SEPARATING PREFERENTIAL AND MATRIX FLOWS USING SUBSURFACE TILE FLOW DATA

Preferential flow is primary mechanism for faster movement of agricult ural chemicals to groundwater. Subsurface tile flow data were used to quantify the contributions of preferential flow and matrix flow at the field scale over rime. Tile flow hydrographs were constructed using h ourly tile flow data related to rainfall events that were equal to or greater than 25.4 mm. A hydrograph separation technique was applied to separate preferential and matrix flow components of the subsurface ti le flows. On the average, preferential flow was found to contribute ab out 13% of the total subsurface tile outflow for all the rain events, and an annual contribution of 10-20% is reasonable approximation. Howe ver, considerable spatial and temporal variability was observed, even among contiguous plots. Preferential flow for some rain storms was Fou nd to be as high as 60% of the total subsurface tile flow. This study also indicated that for storms greater than or equal to 25.4 mm, the s ubsurface tile outflow, on the average, was about 16% of the total rai n water. However, it was observed that sc?me of the subsurface tiles c ould drain as much as 80% of the rainfall depending upon the initial w ater table and soil moisture conditions.