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Evaluation of the Hot Air Method for Measuring Soil Water Diffusivity¹

ABSTRACT

The Hot Air Method (HAM) is a fairly new and simple method for measuring soil water diffusivity. However, with the introduction of temperatures much higher than originally proposed by Arya et al. (1975), the validity of the underlying assumptions must be questioned. Philip's iterative solution was adapted for desorption out of a semi-infinite column at constant surface water content to evaluate effects of soil type and boundary water contents on the experimental results. Errors due to deviations from assumed isothermal conditions were evaluated with the Philip and de Vries (1957) theory for the large temperature increases that occur in soil columns during execution of HAM. Evaporation losses and water redistribution during sampling of the soil column were evaluated by simulation. Error analysis indicates that, with the present usage of air temperatures up to 250°C and sampling times of several minutes, HAM does not satisfy the prerequisite assumptions. Reports of favorable results from HAM are probably due to mutual compensation of different errors and to considerable smoothing of experimental results. In spite of its great experimental advantages one should be dissuaded from using HAM unless the experimental errors are minimized and the calculation technique is made objective and reproducible. In this paper some improvements for the experimental procedure are proposed.

¹ Contribution from the Dep. of Soil Science and Plant Nutrition, Agricultural Univ., 6703 BC De Dreijen 3, Wageningen, The Netherlands.

² Soil Scientist (presently working for the Dep. of Soil Science and Geology), Soil Scientist, Soil Scientist from Lab. of Physical Geography and Soil Science, Univ. of Amsterdam, respectively.

Received for publication January 3, 1984. Accepted for publication March 22, 1985.

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