Meeting demands (as regards quantity and quality) is one of the main aims of water resource planning and management. To find an equilibrium between human activities and needs, and the availability in particular areas is a key issue in semi-arid regions. As a consequence of an unsustainable development model, particularly agricultural, demands are often greater than the available resources. Many of the water resource problems existing in the EEA semi-arid zones may be solved by looking for a more "realistic" agreement between demands and exploitation.
The agricultural sector consumes an extremely high percentage of water resources in these zones (see Figure 3.1-1), particularly when quantified as the relationship between the water abstracted for agricultural uses and the country surface area). Most crops only require water for a limited number of months, but these do not coincide with the seasonal distribution of precipitation and runoff. The natural water supply ceases in summer and autumn, and this has a negative effect on crops, which means that water courses have to be controlled by the construction of reservoirs.
Figure -1 Freshwater resources and abstractions in semi-arid countries
The differences in yield from year to year, makes it necessary to construct large dams for the purpose of water storage in wet years, so that demands can be met in dry years. Sedimentation or eutrophication are common problems in many of those reservoirs. There are other problems associated to them, such as the reduction of the biodiversity downstream and the interruption of fish migration. The establishment of a common policy for ecological and minima flows to be guaranteed by reservoir management is a key question in order to maintain the aquatic biodiversity.
The conjunctive use of surface water and groundwater is a main factor in order to meet demands in semi-arid zones. However, excessive use of the aquifers can cause over-exploitation problems with the consequent negative environmental, social and economic impact. In coastal areas, excessive water extraction causes the water table to drop, giving rise to the sea water intrusion phenomenon. To consider any options which help save water must be one of the first steps in water management in semi-arid areas. The use of technologies that improve efficiency (e.g. drip irrigation in agriculture) is an example of saving water. Another possibility involves carrying out research into new crop varieties with lower water requirements being temporally coincident with the wet season. These new crops should be consistent with the European Joint Agricultural Policy, that hopefully will allocate each type of crop to the appropriate type of climate (INAG, 1995a). Sometimes, however neither the saving measures nor suitable management of water resources of the basins are sufficient. It would be necessary to transfer water from basins with a surplus to those with a deficit by means of works such as aqueducts.
In urban areas, the main problems concern water supply, wastewater treatment and flood prevention. The water supply can be guaranteed by storage in reservoirs, or by pumping groundwater when possible. Wastewater must be treated before being returned to a river whose discharge is very low either all year round or during the dry season. Once it has been treated it can be used to recharge aquifers, by percolation from shallow pools or injection wells, but such practice is not common in semi-arid areas. After being properly treated, wastewater can also be used for irrigation purposes, a practice which has been used recently in Greece. Floods bring about a high element of risk, especially in the case of steeply sloping basins, because of the high velocity that the water reaches. The transport of considerable amounts of sediment is an additional danger. The strategy for dealing with floods is based on the application of structural measures (reservoir construction, channelling, dikes, etc.) and non-structural measures, (flood warning systems, management of flood areas, etc.). Erosion and soil loss in the basins, is another problem closely associated with the flood phenomenon. This problem is remarkable in the Mediterranean zones, which are periodically affected by forest fires that accelerate the erosion process as a consequence of the loss of vegetable cover. The reduction of water resources coupled with the erosion problem, can lead to desertification in many areas inside the semi-arid regions, with the consequent socio-economic repercussions.
A greater in-depth review of some of the problems mentioned above is described in the following sections.