The potable water supply concept reflects relationships between distribution of available water resources that determine hydrogeological attributes of the region, and distribution of water service areas. The distribution of the water resources is often uneven, which is one of reasons of development of extensive long distance water supply systems. The long distance water supply systems connect deficit areas with surplus water resources areas. For example, the yield of large capacity wells in Danube region near Bratislava is several times higher than the real demand in this region. In this case, water from the wells is transferred by multiple pumping over a long distance (tens or hundreds of kilometres) to the areas with shortage of sources. The Little Carpathians foothills, where local water sources (springs) do not give a sufficient yield all year long. Most of these springs offer high-quality water, but fluctuating yields of the sources and do not guarantee continuous water supply. The sources are important for its quality and yield as well as for their elevation. The elevation potential allows gravity water supply without a need for pumping. However, the water supply systems do not even use average yields of the springs. The balance capacity of the springs is technically designed for minimum yields. Therefore, several reasonable questions arise: Is it efficient to pump and transport water over a long distance even in the time of maximum yields of the gravity springs? How can we determine efficiency rate of use of such yield in its extreme status? The answers can be found in the analysis of the necessary technical measures and investments, and calculation of operating costs of a system dependent on variable yield of gravity springs.

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