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 numerical models to simulate how 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(theta) = -d theta/dT where S-W is the thermogradient coefficient, D-T is the thermal water diffusivity, D(theta) is the isothermal water diffusivity, and d theta/dT is the rate of change of water content with temperature. The relative constancy of D-T allows D(theta) to be estimated as D(theta) = -D-T(dT/d theta) where dT/d theta 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(theta) can then be applied to estimate hydraulic conductivity, K(theta). Soil columns ranging in length from 5 to 10 cm, exposed to thermal gradients of 1 degrees 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.