Introduction

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.

Definition and indices of aridity

In his classic book Petrov (1976) equates aridity with a deficiency of rainy days and ground moisture. Meigs (1953), whose classification of arid lands is the one most widely used today, writes that “many distinctive environmental traits distinguish arid lands, but one is essential – lack of precipitation.” Nevertheless, rainfall alone is clearly insufficient for defining climatic boundaries. In Australia, for example, the generally accepted limit for an arid zone is 250 mm, but some areas of the tropical northwest are clearly arid, although mean annual rainfall exceeds 500 mm (Mabbutt 1979). For the Sahara, Dubief (1959) suggests that the 50 mm and 100 mm isohyets, respectively, be used to delineate its extra-tropical northern border and its tropical southern border.