The number of phosphorus loss events will increase with variability and seasonality in far future climate scenarios.

Climate change is likely to add further pressures to water quality degradation across the globe. The development of robust climate-smart mitigation measures necessitates understanding the impact of extreme hydrological events on catchment hydrology and nutrient losses. Here, empirical modelling (EM) was applied on 14 years of sub-hourly water quality and weather data from six hydrologically diverse agricultural catchments in Ireland to understand the climatic factors that trigger an increase in phosphorus (P) losses [manifested as increase of 0.01 mg L- 1 in total phosphorus (TP) and increase of 0.005 mg L- 1 in total reactive phosphorus (TRP) over one day]. Plausible future P-loss due to extreme weather events was then modelled using climate change scenarios (from 2010 to 2100) for medium and high emission pathways, i.e. Representative concentration pathways (RCP) 4.5 and RCP8.5, respectively. EM identified three climatic conditions that trigger TP and TRP losses across all study catchments, namely: (i) cumulative effective rainfall > 5 mm over five days followed by effective rainfall > 5 mm in one day; (ii) effective rainfall > 5 mm in one day, and; (iii) effective rainfall over ten mm in one day. Together, these criteria captured up to 80% of the events across all catchments despite their different characteristics. From the projected climate change scenarios, the frequency of triggering events and their associated discharge rates, increases significantly towards the end of the century in all catchments, especially under RCP8.5. The sensitivity of catchment response to the changing weather patterns and the monthly trend of precipitation throughout the century strongly depended on catchment characteristics. The hydrologically flashy catchments in the dataset tend to be most sensitive to climate driven changes, returning the highest percentage increase of annual P-loss events in both RCPs. Considering far-future scenario, there would be 10-66% increase in the number of P-loss events under RCP4.5, and 28-67% under RCP8.5, taking into account the potential underestimation of projected precipitation probability. Assuming no changes in P-inputs in the future scenarios, the projections also indicated average discharge of up to 8.5 mm per a single triggering event that would directly contribute to increases in P-concentrations and mass loads leaving the catchments. Changes in climate are likely to compound already significant challenges in improving/ maintaining good water quality. It is therefore critical to incorporate the influences of climate change on nutrient losses in developing mitigation/adaptation strategies that are tailored to catchment-specific characteristics.