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.