Climate change in the Middle East area including Jordan has started to be reflected in decreasing precipitation and increasing temperatures with their impacts on the availability of surface and groundwater. This article aims to evaluate the impacts of decreasing or increasing precipitation by 10% and 20% on the quantities of flood runoff based on recorded precipitation and runoffs of catchments during the past 60 to 70 years of observation, during which the precipitation in individual or a few years increased or decreased by tens of percentages relative to the long-term average precipitation. The results of quantification show that in Jordan as a whole, decreasing precipitation by 10% and 20% has historically (during the recording period) resulted in reductions in flood flows by 26.2% and 52.8% and that increasing precipitation by 10% and 20% has resulted in increases in flood flows by 26.4% and 56.5% respectively. These results look somehow paradox, because the general perception is that flood runoff changes in the same percentage like precipitation although flood flow is not linearly correlated with precipitation but exponentially. Decreasing precipitation in the water-scarce stressed country, Jordan due to climatic changes, will have strong implications on rain-fed and irrigated agriculture and on household water supplies with very severe socio-economic percussions expressed in increasing unemployment and poverty which may lead to social and political unrest. Therefore, proactive measures have to be implemented before disasters hit. Such measures are limited in Jordan to seawater desalination, intensified water harvesting and improved water use efficiency in agriculture.
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