The semi-arid zones of the world are fragile ecosystems which are being sub- stantially modified by the activities of mankind. Increasing human populations have resulted in greater demands on semi-arid zones for providing human susten- ance and the possibility that this may enhance desertification is a grave concern. These zones are harsh habitats for humans. The famines that resulted from drought during the late 1960's and the 1970's in the African Sahel illustrated the unreliability of present agricultural systems in this zone. Large fluctuations in ag- ricultural production have occurred...