Using RZWQM2-P to capture tile drainage phosphorus dynamics in Ohio

Phosphorus (P) loading from tile-drained agricultural lands is linked to water quality and aquatic ecosystem degradation. The RZWQM2-P model was developed to simulate the fate and transport of P in soil-water-plant systems, especially in tile-drained croplands. Comprehensive evaluation and application of RZWQM2-P, however, remains limited. This study evaluates RZWQM2-P in simulating P dynamics using extensive data and assesses the potential of management practices for mitigating P losses. Subsurface drainage and surface runoff flows were monitored at a tile-drained site from 2017 to 2020 in Ohio, and the water flow and P loss data were summarized on a daily basis. RZWQM2-P was calibrated and validated using those observed data and was subsequently used to assess the effectiveness of controlled drainage (CD) and winter cover crops (CC) in reducing P losses. The model satisfactorily simulated dissolved reactive P (DRP) loss from tile drainage on daily and monthly bases (Nash-Sutcliffe efficiency [NSE] = 0.50, R² = 0.52, index of agreement [IoA] = 0.84 for daily; NSE = 0.73, R² = 0.78, IoA = 0.94 for monthly) and total P (TP) loss on a monthly basis (NSE = 0.64, R² = 0.65, IoA = 0.88), but the daily TP simulation was less accurate (NSE = 0.30, R² = 0.30, IoA = 0.59). Simulations showed that winter rye CC reduced DRP by 16% and TP by 4% compared to the base scenario, whereas CD increased DRP (60%-129%) and TP (5%-17%) losses at three tested outlet elevations compared to free drainage. RZWQM2-P can capture P dynamics in tile-drained cropland and is a promising tool for effective P management.