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To assess the balance between water availability and demand, and to identify water stress areas, indicators that capture elements of the water balance are useful and simple tools. The spatial and temporal scales of application of all such indicators, as well as their methods of calculation, are crucial yet cautious interpretation should be applied to avoid biased conclusions.

Research suggests that 20-50% of the mean annual river flow in different basins needs to be allocated to freshwater-dependent ecosystems to maintain them in fair conditions (Smakhtin, Revenga, and Döll 2004). Excluding this volume from the available for exploitation water may result in changing the severity level of water scarcity conditions. Returned water (into the same hydrological unit where abstraction occurs) can also affect the water stress level of an area. Depending, of course, on the water quality and location where the return occurs (e.g. upstream enough to be exploitable by other users downstream) this volume may be an important addition to the system alleviating potential problems, and thus needs to be taken into account when calculating the overall balance between availability and demand of a region to define the relevant water scarcity. Finally, the temporal scale of analysis of water stress conditions is important, since the problem may not be apparent at an annual scale yet be acute at seasonal scale, especially during summer where the availability is usually lower and the demand picks up.

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