Hydrology

Increasing seasonality in hydrological regimes can be expected with decreased summer and increased winter flows likely.

Flood risk is expected to increase across Ireland while increases in the frequency of drought conditions is also expected.

Identifying a clear signal of climate change in observed precipitation and river flows is difficult because of natural variability. Observational records show that the magnitude and timing of floods are changing; however, there is uncertainty in attributing the cause of these changes, whether due to climate change, natural climate variability, data quality or internal catchment modifications (e.g. land use change, arterial drainage).
An analysis of river flows over a period of more than 50 years of data (1972-2017) indicates an increase in river flows across most of the country. An analysis of a shorter period from 1992 suggests an increase in drought conditions, particularly in the east of the country.

Rainfall and river flow projections for mid-century and end of century are uncertain, but suggest increases in flood magnitude and frequency, increases in spring and summer drought magnitude and frequency (future changes in drought duration are uncertain), and increases in river flows in winter and decreases in summer.
The response of individual catchments will be determined by individual catchment characteristics (e.g. groundwater versus surface water dominated catchments). For example, summer reductions for groundwater dominated catchments are not as severe as those projected for surface water dominated catchments.
Using impact models (conceptual runoff models), a robust signal of increasing seasonality in hydrological regimes is evident with increases in winter and spring streamflow likely and a decrease in summer. A 20% increase in the amount of water flowing through rivers are expected for the majority of catchment by mid-late century while for summer decreases of over 40% (those with little groundwater storage in particular) have been simulated for the end of the century.
Projected increases in winter flows coupled with likely increases in extreme precipitation events are likely to lead to an increased flood risk. However, catchment response time will be critical in determining the changing nature of extremes and those catchment with fast response times are likely to be most at risk.