Improving hydrological modeling to close the gap between elevated CO2 concentration and crop response: Implications for water resources and water quality

Phosphorus (P) loss from agricultural lands is a major contributor to surface water quality deterioration. Elevated atmospheric CO₂ concentrations (eCO₂) may affect P loss directly by altering hydrological processes and indirectly by influencing soil P cycling. However, the combined effects of these two mechanisms on P loss remain considerably uncertain. This study employed a more physically-based SWAT-CO₂ model, which incorporates a nonlinear g_s-CO₂ and a LAI-CO₂ function, to project P loss from corn fields in the macro-scale watershed (~500,000 km²) of the Upper Mississippi River Basin (UMRB) under eCO₂ and future climate change. Results showed that the modified SWAT-CO₂ model predicted 7.9% less total phosphorus (TP) loss than the original SWAT at 825 ppm CO₂ during the baseline (1985-2014). Future TP loss projections deviated between models compared to the baseline (30.3% increase by the modified SWAT-CO₂ vs 40.1% increase by the original SWAT under high emission scenario in the 2071-2100 period). Moreover, different forms of P loss exhibited distinct change patterns over time for both models. Soluble phosphorus (SOLP) loss increased 16.5%-58.8%, while organic phosphorus (ORGP) loss changed from -11.1% to 38.8% across all SSP scenarios. As a result, economic costs for reducing TP loss to low risk were projected to rise, with costs during the 2071-2100 period exceeding those during both 2041-2070 and the baseline periods, particularly under SSP5-8.5 scenario. These findings highlight the importance of eCO₂ in predicting P loss and underscore the need for increased economic investment to achieve P-related sustainable environmental development goals.