Drought events in Southern Portugal from the 12th to the 19th centuries: integrated research from descriptive sources

Historical research of extreme climate events, such as droughts, that occurred before the quantitative period initiated with the installation of weather station networks, is dependent on the availability of descriptive sources such as newspapers, archives, memories or other written reports. This study is based on a thorough survey and analysis of available sources of information, reporting drought events and impacts occurring in the southern Portuguese region of Algarve since medieval times. Most sources were compiled from published references, coupled with a thorough research of periodic newspapers published during the 19th century, which significantly increased the volume of available information. Results were validated by comparison whenever multiple sources were available, including early instrumental records and a study using a similar methodology for the neighbouring region of Inner Lower Alentejo. This study concludes that frequent drought events occurred in the Algarve during the period covered. The same applies to the Alentejo, but many of the identified periods differed, exposing different climatic genesis of most events between the two neighbouring regions. Comparison with instrumental records show that much of the data collected in 19th century newspapers may refer to dry spells rather than long-term droughts. Finally, the study allows to conclude that descriptive sources have, in general, a large informative potential where no quantitative sources are available, but lack complementary methods both for validating collected data, and for reducing subjective and arbitrary criteria that lead perception-based description of such extreme events.