ABSTRACT
Scope and definitions The terms arid and semi-arid, as related to land-use, have been used rather loosely and with differing meanings in recent literature on the ‘arid zones’. In this chapter, a more precise connotation is proposed for these terms, based upon position in relation to a particular threshold of expected or effective rainfall, a threshold which will vary from one region to another according to the growing season of plants, to the nature of the climate (summer rainfall, winter rainfall, monsoonal, etc.), and to altitude, latitude and other factors. On one side of such a threshold, the conditions are said to be arid, and extensive grazing is the only reliable form of land-use. On the more humid side of the threshold, conditions are said to be semi-arid, and crop cultivation using dry-farming techniques and drought-resistant or drought-escaping crops can be recommended. Naturally this threshold is not precisely along any fixed isohyet or the line of any given expected rainfall. There is a transition zone in which rainfall and therefore crop cultivation are quite unreliable, but into which agriculture is frequently forced under a variety of social and economic pressures.
