Assessments of global water resources are uncertain. This refers to all data, but in particular to groundwater and water in permafrost areas. Vast volumes of water contained deeply in the Earth crust are not included in typical assessments. However, even the continental runoff cannot be reliably estimated. The differences between water resources assessments for continents done by different experts reach several tens of percent, being particularly strong for South America (highest estimates are of the order of 150% of lowest estimates). Older assessments are still quoted in recent works, because these stem from the time when hydrological observation networks were better developed and contained many more stations than today. There is an acute lack of newer data from several countries, due to the growing inadequacy of declining hydrological networks. Hydrological data collection and analysis worldwide are not keeping pace with the actual water development and management needs, despite the increasing demands for water and the growing water stress, calling for improvement in efficiency of water management. Hence, according to some experts, we are guessing rather than assessing the water resources.
More than half of the river gauges of the world are located in Europe and North America, where observation series are longest. In contrast, water resources estimates are most erroneous in a number of regions of Africa and Asia, where hydrological networks are weakly developed, and the situation is not improving.
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