Assessing the impact of grazing on nutrient export from pastureland in an ungauged basin in Manitoba under climate change using an integrated modelling approach

Assessments of climate change impacts on hydrology and nutrient export from pasturelands are not readily available in Canada. The objective of this study was to implement an integrated modelling approach to predict the impact of climate change on the hydrology and nutrient fluxes on a 46.6 km2 pasture-dominated ungauged watershed in Manitoba. The Integrated Farm System Model (IFSM) was used to predict the pasture yield/residues, and manure production. To simulate the hydrological response, the Cold Regions Hydrological Model (CRHM) platform and the Hydrological Predictions for the Environment (HYPE) model were used. Predictions of nutrient exports to stream were also carried out using the HYPE model. Model assessments were conducted for the historical reference period between 2000 and 2019, and for the near (2020 – 2049) and the distant future (2050 - 2079) periods using climate projections under two Representative Concentration Pathways (RCP). Satisfactory model performance was obtained for the three individual models using available observations. Under a changing climate, pasture yields were anticipated to decrease by approximately 2% to 6.8%, with an exception that a 2% increase in pasture yield was predicted in the near future period under RCP 4.5. There was no significant changes observed in manure production. Both CRHM and HYPE models predicted a 20% and 40% reduction in stream discharge in the near future under RCP 4.5, respectively. Meanwhile, CRHM predicted an increase in stream discharge by 26% and 39% in the near future and distant future periods under RCP 8.5. In contrast, HYPE had a marginal increase in stream discharge, averaging 2% under RCP 8.5. An earlier onset of spring snowmelt was predicted by both CRHM and HYPE, ranging from 6 to 21 days in future periods compared to the reference period, implying a potentially longer growing season. Lastly, a 33% and 4.5% reduction was observed in the projected total nitrogen load in future periods under RCP 4.5 and RCP 8.5, respectively. The total phosphorus loads, on the other hand, had a significant average reduction of 50% and 35% under RCP 4.5 and RCP 8.5, respectively. It was found that the general pattern of nutrient export (N and P) closely mirrored that of the stream discharge. Overall, the predicted reduction in nutrient export implied a lower risk of eutrophication in the study area.
Multi-model simulations, Water quality assessment, Climate change