Defining aridity: the classification and character of dryland climates

In this book the term dryland is used to designate those regions that have a pronounced dry season during at least part of the year. This encompasses vastly diverse regions that, in the literature, are variously termed arid, extremely arid, hyper-arid, desert, semi-desert, semi-arid, semi-dry, and subhumid. The essence of the problem of classification is ultimately the definition – and quantification – of aridity. There are at least as many definitions as there are deserts. Most definitions of aridity relate to climate, but people who are interested in vegetation, landforms, soils, or hydrology are likely to have quite different concepts of aridity and drylands. The best aridity index or climate classification scheme is that which is most appropriate for the task at hand, be it understanding vegetation growth, modeling climates, or assessing agricultural potential or human comfort. In this chapter, an overview of the most common aridity indices and climate classification schemes is provided. The ways in which these various systems define and distinguish the various dryland climates are emphasized.