ABSTRACT
Dryland farming or crop cultivation with limited marginal precipitation is characteristic of and most extensively practised in the semi-arid areas of the world (Hall et al, 1979). While semi-aridity implies conditions neither totally wet nor completely dry, the agricultural significance of the concept can only be expressed in terms of the sufficiency of available precipitation for cultivation. The semi-arid areas lie between the deserts, where the production of rain-fed crops is not possible during the majority of years despite efforts to conserve and maximize available moisture, and the humid areas, where drought does not substantially limit crop production in the majority of years. As Thornthwaite (1948), Bailey (1979) (see Fig. 12.1) and others have demonstrated, the annual precipitation amount defining the humid–arid boundary depends on those atmospheric conditions, particularly temperature, which determine evapotranspiration rates; and a rainfall amount sufficient to produce a crop in a cool climate may be inadequate in a warmer one.
