Though most semi-arid lands border the arid or desert climates of the world, they support a diversity of cropping (mostly with annual crops). Because of this diversity their climate should be quantified and any variations within them delineated. Such drylands have either one or two wet seasons. Bimodal rainfall regimes are characteristically found in latitudes between the wet tropics and the unimodal more temperate, semi-arid zones. Their distribution, however, is governed more by pressure systems and land masses.

...Within these environments, the length of growing season and the kind of cropping system are determined not only by rainfall distribution but by soil water-holding characteristics and topographic position. The latter affects runoff to and runon from neighbouring land. The wetter regimes within the 119-day growing season allow some relay cropping on soils with favourable water-holding characteristics, but single cropping is more typical unless runon water is available.