Historical Droughts in Irish Catchments: Flow Reconstructions, Drought Characteristics and Impact Analysis

Long-term hydrometric records are required to identify and categorise drought events, their characteristics and impacts. In Ireland, a lack of long-term flow data combined with the paucity of extreme droughts in recent decades has impeded understanding of drought and its impacts. To address this, 250 years of monthly flow reconstructions were generated for 51 river catchments across the island of Ireland for the 1766-2016 period, by employing gridded precipitation and temperature data and an ensemble of hydrological models. Catchment-based standardised precipitation and streamflow indices (SPI; SSI) were derived from reconstructions and analysed to identify the most extreme drought events, spatial and temporal differences in drought characteristics (severity, duration, accumulated and mean deficits) and drought propagation dynamics. Subsequently, a novel database of land-based and hydrological-based drought impact articles (1900-2016) was linked to the derived indices using logistic regression models, finding distinctive regional drought impact likelihoods. Overall results show that Ireland is prone to drought, confirming that recent decades, in which positive North Atlantic Oscillation conditions and related wetter weather have been more prevalent, are unrepresentative of the long-term record. Three regionally distinct catchment groupings were identified with hydrological drought more frequent in the wetter northwest as opposed to the drier east/southeast, a consequence of the flashier features of catchments in that region. Once established however, drought in the south-east tends to last longer and results in greater accumulated deficits due to the larger areas and greater groundwater storage of catchments in the region. Southwestern catchments showed characteristics intermediate of the other two regions. The most extreme island-wide droughts occurred in 1803-1806, 1854-1859, 1933-1935, 1944-1945, 1953-1954 and 1975-1977. Trend assessment found a tendency towards shorter more intense droughts over time. Finally, linking drought impacts (via newspaper articles) and indices found that SPI-3 and SSI-2 perform best for drought impact analyses and that northwestern catchments are more vulnerable to drought impacts, particularly hydrological-based impacts, with increases in the likelihood of reported impacts having occurred in the region post-1961, in contrast to reductions elsewhere. The study findings considerably advance understanding of drought, its dynamics and impacts at the catchment level in Ireland and, together with the flow reconstructions, will be of interest to hydrologists and water managers in Ireland and further afield