Anion transport in a piedmont ultisol .1. Field-scale parameters

Tile drains have been suggested as a method of measuring held-scale transport parameters, but lateral how to the drains in the saturated zone can introduce error in the measurement of unsaturated zone parameters. Our objective was to use a two-layer transfer function model to determine held-scale unsaturated zone transport parameters from tile drain breakthrough curves measured in two 12.5 by 30.5 m plots. Plots were irrigated to steady-state tile drain flow, Cl was added as a tracer, and irrigation was continued for up to 600 h, The first layer of the transfer function model consisted of the unsaturated zone and was described by the convection-dispersion equation (CDE) probability density function (pdf). The second layer consisted of the saturated zone and was described by the gamma distribution pdf. The CDE parameters were determined from a fit of the two layer model to the drainage pdf in each tile drain. Transport parameters were similar for the two plots. Mean field-scale dispersivity (lambda) for the unsaturated zone (0-20 cm) was 5.3 cm for Plot 1 and 3.4 cm for Plot 2. This was much less than lambda calculated for the overall system by moment analysis (lambda = 23.7 and 19.5 cm in Plot 1 and 2, respectively) suggesting that most of the dispersion of the tile drain breakthrough curves was due to lateral how in the saturated zone. This illustrates that tile drain breakthrough curves can be used to determine field-scale unsaturated zone transport parameters provided a model is used that accounts for two-dimensional flow in the saturated zone.