Measurement of hydraulic properties and mobile water content of a field soil

The last years have seen the development of field methods that are capable of being applied to the direct measurement of simultaneous water and solute transport properties of soils. A tension disk infiltrometer was used to determine the ratio (f = theta(m)/theta) between mobile water content (theta(m)) and the total volumetric water content (theta) of a soil and to characterize the hydraulic properties. The soil is a heterogeneous stony soil of a cultivated plot, recently plowed. The hydraulic conductivity and the sorptivity were obtained using a new approach that considers the transient three-dimensional infiltration from a disk. The measurement of the mobile water content was performed by adding KCl tracer after prewetting the soil with water. Results show an important nonlinearity in both conductivity and sorptivity of the soil with applied pressure heads. The mobile water content ratio changes with the applied water pressure, and it was found to be a function of the effective mean pore size, lambda(m). Results show a transition from a capillary-dominated to a gravity-dominated flow. The soil water flow changes to a macroporosity flow when the water pressure head increases from - 100 to 0 mm, resulting in an abrupt increase in the mobile water content ratio from 0.11 to 0.37. The proposed f(lambda(m)) relationship is an 5-shaped analytical equation. It appears that for a given porous network topology, the mobile water content depends both on the dynamics of the water movement and on the connectivity of the porous network.