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Determining Effective Soil Water Diffusivities from One-Step Outflow Experiments¹

ABSTRACT

A method is developed whereby effective or weighted mean diffusivity, , can be estimated by using outflow data. These estimates can further be analyzed for soil water diffusivity, D(), with the help of a step-wise function and the relationship given by Crank (1956). The suggested procedure for estimating soil water diffusivity is more accurate than previous methods because no assumption of constant diffusivity either over the pressure increment or over the length of the soil core is required. Moreover the procedure does not use the early part of the outflow curve and thus reduces the effect of membrane impedance on the estimates of D().

The method was tested by simulating outflow experiments on a digital computer, and using data obtained to calculate diffusivities. Our estimates of diffusivity were in better agreement with the actual diffusivities than those obtained with the earlier "one-step" method.

¹ Contribution from the North Central Region, ARS, USDA, St. Paul, Minn. 55108, in cooperation with the Minnesota Agr. Exp. Sta., Paper no. 8530, Scientific Journal Series.

² Research Fellow, Univ. of Minn.; Senior Principal Research Scientist, CSIRO, Div. of Soils, Adelaide, Australia; Research Soil Scientist, USDA, and Professor, Univ. of Minn., St. Paul.

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