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Soil Water Evaporation, Isothermal Diffusion, and Heat and Water Transfer¹

ABSTRACT

Temperature and water distribution data were taken for a 9- by 11- by 20-cm soil column where wind was the evaporation agent and for a similar soil column where radiation was the evaporation agent. A Webster silty clay loam soil characterized by a soil-water retention curve and an inflow diffusivity versus water content curve was used in the experiments. The experimental results were compared with results calculated from a solution of the diffusion equation normally used to describe isothermal movement of soil water and from a heat and mass transfer solution which accounted for temperature effects. It was found that the isothermal diffusion equation would describe the cumulative evaporation for the wind and for the radiation treatments. The isothermal diffusion equation did not predict the formation of a surface layer of dry soil, and thus, did not describe the water distributions for the radiation treatment for times greater than 20 hours or for the wind treatment for times greater than 200 hours. A heat and mass transfer equation did predict the development of a dry surface layer and did describe the water distribution where temperature gradients were important.

¹ Journal Paper no. J-6272 of the Iowa Agr. & Home Econ. Exp. Stat., Ames, Iowa. This research was supported by U. S. Atomic Energy Commission Contract At(11-1)-1269, Office of Water Resources project A-003-IA, and the Iowa Agr. & Home Econ. Exp. Sta. projects 998, 1003, 1235, 1276, and 1653.

² Formerly Research Associate, Iowa State University, now Assistant Professor of Soil Physics, Cornell University, Ithaca, New York; Professor of Soils and Physics; and Professor of Agricultural Climatology, respectively.

Received for publication June 9, 1969. Accepted for publication November 18, 1969.