The most obvious and the most common solution to that problem consists of storing surface water behind dams, but storage of water in the ground may be a valuable alternative to surface storage systems, although not always systematically considered when planning water development. Yet surface reservoirs have many drawbacks, especially.

  • evaporation:large open water areas are exposed, during several months and even years, to high evaporation rates leading to water losses sometimes exceeding 20 percent of the average annual runoff. Losses may be even higher when the width of the impounded valley is considerable, and induces a larger open water area.
  • Sedimentation:soil erosion in the catchment results in siltation in the surface reservoirs and in the equivalent reduction of the storage capacity. The soil vulnerability to erosion, and therefore the importance of the siltation problems in surface reservoirs, grows as the vegetation cover shrinks, so the more arid the climate, the less the vegetation cover, the higher the probability of sediment accumulation in the surface reservoirs. Draining part of the mud from the reservoirs is occasionally possible through specially designed pipes placed at the bottom of the dam, but each operation is water consuming (to flush the mud) and may be detrimental to downstream environment.