Introduction

Salinity

The term salinity describes the presence of soluble salts in soil or aqueous solution. Whilst the ions Na+ and Cl- of sodium chloride are often dominant contributors to salinity, other ions can also provide an important addition to the solute concentration in soils. As the concentration of saline salts in soil increases, plant or tree growth is inhibited, and, at sufficiently high concentrations, death may occur (see the frontispiece to this chapter, showing a saline stream, Quairading, Western Australia). Physical characteristics of soil are also deleteriously affected by salinity: hydraulic conductivity can decline dramatically, and, on drying, soils can become very hard and unsuitable for plant growth.

Especially in those regions of the world where annual rainfall is low and evaporation rates potentially high, areas of naturally saline soil can be found. This is referred to as ‘primary salinity’, and a salt concentration by weight of only some 0.1% to 0.3% in surface soils can substantially reduce the growth of most plants and trees. It has been estimated that about 1000 million hectares, or 7% of the world's land area, is affected by salt to some extent (Dudal and Parnell, 1986).

However, what has emerged, particularly in recent times, as a major environmental issue in sustainable land management is the large-scale increase in salt-affected land caused by human activity. This is referred to as ‘secondary salinity’.

This book is intended to provide the basic physical knowledge required to understand the processes involved in the sustainable use of the earth's land and water resources. Description of the physical science of soil and water processes is carried through to application at the watershed scale. Consideration of processes at this scale is necessary since this is the scale at which land-management decisions begin to be made and at which activities with environmental and water-quality implications occur.

The book is introductory in the sense that no prior knowledge of physics or calculus is assumed. Arithmetic and elementary algebra are used. No experience of computer-spreadsheet use is required of the reader, though the utility of such aids to calculation is illustrated on a few occasions. Though elementary in this sense, in some issues consideration is given to ideas at the frontier of research and understanding.

How theory can be applied to field data is illustrated using many examples. Exercises that further illustrate the application of theory are given at the end of each chapter. Answers to all exercises are given in the book, and fully worked solutions are available on the world-wide web at http://publishing.cambridge.org/resources/0521536790.

The methodologies of environmental physics and engineering dominantly employed in this book can provide only one link in the chain of activity necessary to deal effectively with typically complex environmental issues. Other links or areas of knowledge required to approach the environmental issues addressed are at least hinted at.