METHOD TO ESTIMATE WATER DIFFUSIVITY AND HYDRAULIC CONDUCTIVITY OF MODERATELY DRY SOIL

Water diffusivity and unsaturated hydraulic conductivity of moderately dry soils are seldom measured directly, yet are often needed by numer ical models to simulate how processes. We propose a relatively simple method to obtain water diffusivity or hydraulic conductivity in modera tely 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(theta) = -d theta/dT where S-W is the thermogradient coefficient, D-T is the thermal water diffusivity, D(theta) is the isothermal wate r diffusivity, and d theta/dT is the rate of change of water content w ith temperature. The relative constancy of D-T allows D(theta) to be e stimated as D(theta) = -D-T(dT/d theta) where dT/d theta is measured a t various locations along the soil column. Using water retention data, the steady water content profile can be transformed into a suction he ad profile and the above procedures for estimating D(theta) can then b e applied to estimate hydraulic conductivity, K(theta). Soil columns r anging in length from 5 to 10 cm, exposed to thermal gradients of 1 de grees C/cm for periods as short as 7 d, at initial suction values rang ing 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.