Accelerated climate-induced risks of nitrogen-loss from agricultural catchments: Empirical modelling of extreme weather events using climate change scenarios.

Nitrate pollution from agriculture imposes a major threat to global aquatic ecosystems globally, and its impacts are expected to intensify as climate change alters weather patterns, catchment hydrology, and soil processes. Understanding the timing and extent of extreme-weather-events and their impacts on nutrient losses is essential in developing climate-smart adaptation/mitigation measures, in order to avoid further increases in nitrate pollution in receiving water bodies. This study applies an empirical modelling (EM) approach to 15 years (2010-2024) of high-temporal resolution weather and water quality data from six hydrologically-diverse agricultural catchments in Ireland to i)identify climatic conditions associated with increased Nitrogen (N) losses (expressed as NO3N), and ii) estimate the future occurrence of similar N-loss events using climate change projections under different emissions scenarios. Climate projections were derived from an ensemble of models forced by two greenhouse gas concentration pathways: RCP 4.5 (moderate emissions) and RCP 8.5 (high emissions), for three future time periods. Results show a significant increase in both the duration of warm/dry periods, and the annual frequency and intensity of wet/very wet days, particularly under RCP8.5 by the end of the century. The EM identified key temperature and precipitation indices triggering N losses: average air temperature>15°C over 5 consecutive days explained up to 53% of observed N loss events, while effective rainfall (ER) exceeding five mm in one day, and on the preceding day were associated with up to 67% and 77% of loss events, respectively. Future projections indicate large increases in the frequency of conditions associated with N loss, with temperature and precipitation related triggering events projected to reach 17 to 122 and 17 to 79 events per year, respectively, under RCP 8.5 toward the end of the century. Catchment sensitivity was found to depend on catchment characteristics (i.e. drainage status, soil chemistry, farming practices, etc). These findings highlight the need to explicitly account for climate change when addressing agricultural nutrient losses and water quality, meaning that effective adaptation and mitigation measures must be climate-resilient and tailored to specific catchment typologies.