of low rainfall. On the other hand, surplus surface water may be used in overdraft areas to increase the groundwater storage by artificial recharge. Moreover, surface water, groundwater or both, depending on the surplus available, can be moved from water-plentiful to water-deficit areas through canals and other distribution systems. On the whole the integrated system, correctly managed, will yield more water at more economic rates than separately managed surface and groundwater systems.
2.2 Water Storage
In conjunctive use, the two most important issues that planners have to face concern the storage of surplus water and the optimal allocation of water withdrawals.
With regard to the first problem, a question that needs to be answered is where to store water and which reservoirs to develop: surface or subsurface?
2.2.1 Subsurface water storage
The advantages of subsurface over surface reservoirs are:
surface reservoirs are lost forever once they are silted up, while underground storage capacities remain practically unaffected by development;
yields from groundwater storage, less affected by evaporation and leakage, are more dependable than yields from surface reservoirs;
groundwater is less prone to pollution than surface water, and if polluted, pollutants can be diluted during underground movement;