Impact of saturated hydraulic conductivity on the prediction of tile flow
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Preferential flow through macropores and other structural voids in field soils most often occurs at or near saturation. Our earlier research revealed significant differences in the value of the saturated hydraulic conductivity (K(s)) of a glacial till soil in central Iowa when obtained with five different measurement techniques. The five techniques included one laboratory constant-head permeameter method and four in situ methods: disc permeameter, Guelph permeameter, velocity permeameter, and double-tube permeameter. Differences in measured K(s) values were attributed to differences in sample size, the existence or absence of open-ended macropores, and measurement principles. In this study, we used the different K(s) estimates in a two-dimensional numerical model, CHAIN 2D, to predict water flow into a subsurface tile drain in the same field. Comparisons between predicted and observed tile flows were made during four crop growing seasons. Preferential flow observed in the tile drain during large storm events was predicted best by the model when using K(s) values measured with the disc permeameter method, which least disturbed the boundary conditions of the flow field and better accounted for the macropore structures of the field soil. Quantitative and qualitative findings suggest that the disc permeameter was best suited for the field site.
