Adam Scott Ward , Molly Cain, Praveen Kumar, Dongkook Woo, Laura Keefer

Threshold changes in rainfall-runoff generation commonly represent shifts in runoff mechanisms and hydrologic connectivity controlling water and solute transport and transformation. In watersheds with limited human influence, threshold runoff responses reflect interaction between precipitation event and antecedent soil moisture. Similar analyses are lacking in intensively managed landscapes where installation of subsurface drainage tiles has altered connectivity between the land surface, groundwater, and streams, and where application of fertilizer has created significant stores of subsurface nitrogen. In this study, we identify threshold patterns of tile-runoff generation for a drained agricultural field in Illinois and evaluate how antecedent conditions—including shallow soil moisture, groundwater table depth, and the presence or absence of crops—control tile response. We relate tile-runoff thresholds to patterns of event nitrate load observed across multiple storm events and evaluate how antecedent conditions control within-event nitrate concentration-discharge relationships. Our results demonstrate that an event tile-runoff threshold emerges relative to the sum of gross precipitation and indices of antecedent shallow soil moisture and antecedent below-tile groundwater moisture deficit, indicating that both shallow soil and below-tile storages must be filled to generate significant runoff. In turn, event nitrate load shows a linear dependence on runoff for most time periods, suggesting that subsurface nitrate export and storage can be estimated using runoff threshold relationships and long-term average nitrate concentrations. Finally, within-event nitrate concentration-discharge relationships are controlled by event size and the antecedent tile flow state because these factors dictate the sequence of flow path activation and tile connectivity over a storm event.