Antecedent Conditions Control Thresholds of Tile‐Runoff Generation and Nitrogen Export in Intensively Managed Landscapes. Issue 2 (7th February 2022)

• Record Type: Journal Article
• Authors: Cain, Molly R.; Woo, Dong Kook; Kumar, Praveen; Keefer, Laura; Ward, Adam S.
• Is Part Of: Water resources research. Volume 58:Issue 2(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-02-07
• Subjects: subsurface drainage -- storm response -- nitrate -- agricultural water management -- concentration‐discharge relationship -- hysteresis; Hydrology -- Periodicals; 333.91
• DOI: 10.1029/2021WR030507
• Languages: English
• ISSNs: 0043-1397

Abstract

Abstract: 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. Plain Language Summary: Improving nutrient management in intensively managed landscapes requires understanding of how human alterations for agriculture have influenced nutrient transport from and storage within the landscape. In addition to creating large subsurface stores of nitrogen through fertilizer application, humans have altered the drainage structure of intensively managed landscapes by installing subsurface drainage (commonly "tiles") to maintain optimal moisture conditions for crops. Although highly engineered systems, it is unclear how tiles influence the timing and magnitude of water and nutrient export from these landscapes. Here, we identify how prestorm wetness conditions control rapid, nonlinear changes in tile flow (thresholds). We find that a tile flow initiation threshold results from the sequential filling of first a depleted shallow soil storage and then deeper below‐tile groundwater storage. Further, nitrate export reflects tile‐runoff thresholds, indicating that the factors controlling tile‐runoff are also primary controls on tile nitrate export. Key Points: A tile‐runoff threshold emerges as a function of both above‐tile and below‐tile antecedent storage Total storm event nitrate load is primarily controlled by the same factors that dictate event tile‐runoff Within‐event nitrate concentration‐discharge relationships reflect a threshold of soil water mobilization