Water flow and chemical transport in a subsurface drained watershed

Tile-drainage in the Corn Belt has a great impact on surface water quality, at all scales (field, watershed and drainage basin) especially by increasing nitrate loads in the Mississippi River. An improved understanding of flow and transport is essential to better manage these ecosystems. However, such systems are extremely difficult to characterize and model since the regulating processes are dominated by preferential flow mechanisms that exhibit great spatial and temporal variations. The objectives of the present effort are to (1) Develop experimental data set describing flow and transport in tile-drained fields suitable for model development and evaluation, (2) Evaluate observed spatial and temporal variations in flow and transport processes to identify empirical relationships between variables, (3) Characterize solute travel time distributions relative to chemical application distance from tile drains, and (4) Provide an improved understanding of relationships between tile effluent response and climatic and soil variables. To achieve these objectives, environmental monitoring (e.g., rainfall, soil moisture, evaporation, flow, effluent sampling) over an extended period was performed within four replicate (27.4 x 30.5 m) tile drained field plots designed to provide hydraulic isolation. On two of the plots four chemicals were surface applied, one to each of four 1-m wide bands which ran parallel to the drains. The spatial distributions of chemicals within the soil were characterized by coring five days and one year after application, and tile effluent samples were collected and analyzed to provide effluent time-series. Chapter 4 provides a detailed analysis of tile flow response under the influences of variable climatic and hydrologic conditions, and identifies factors that regulate preferential flows. Chapter 5 describes the water balance model development and provides an analysis for each of 15 drainage cycles, with a clear demarcation identified between summer and winter cycle responses. A precise water balance was calculated which included development of a new method for modeling bare soil evaporation during repeated wetting and drying cycles. This method performed well in comparison with the HYDRUS numerical model. Chapter 6 evaluates the chemical transport processes with a special focus on preferential flow and transport, providing insight into associated governing factors. Finally, the last chapter provides conclusions and suggestions for future research.