Field measured mobile water fraction for soils of contrasting texture

For the purpose of modeling solute transport, soil water has often been sim ply divided into an essentially mobile fraction, theta (m), which is active in solute transport, and an apparently immobile fraction, theta (im). Dist inction between theta (m) and theta (im) was sought using the disc permeame ter technique. This study examines unsaturated estimates of mobile water co ntent at suction heads, h, of 20, 40, 80, and 120 mm for several soils rang ing in texture from sand to clay. Following infiltration of 35 mm depth of 0.01 M KBr into initially dry soils, soil samples were collected from below the base of the disc permeameter and analysed for tracer concentrations wh ich enabled partitioning of mobile and immobile water. Hydraulic conductivi ty and sorptivity were also derived from the infiltration data. The results show the expected non-linearity of hydraulic conductivity and sorptivity w ith suction. The mobile water expressed as a fraction, f, of the volumetric water conten t theta (f = theta (m)/theta) was generally found to range from 0.7 to 0.95 , with an average of 0.85. The exception was one site for which f was appro ximate to0.50. These values of f are comparable to those derived from leach ing studies reported in the literature. An important finding of this work i s that within the range of suctions measured, the mobile fraction was indep endent of suction. A possible explanation for this observation is that the soil capillary forces were dominant during the time scale of the experiment and therefore rapidly drew the invading solution. This finding could have important implications for fertiliser application. Furthermore, this result suggests that the assumption of a negligible solute transfer coefficient, alpha, between the mobile and immobile domains may be valid within the time scale of this method of measuring the mobile water content.