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DIVISION S-1—SOIL PHYSICS

Determination of Preferential Flow Model Parameters

National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011

* Corresponding author (logsdon{at}nstl.gov)

Solute transport models that include a preferential flow component require many input parameters. There are well established procedures to determine micropore parameters, but procedures to determine macropore parameters are not well established. The objective of this paper was to evaluate methods to independently measure macropore parameters. The test model used was MACRO, a transient-state, two-flow domain model. The key macropore parameters in the model are saturated and boundary hydraulic conductivities (K_s and K_b), the absolute value of the boundary head between macropore and micropore domains (h_b), the exponent (n*) of the relation between K (variables are defined in the appendix) and water content (), the macropore fraction (_sma), and the half spacing (d) between equivalent parallel fractures. As an example this study used soils in the Des Moines lobe (Mollisols with textures ranging from sandy loam to silty clay). Data used to calculate model parameters included wet-end K--h and K(h), and results from image analysis. For the MACRO model, the parameters fit the equations best when h_b was assumed to be 30 mm. For the measured data with assumed h_b = 30 mm, n* had a median of 2.1 and a range from 0 to 5.2, median K_b was 15 mm h^-1 with a range from 1 to 100 mm h^-1, and the median K_s was 122 mm h^-1 with a range from 7 to 741 mm h^-1. The calculated d ranged from 1 to 847 mm, and _sma ranged from 0.001 to 0.053 m³ m^-3. Depending on the data available, the various techniques can be used to determine input parameters for preferential flow models.

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