Changes in the Moisture of a Soil Continuously Irrigated Over a Small Surface

The paper contains a description of water movements in the soil when the irrigated surface is restricted by a 40-cm-diam. hollow dug in the soil. Vertical and lateral water movements have been followed with a neutron-moisture meter designed for deep measurements. Several tubes, 1 m apart and extending to a depth of 3 m, are placed in a brown, clay-limestone soil on the site of the Cadarache Nuclear Research Centre. The hollow is dug round die central tube and a constant water level maintained in it. Water profiles taken at regular intervals of time over a period of several months show the infiltration on the central tube and diffusion on the outer tubes. On the central profile, the rate of advance of the infiltration front decreases with time. After some days, equilibrium is established between the water supply and the diffusion and evapotranspiration losses, and the soil moisture tends towards a constant value. Examination of the other profiles indicates vertical and lateral water movements and gives qualitative and quantitative information on the volume of moistened soil and the moisture limit. The phenomena observed can be explained by competition between gravity and suction forces and by the fact that the central profile, which is constantly under water, is not subject to evaporation, whereas in the others evaporation causes a rising movement of moisture. These investigations form part of the studies on the radiological safety of sites, for which purpose it is important to know the water movements in the first few metres of soils likely to be contaminated by the spilling of radioactive liquids. (author)