ABSTRACT
Water is an extraordinary substance. It is found in all environments on Earth, and its concentration always has a major influence on physical, chemical and biological properties. Thus, whether we are studying desert environments, climatic feedbacks in the atmosphere, or formation of basaltic magma in the crust, water concentration is a master variable. If we want to understand the environment, we must understand water. The approach we have chosen to achieve this is to present a model for water. But first, what do we mean by ‘model’? In science, this term is shorthand for a package of ideas and rules which allows us to explain and predict behaviour under a variety of circumstances. The value of a model is in simplification. Instead of learning all the complex patterns of behaviour, we can learn a few simple rules that allow us to work out what will happen. Before looking in detail at the properties of water, let us first look at its distribution on Earth.
