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Soil Physics

Measured and Predicted Solute Transport in a Tile Drained Field

^a International Water Management Institute IWMI c/o ICRISAT Patancheru, AP 502 324 India
^b Dep. of Agronomy, Iowa State Univ., Ames, IA 50011
^c USDA-ARS, National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011
^d Dep. of Agricultural and Biosystems Engineering, Iowa State Univ., Ames, IA 50011

^* Corresponding author (rhorton{at}iastate.edu)

Most solute transport measurement techniques are tedious and require extensive soil excavation. A field experiment was conducted to evaluate whether surface transport properties determined by a nondestructive time domain reflectometry (TDR) technique could be used to accurately predict tile flux concentrations. A 14 by 14 m field plot selected above a 1.1-m deep tile drain was studied. Low electrical conductivity (EC) water was sprinkled on the plot surface, and after reaching a steady-state condition, a pulse of calcium chloride solution (16.3 cm) with an EC of 23 dS m^–1 was applied through the same sprinklers. Time domain reflectometry equipment was used to record the change in EC of surface ( top 2 cm) soil at 45 locations. The EC of the tile drainage flow was measured continuously with an EC probe. The surface convective lognormal transfer (CLT) function parameters, log mean irrigation depth, µ_I, and its standard deviation, _I, were found to be 3.44 and 0.94 [ln(cm)], respectively, for a reference depth of 110 cm. These surface parameters were used in a one-dimensional (1-D) CLT model and in a two-dimensional (2-D) model (CLT vertical function combined with exponential horizontal transfer function) to predict the tile flux concentrations. The 1-D CLT model predicted an earlier arrival time of chemicals to the tile drain than observed values. The root mean square error, RMSE, of the 1-D CLT predictions was 0.123, and the coefficient of efficiency, E, was –0.47. The 2-D model predictions of tile flux concentrations were similar to the observed values. The root mean squared errors (RMSE) and E were 0.023 and 0.94, respectively. The findings suggest that in this field soil, the surface solute transport properties determined by TDR could be combined with a 2-D transport model to make reasonable predictions of tile flux concentrations.

Abbreviations: 1-D, one dimensional • 2-D, two-dimensional • CDE, convective-dispersion equation • CLT, convective lognormal transfer • E, efficiency • EC, electrical conductivity • MIM, mobile-immobile • pdf, probability distribution function • RMSE, root mean squared error • TDR, time domain reflectometry

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