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Shrub encroachment is one of the major problems in arid and semi-arid savannas of southern Africa. Especially artificial waterholes in the Kalahari often show a distinct increase in woody vegetation cover. However, the formation of shrub encroached piosphere zones is not always unidirectional among different environmental parameters. It is controversial whether piosphere zones expand in the course oi time and what the impact of different stocking rates and rainfall scenarios on this process is. On the basis of a spatially explicit, grid based simulation model the piosphere dynamics at two different localities (a high and a low rainfall scenario) in the southern Kalahari is compared. The model results show that under the high rainfall scenario distinct piosphere zones and shrub encroachment develop for a wide range of stocking rates. The piosphere expands in the course of time. Under the low rainfall scenario only high grazing pressure leads to shrub encroachment within timespans below 80 years and the piosphere expands and contracts in response to the highly variable rainfall.