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Method to Estimate Water Diffusivity and Hydraulic Conductivity of Moderately Dry Soil

Lab. of Experimental Hydrophysics and Physical Chemistry of Soils, Agrophysical Research Inst., Grazhdansky pr. 14, St. Petersburg, Russia 195220

G. W. Gee^*

Pacific Northwest Lab., P.O. Box 999, Richland, WA 99352

*Corresponding author (gw__gee{at}pnl.gov).

ABSTRACT

Water diffusivity and unsaturated hydraulic conductivity of moderately dry soils are seldom measured directly, yet are often needed by numerical models to simulate flow processes. We propose a relatively simple method to obtain water diffusivity or hydraulic conductivity in moderately dry soils. Steady-state profiles of water content and temperature in a closed, non-isothermal soil column are used to determine S_w = D_T/D() = – d/dT where S_w is the thermogradient coefficient, D_T is the thermal water diffusivity, D() is the isothermal water diffusivity, and d/dT is the rate of change of water content with temperature. The relative constancy of D_T allows D() to be estimated as D() = -D_T(dT/d) where dT/d is measured at various locations along the soil column. Using water retention data, the steady water content profile can be transformed into a suction head profile and the above procedures for estimating D() can then be applied to estimate hydraulic conductivity, K(). Soil columns ranging in length from 5 to 10 cm, exposed to thermal gradients of 1°C/cm for periods as short as 7 d, at initial suction values ranging from 0.1 to 1.5 MPa, can be used to estimate water diffusivity and hydraulic conductivity with uncertainties of a factor of five or less in the suction range from 0.03 to 3 MPa.

Received for publication October 11, 1993.

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