BOOK REVIEW:
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, $91.

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 solution.

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 catchment studies.

Author: George M. Hornberger
Department of Environmental Sciences
University of Virginia
Charlottesville, USA