There is very little data preserved for precipitation in Portugal during the 19th century but the Civil Protection Service of Amadora (a municipality in the north Lisbon region and distanced by 10 km. from the Atlantic) has kept accurate meteorological records after January 1916. Using modern computer methodology, Professor Nuno Leitão of the Interdisciplinary Centre for Social Sciences painstakingly interpolated these with the North Atlantic Oscillation index for the period 1836 to December 2015 provided by US National Oceanic Atlantic and Atmospheric Administration . Irregularities were evened out and other data introduced from which a table resulted to show the a yearly rainfall from 1831 to 2020.
The yearly average for 186 hydrological years proved to be 719 mm. The five wettest years were 1867, 1880 1968 1995 and 1997 with ranges from 1046 mm. tp 1216 mm. The driest years were 1943, 1944, 1982, 2004 and 2018 with ranges from 263 to 407 mm. Overall the tendency was for less annual rainfall towards the end of the term of 186 years and for the wettest months to move from the Autumn to Winter. For the period 1991 to 2020 average yearly precipitation fell to 678 mm. with the wettest month always being January with a maximum of 352 mm.
Putting to one side the debate concerning the influence on climate by humankind´s polluting activities, one can easily deduce that the third decade of the 21st century will see much less precipitation and that this will mostly be confined to the Winter months. To combat this forecast of increasing drought it behoves the government to introduce now stern measures to (1) conserve rainwater in reservoirs, (2) provide desalination plants for supply to commercial entities and (3) ensure that waste water from both domestic and commercial premises is recycled before being used for irrigation.
Such measures are going to cause grief and consternation especially to the privileged classes but there must be imposed a restrictive use of potable water which must be available cheaply to a pre-determined level for the occupants of each household ; beyond this basic need the costs per person should rise sharply. The use of potable water for luxuries such as swimming pools should be highly taxed and eventually cease when recycled and desalinated supplies become available
It would be timely for our hydrological engineers to study the precautions set in place by the state of Israel which shares with Portugal many geographical similarities. A recently published study with video supported projections prints a vivid scenario of the dual potentially devastating hazards of rising sea levels and falling precipitation which will cause fresh water to become a very expensive and coveted commodity. Perhaps it would be appropriate on Sunday 16 June, (the 7th and final day of Sukkot) that we should join our Sephardic Jewish ancestors in their prayer Tikkun Hageshem which calls upon their God to remember the righteousness of His creation and the great gift of flowing water .
by email, Roberto
EDITOR, The Oscillation of Precipitation and Drought
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