Variability of the hydrological regime in Irish Rivers under Climate Change

Climate change is altering the hydrological cycle, with implications for riverine ecosystems that are expected to vary over space and time. This study assesses projected changes in streamflow regimes for 27 Irish rivers across three future periods (2006–2035, 2036–2065, and 2066–2095) relative to a baseline period (1976–2005). Changes in hydrological behaviour are quantified using seven flow regime metrics that represent the magnitude, variability, frequency, and duration of the flow. These metrics include the 5th percentile daily streamflow (Q₅), 99th percentile daily streamflow (Q₉₉), maximum monthly mean streamflow (MaxMonthQ), Richards–Baker Flashiness Index (RBI), interdecile range ratio (QmaxIDR), low-flow event duration (LowDur), and the number of high-flow events (HighNum), capturing changes in both low- and high-flow conditions. The metrics are derived from daily streamflow simulated using the SMART model driven by five regional climate models under both RCP4.5 and RCP8.5 scenarios.

The results, based on the ensemble median, indicate a consistent decline in low flow magnitude across all catchments and periods, with larger reductions under RCP8.5. In contrast, LowDur exhibits both increases and decreases depending on catchment and period relative to the baseline. High-flow magnitude increases at all but two stations, while changes in the frequency of high-flow events (HighNum) are mixed across periods and catchments. MaxMonthQ shows an overall increasing trend. QmaxIDR increases across most rivers indicating greater flow variability. The result show a widespread moderate increase in RBI, suggesting a progressively flashier flow regimes across Irish rivers under future climate scenarios. These findings help identify rivers and future periods that are the most vulnerable to alternation of the hydrological, highlighting the increased risks of both drying and flooding and their potential effects on aquatic ecosystems under climate change.