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Steady Flows of Water and Salt in Uniform Soil Profiles with Plant Roots¹

ABSTRACT

Steady upward and downward flows of water in the presence of uptake of water by plant roots are analyzed. Qualitative features of possible pressure head, total head, and water content profiles for steady conditions are discussed. Specific calculations are based upon an exponential increase of the hydraulic conductivity with the pressure head and an exponential decrease of the rate of uptake with depth. For downward flows, the distribution of the matric flux potential can be expressed as a function of the rate of infiltration, the leaching fraction, and two parameters characterizing the soil and the uptake distribution, respectively. Although the water content in the root zone may be nearly uniform, the matric component of the flux may be significant. For upward flows, the distribution of the matric flux potential can be expressed as a function of the rate of infiltration, the rate of transpiration, the hydraulic conductivity of the saturated soil and the same two parameters characterizing the soil and the uptake distribution. The depth at which the flux vanishes, the minimum value of the matric flux potential for a given depth of the water table, and the maximum depth of the water table are evaluated. Finally, neglecting the effects of salt precipitation and dissolution and of diffusion and dispersion, the distribution of salts associated with the steady downward flows is evaluated.

¹ Contribution from the Western Region. ARS, USDA, P.O. Box 672, Riverside Calif. 92502.

² Soil Scientist at the U.S. Salinity Laboratory, Riverside, Calif.

Received for publication March 18, 1974. Accepted for publication July 3, 1974.

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