Land to water phosphorus transfer processes under climate change

Water quality in European rivers is degraded by nutrient loss to waters, and such problems can be exacerbated by climate change. Climate smart mitigation measures are needed and these require insight into the underlying processes of nutrient loss under future weather conditions. To address this, the aim of this study was to assess how a changing climate may alter phosphorus (P) mobilisation, delivery and impact in two hydrologically contrasting agricultural river catchments (ca 11 km²) in Ireland. As part of the WaterFutures project and the Agricultural Catchments Programme, The Simply P model was calibrated with 10 years of high frequency data of hydro-chemo-metrics for the two catchments. Five downscaled Global Climate Models (CNRM-CM5, EC-EARTH, HadGEM2-ES, MIROC5 and MPI-ESM-LR) were used to simulate two far-future climate scenarios, one intermediate emission pathway (RCP4.5) and one intensive emission pathway (RCP8.5). The scenarios were used to estimate P concentrations and loads for the coming century. A newly developed P Mobilisation index (ratios of concentration percentiles) and P Delivery index (ratios of mass load percentiles) was used to assess changes in P transfer for the modelled P concentrations and P loads.

In a hydrological flashy catchment, it was estimated that climate change alone may increase mean annual total P (TP) concentration from 0.120 mg/L monitored between 2010-2019 to 0.184 mg/L by 2070-2100. A corresponding increase in Delivery index by around 25% and 40% (for RCP4.5 and RCP8.5, respectively) but no change in Mobilisation index suggests that the impact is mostly due to enhanced hydrological connection and/or reduced P retention. The mean annual total reactive P (TRP) concentration was estimated to show minor decreases from 0.079 mg/L to 0.075 mg/L. A corresponding decrease in the Mobilisation index by around 5% and 10% (for RCP4.5 and RCP8.5, respectively) but an increase in Delivery index by 25% and 40% suggests a possible decrease in soil P detachment and/or solubilisation, limiting the increased delivery potential. The same analysis on data from a groundwater-fed catchment suggests that climate induced changes in TP and TRP concentrations were mostly related to delivery processes for TP.

The underlying processes for P losses associated with climate change are likely to be different for TP and TRP and for catchments with different hydrological controls. Such information helps to target more resilient land use mitigation methods and further design these for scenarios of future weather conditions and land use.