Rogation ceremonies: key to understand past drought variability in northeastern Spain since 1650

In the northeast of the Iberian Peninsula, drought recurrence, intensity, persistence and spatial variability have been mainly studied by using instrumental data covering the past ca. 60 years. Fewer studies have reconstructed drought occurrence and variability for the preinstrumental period using documentary evidence and natural proxies. In this study, we compiled a unique dataset of rogation ceremonies, religious acts to ask god for rain, from 13 cities in the northeast of Spain and investigated the annual drought variability from 1650 to 1899 AD. We converted the qualitative information into three regionally different coherent areas (Mediterranean, Ebro Valley and Mountain) with quantitative, annually resolved (December to August) drought indices according to the type of religious act. We found common periods with prolonged droughts (during the mid and late 18th century) and extreme drought years (1775, 1798, 1753, 1691 and 1817) associated with more blocking situations. A superposed epoch analysis (SEA) was performed to test the regional hydroclimatic responses after major tropical volcanic eruptions. The SEA shows a significant decrease in drought events one year after the volcanic events, which might be explained by the decrease in evapotranspiration due to decreases in surface temperatures and, consequently, the higher water availability that increases soil moisture. In addition, we discovered a common and significant drought response two years after the Tambora volcanic eruption in the three regional drought indices. Documented information on rogations thus contains important independent information to reconstruct extreme drought events for specific seasons in areas and periods for which instrumental information and other proxies are scarce.