ABSTRACT
Rain-fed agriculture predominates throughout the humid tropics. As rainfall decreases towards the semi-arid zone, the chances of crop failure increase, until below about 250 mm per annum, successful cultivation is not really possible on a long-term basis. Along this same gradient the importance of irrigation increases. Rain-fed agriculture has the great advantage over irrigation that it requires no infrastructure to be built. It is therefore a much cheaper form of crop production and is utilised by farmers where- ever possible. In the semi-arid zone there are great differences in crop production between the wetter and drier margins. In the wetter parts low yields or crop failure due to drought may occur only infrequently, perhaps less than one year in every ten, while in the drier parts crops might fail three or four years in every five.
