3.2. Water surface exploitation

The river yields in semi-arid zones show major fluctuations, both on a seasonal and annual basis. Dam building, or other hydraulic works, is the way usually to control water quantity, as far as meeting demands is concerned.

Table 3.2-1 shows the number of large dams in the countries with semi-arid regions (ETC/IW, 1995):

Table 3.2.-1 Large dams in countries with semi-arid regions.


Surface (km2)

Number of large dams

km2/large dam


















There are 847 large dams in Spain (ETC/IW, 1995). They have been built in order to provide an availability of 40% of its natural water resources. In contrast, most of the member countries of the European Union can make use of that same percentage of its resources, which are in themselves abundant, in a natural way. This means that they do not need to invest so heavily in the construction of such public works. More than 80% of the resources destined for drinking water and irrigation purposes come from works funded by governments. (Gil, 1995).

Storage and regulation by reservoirs do not always solve the problem of water scarcity in areas where dry periods are particularly damaging to the natural life and also to the human daily life. In those cases, water transfer from areas with a surplus to those with a deficit must be undertaken. Those sort of works involves political, social, economic and environmental considerations which need to be included as elements of resource planning to which governments have to resort occasionally. This happens when the water demands in semi-arid zones are to be met and development continued or maintained. Social groups living in wet areas find it difficult to comprehend this type of operation, and all the studies, works and investments involved.

When facing the problems that water scarcity causes to semi-arid regions in the day-to-day life, water transfer could prove to be, in some cases, an effective and permanent solution in the short and medium term. Such is the case of many of the existing hydrological plans for the developed or developing semi-arid regions of Europe. These plans include either water transfer or salt water treatment as solutions. However, it should be borne in mind that on many occasions, drought periods affect large parts of Europe at the same time, which means that in a natural scarcity situation inter-basin transfers can lose their functionality and strategic value.

Water transfers are not always related to small-scale transfers in which water is conveyed from one small sub-basin to an adjacent one. They also may involve water transfers between large basins, from wet areas to semi-arid areas or those having water scarcity problems. Both small-scale and large-scale transfers mean not only a loss of water in the basins that provide the water, but also social and economical costs.

The economic and social compensation policies for the water providing basin and the intermediate ones which transport and control the transferred resources, usually prove to be a further complication to an operation that attempts to alleviate the water deficiencies in the semi-arid areas.

Monitoring of quantity and quality is generally strictly controlled, even on a political level, with a view to determining whether the transfer is carried out on a seasonal or annual basis. In addition to that, a prediction is also required for controlling the duality between future availability and demands, particularly in summer months. .An accurate short-term prediction of future resources identifies the best time at which water should be transferred and, therefore, avoids possible imbalances arising from water needs prior to the maximum demand in summer.

Although interbasin transfers constitute an element of water resources planning, there are also several disadvantages inherent to such transfers, such as major investments in construction works (aqueducts), leak losses and evaporation losses during the transfers and negative environmental impact. One undesirable situation from the ecological point of view could be the intrusion of foreign species in ecologically delicate equilibria such as those within semi-arid areas, as a result of water transfer between the north and south of Europe (INAG, 1995a).

The corrective measures contemplated in Spain to re-address the existing basin imbalances, as included in the Anteproyecto de Ley del Plan Hidrológico Nacional (National Hydrological Plan Bill (MOPTMA, 1993), involve plans to set up an Integrated System for National Hydraulic Balance (SIEHNA), which will consist of works that will make it possible to interconnect basins and to transfer resources from one to another. The envisaged redistribution of resources is as follows: at present (550*106 m3), in 2002 (2,400*106 m3) and by the year 2012 (3,800*106 m3) (Baltanás, 1985). The only current activity of this kind taking place involves the transfer of water from the Tagus Basin to the Segura Basin (Acueducto Tajo-Segura) -which was designed to transport 33 m3/sec., equivalent to a continuous flow regime of 1,000*106 m3/year. So far an average of 320*106 m3/year has been transferred. The aqueduct for taking water from the River Ebro to supply Bilbao and the Tarragona area and a further system which transports water from the River Segura to the southern reaches of the River Júcar (MOPTMA, 1993) are also operative at present.

Portugal only has small water transfers between catchments Mondego-Tejo (Alto Ceira Reservoir to Sta Luzia Reservoir) for hydropower production, Sado-Morgavel, for water supply purpose, and Guadiana-Algarve (Beliche-Odeleite system under construction).

In Sardinia there is one water transfer. It joins up the Flumendosa catchment area and the Campidano plain. A 147 km-length open channel and a 23 km-length underground tube convey the water from the Flumendosa and Mulargia reservoirs (2,5*106 m3 capacity) for domestic, agricultural, industrial and tourist uses.

In Greece, two dams (Mornos dam and Evinos dam) have been built to transfer water between basins. The first of them supply the Attiki region from the Mornos catchment area. The second one transfers water from the Evinos river to the Mornos catchment area, in order to satisfy the growing water demand of Athens (Attiki area). There is another transfer system (under construction) in the region of Acheloos which will transfer water for irrigation purposes.

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