Even with a drastic reduction of greenhouse gas emissions now, the consequences of the present trend of climate changes will continue in subsurface waters beyond the twenty-first century, because of the delayed response of all large reservoirs such as the atmosphere, oceans, ice shields and ground-water systems. All input changes in such large reservoirs will trigger a transient behaviour, as is well known in hydrogeology from desertification processes and the change of hydrogeologic systems from exorheic to endorheic.

Every assessment of future strategies of ground-water management in a world of climate changes has to consider the transient response of subsurface systems, which leads to an over- or under-estimate of the groundwater yield. This is specially valid for fragile ecosystems such as dry (arid, semi-arid) and cold lands (permafrost) and should also be debated in connection with deep groundwater extraction everywhere (Ghergut et al. 2001).

Since emerging groundwater in dry lands is lost by unproductive evaporation, on the one hand, and evaporation increases groundwater salinity, on the other hand, it is also recommended that these resources are explored and exploited in accordance with the transient outflow characteristics. In this way over-exploitation of the groundwater resource exceeding 'natural groundwater stress' can be excluded.

Hence, the re-assessment of groundwater resources with respect to climate changes is not only a question of present input/output relations but also of past, transient input responses.

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