Understanding tile drainage’s effect on streamflow in the agricultural Midwest

Understanding the movement of water across the land surface and into channel networks is critical for accurate flood prediction, effective water resource management, and the protection of water quality. In agricultural regions such as Iowa, subsurface tile drainage systems— networks of perforated pipes installed beneath fields—play a key role in regulating soil moisture and routing water to streams, influencing both the timing and magnitude of runoff. There is a growing interest in the scientific community in understanding the effect of tile drainage on the transport of agricultural nutrients and other pollutants across a basin’s river network. This study builds upon a recent study where a tile drainage module for the WRF-Hydro model was calibrated automatically at large regional scales (e.g., the Upper Mississippi River basin) to estimate streamflow. Here we addressed the underexplored model applicability to a smaller domain: the South Fork Iowa River basin, a heavily tile-drained watershed. Here is proposed a novel approach where model calibration is performed manually, with a dual focus on reproducing observed streamflow hydrographs and aligning modeled runoff partitioning with expected hydrologic behavior. The model is also forced with more refined local tile drainage maps. Model simulations were produced at hourly temporal resolution over a 14-year period between 2008 and 2021, and include outputs of streamflow, soil moisture, and all necessary elements to calculate water budget. Results indicate that tile drainage accounted for approximately 24% of the total runoff volume over the simulation period and contributed up to 30% of total streamflow during peak flow events. These findings underscore the hydrologic significance of tile systems and highlight the need for accurate representation of their behavior in fine-scale watershed models.Improving WRF-Hydro’s ability to simulate tile drainage dynamics in small agricultural basins enhances its utility for flood forecasting and water quality assessment. This framework also lays the groundwork for evaluating the hydrologic impacts of conservation practices and informing land and water management strategies under future climate and land use scenarios in the agricultural Midwest.