Stable water isotopes show Irish river response to droughts and floods

Climate change is projected to increase the incidence of precipitation extremes around the world and will affect the river systems that populations rely on for drinking water, energy, and additional resources. Across the Republic of Ireland, regular year round effective rainfall helps stabilize river flows, which act as a key source of drinking water. Projected increases in potable water demand, coupled with more seasonal precipitation patterns, suggest that water management strategies need to be reevaluated. Identifying those catchments most at risk from these changes is a fundamental element in this evaluation. We used stable isotopes of oxygen (δ¹⁸O) and hydrogen (δ²H) to investigate hydrologic changes from river headwaters to mouth in drought and flood conditions for two major Irish catchments: the Corrib and Shannon. Both river systems include large lakes and flow‒controlling mechanisms that moderate discharge. We collected samples from both rivers and their major tributaries during seven sampling campaigns from 2018–2020. We used δ¹⁸O and δ²H to calculate the evaporative fraction during the summer 2018 drought and as hydrologic tracers after Hurricane Lorenzo in 2019 and extreme winter floods in 2020. Results showed that the River Corrib δ¹⁸O and δ²H values were the most enriched in summer 2018. The River Corrib isotopic signature, coupled with low water levels in Lough Corrib, suggested that isotopic enrichment was due to evaporation. We also calculated the greatest evaporative fraction for the River Corrib, which was supported by δ¹⁸O, δ²H, and lake level data. Conversely, lakes in the Shannon catchment seemed to promote a homogenized river δ¹⁸O and δ²H signatures in drought conditions. After Hurricane Lorenzo and extreme winter flooding, consistent δ¹⁸O and δ²H data reflected a depleted rainfall signature at most sample locations. However, after Hurricane Lorenzo, enriched river δ¹⁸O and δ²H values below lakes demonstrated increased storage in Shannon lakes. These data reflect the potential storage capacity and contrasting vulnerability of these Irish river systems in extreme hydrologic conditions.