Simulating water balance and nutrient losses in a pastoral catchment using the SWAT+ model: added value of isotope data

New Zealand's freshwater is under increasing pressure from intensified dairy farming. This study assesses agricultural nitrogen loads from small pastoral catchments contributing to eutrophication and periodic algae blooms in a lake. Using the Soil and Water Assessment Tool (SWAT+), we simulated hydrological processes and nutrient loads, incorporating water (δ2H, δ18O) and nitrate (δ15N, δ18O) isotope data to improve model performance. The SWAT+ model was calibrated and validated with flow and nitrate data, achieving R2 values of 0.65–0.90, Nash-Sutcliffe efficiency (NSE) of 0.46–0.82, and percent bias (PBIAS) between 10.24% and 33.48%. The study demonstrates that stable isotope insights can enhance SWAT+ modelling accuracy and highlight the need for better fertilizer management to reduce nutrient loads.