A physically-based flow model has been developed to specifically address the influence of irregular tile networks on Midwestern watershed hydrology. Several issues were identified as pertinent to this specific task: solving the problem of flow to a single tile drain, addressing irregular tile networks at the watershed scale and integrating the dominant hydrologic processes. An analytical solution to the problem of subsurface flow to a single tile was proposed and evaluated. This solution was incorporated into a description of infiltration which accounts for variable moisture conditions and complex precipitation patterns. Together, the tile flow solution and the comprehensive infiltration treatment constitute the subsurface flow component of the overall watershed-scale model. These account for the spatial conditions encountered at the field level, particularly the presence of tile lines which drain the soil profile and affect the subsurface water balance. The tile system and the stream channels in the watershed are modeled as one integrated directed network. The comprehensive network of links and nodes routes water that is drained by individual tile lines to the watershed outlet, yielding transient responses to precipitation along every node in the network. A grid-based description of the entire watershed served as a framework to integrate the subsurface module with the comprehensive directed network. The model was able to overcome the need to prescribe a spacing parameter for tile drainage and, therefore, is applicable to irregular tile drainage systems. When applied to a watershed in east-central Illinois (Upper Little Vermilion River watershed), the model was able to simulate observed trends, such as the minimal surface runoff and increased infiltration.
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