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by Dr. T.D. Davies, University of East Anglia, Norwich (U.K.)

Seasonal snowcover is an important part of the hydrological cycle for large areas of the Earth's surface. Since snowcover represents a reservoir for atmospheric deposition, its melting can produce significant changes in soilwater and surface water composition. The composition of the meltwater will depend, to an extent, on the evolution of the snowcover over the winter period. The major hydrogeochemical role which seasonal snowcover plays means that it performs an important dynamic function in many temperate, alpine and subarctic ecosystems. The growing realisation that amongst the potential impacts of possible future climate change is a shifting pattern of seasonal snowcover behaviour, has highlighted the need for an improved understanding of the processes which govern the composition of the snowcover and its meltwater. The appearance of this book (NATO ASI SERIES G28) is, therefore, timely and very welcome.

A number of international experts have provided up-to-date syntheses of current research and presented new results from research programmes. The result is a rigorous and concise integration of the nature and causes of compositional change in seasonal snowcovers. The initial chapters deal with the composition of snowfall and how the composition of the existing snowcover can be changed before melting occurs. The linking of chemistry and physics, in seeking explanations for snowcover compositional change, is one of the recurring themes of the book. Amongst the wide range of snowcover impurities covered in the book, trace organic contaminants are, potentially, amongst the most important and yet, hitherto, they have been relatively ignored. Dry deposition processes are assessed, and the problems of applying measurement techniques to snowcover considered. Over limited areas individual urban sources can directly affect the composition of regional snowcover. Small-scale compositional variability can also result from the sublimation or resuspended, blowing snow particles; some novel research on this topic is reported, for which there is much scope for future work.

In order to fully understand the chemical transformations of snowcover upon melting, it is necessary to couple models incorporating many flow routes to solute transport models. This is difficult because of the lack of suitable chemical models and the complex geometry of wetting fronts. However, explicit physical-chemical models are being developed and are outlined in the book. As the snowcover starts to melt, very concentrated solutions are present in small liquid inclusions and the material presented in the book demonstrates the need to clarify the thermodynamics for these thin layers of solution. Of particular significance for seasonal snowcover is biological/microbiological activity. Animals and invertebrates can contribute significantly to compositional change. The pioneering work on this topic presented in the book also demonstrates that the activity of snow algae needs to be considered specifically in snowcover evolution.

Snow and firn analyses have been used to reconstruct past atmospheric conditions, but more consideration needs to be given to the understanding of depositional processes and in-pack (core) processes for full confidence in the interpretations. One chapter reviews work on high-altitude, mid/low latitude glaciers, and examines the type of information available. A second chapter is more specifically oriented towards high latitudes and assesses the reality of links between historical records and contemporaneous atmospheric conditions. Looking to the future, rather than to the past, there is a topical consideration of the implications of future climatic change on seasonal snowcover, and an assessment of possible changes in melt regimes and potential perturbations to aquatic ecosystems in NE North America.

Finally, a contribution from a panel of expert reviewers plots the way forward for research on seasonal snowcovers to conclude a book which makes a serious attempt to bring together various researchers on seasonal snowcover to present an integrated state of the science.
Reference books: C190, C252, C268, C297, G6, G28

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