are
so prevalent in all of hydrology are realized in tracer studies
under these conditions. Answers to the problems are not straightforward
and will require new research. This book convinces one that a host
of clever scientists are using isotope tracers effectively to address
complex problems in catchment science but that many problems must
be resolved before worrying about "under-utilization of isotopes."
Each chapter is authored by one or more individuals who have devoted
much professional effort to understanding catchment processes using
isotope tracers as a tool. Thus, the chapters uniformly are authoritative.
Kendall and McDonnell also have done an excellent job minimizing
chapter-to-chapter inconsistency that often plagues compilations
of papers. The chapters follow a similar template and, for the most
part, do not have gratingly different styles. Almost all of the
chapters are well worth reading, and many will surely become required
reading in graduate courses around the world.
The editors have used the term hydrology in its broadest sense-to
include the waters of the Earth and all interactions with Earth
materials and ecosystems. The book provides a wealth of information
not just for those with an interest in physical hydrology but for
geochemists and ecologists as well. It will be a valuable addition
to the library of scientists in all of the disciplines engaged in
Author: George M. Hornberger
Department of Environmental Sciences
University of Virginia