Water, received from precipitation and stored behind dams for irrigation may have the following drawbacks:

• High evaporation loss.
• Soil erosion in the catchment resulting in siltation and reduced storage capacity.
• Environmental impact of surface reservoirs.
• Expensive distribution system because of the distance between dam and utilization areas.

In contrast, groundwater is not exposed to evaporation; does not suffer from reduction of storage capacity because of siltation; is seldom harmful to environment and offers a natural water distribution up to the users.

When looking at these advantages and disadvantages, groundwater seems to be a better alternative that should be preferred, but this not the case; large and concentrated water demand such as that from large irrigation schemes is usually supplied from surface water storage, and there are various reasons for that choice:

• Groundwater aquifers seldom offer large storage capacity to be able to absorb large volumes of flood in a short period of time, and are unable to return them as significant discharge per unit production system of well or borehole.
• Surface water storage, because of the large investments involved, is often preferred because it offers a much higher political visibility and because high construction costs give an opportunity for private profit, opening the way for improper influence on decision making.

Conjunctive use: Conjunctive use of surface and groundwater consists of harmoniously combining the use of both sources of water in order to minimize the undesirable physical, environmental and economic effects of each solution and to optimize the water demand/supply balance.

Assuming that the mixed solution is part of the national policy, several problems need to be carefully studied before selecting the different options and elaborating a programme of conjunctive use of surface and groundwater:

• Underground storage availability to be determined,
• Production capacity of the aquifer(s) in term of potential discharge,
• Natural recharge of the aquifer(s)
• Induced natural recharge of the aquifer(s)
• Potential for artificial recharge of the aquifer(s)
• Comparative economic and environmental benefits derived from the various possible options.