The most obvious way to save water under scarcity is to re-collect and re-use drain water, in what are called ¿closed growing systems¿. In practice, accumulation of salts in the cycle requires such systems to be flushed from time to time, with consequent waste of water and fertilisers, often ending up in polluting percolation. The rate of salt accumulation depends on many factors, the most obvious being the quality of the water used to re-fill the system. Other relevant factors are the uptake of minerals by the crop and the relative size of water fluxes (transpiration, irrigation) and buffers (substrate, tanks) in the system We have built a model of salt evolution in a closed system depending on these factors. The model has been validated in an experiment with tomato. We show that the model can predict rather well the evolution of most important minerals in the system, when some critical processes are well described. Thereafter we use a couple of examples to show how such a model can be used for determining the best management strategies under various external conditions. We conclude that while it is true that management under scarcity requires more skills than are now common among growers in arid regions, tools can be developed that could warrant economic viability of protected cultivation also in the regions where sustainability is presently in doubt.